draft-ietf-acme-acme-01.txt   draft-ietf-acme-acme-02.txt 
Network Working Group R. Barnes Network Working Group R. Barnes
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Standards Track J. Hoffman-Andrews Intended status: Standards Track J. Hoffman-Andrews
Expires: April 6, 2016 EFF Expires: September 22, 2016 EFF
J. Kasten J. Kasten
University of Michigan University of Michigan
October 04, 2015 March 21, 2016
Automatic Certificate Management Environment (ACME) Automatic Certificate Management Environment (ACME)
draft-ietf-acme-acme-01 draft-ietf-acme-acme-02
Abstract Abstract
Certificates in the Web's X.509 PKI (PKIX) are used for a number of Certificates in the Web's X.509 PKI (PKIX) are used for a number of
purposes, the most significant of which is the authentication of purposes, the most significant of which is the authentication of
domain names. Thus, certificate authorities in the Web PKI are domain names. Thus, certificate authorities in the Web PKI are
trusted to verify that an applicant for a certificate legitimately trusted to verify that an applicant for a certificate legitimately
represents the domain name(s) in the certificate. Today, this represents the domain name(s) in the certificate. Today, this
verification is done through a collection of ad hoc mechanisms. This verification is done through a collection of ad hoc mechanisms. This
document describes a protocol that a certificate authority (CA) and document describes a protocol that a certificate authority (CA) and
an applicant can use to automate the process of verification and an applicant can use to automate the process of verification and
certificate issuance. The protocol also provides facilities for certificate issuance. The protocol also provides facilities for
other certificate management functions, such as certificate other certificate management functions, such as certificate
revocation. revocation.
DANGER: Do not implement this specification. It has a known DISCLAIMER: This is a work in progress draft of ACME and has not yet
signature reuse vulnerability. For details, see the following had a thorough security analysis.
discussion:
https://mailarchive.ietf.org/arch/msg/acme/F71iz6qq1o_QPVhJCV4dqWf- RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for
4Yc this draft is maintained in GitHub. Suggested changes should be
submitted as pull requests at https://github.com/ietf-wg-acme/acme .
Instructions are on that page as well. Editorial changes can be
managed in GitHub, but any substantive change should be discussed on
the ACME mailing list (acme@ietf.org).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 6, 2016.
This Internet-Draft will expire on September 22, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Deployment Model and Operator Experience . . . . . . . . . . 4 2. Deployment Model and Operator Experience . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
5. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 9 5. Message Transport . . . . . . . . . . . . . . . . . . . . . . 9
5.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 9 5.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 9
5.2. Registration Objects . . . . . . . . . . . . . . . . . . 10 5.2. Request Authentication . . . . . . . . . . . . . . . . . 9
5.3. Authorization Objects . . . . . . . . . . . . . . . . . . 11 5.3. Request URI Type Integrity . . . . . . . . . . . . . . . 10
5.4. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.4. Replay protection . . . . . . . . . . . . . . . . . . . . 11
5.5. Replay protection . . . . . . . . . . . . . . . . . . . . 14 5.4.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 12
5.5.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 14 5.4.2. "nonce" (Nonce) JWS header parameter . . . . . . . . 12
5.5.2. "nonce" (Nonce) JWS header parameter . . . . . . . . 15 5.5. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Key Agreement . . . . . . . . . . . . . . . . . . . . . . 15 6. Certificate Management . . . . . . . . . . . . . . . . . . . 14
6. Certificate Management . . . . . . . . . . . . . . . . . . . 16 6.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16 6.1.1. Registration Objects . . . . . . . . . . . . . . . . 16
6.1.2. Authorization Objects . . . . . . . . . . . . . . . . 17
6.2. Directory . . . . . . . . . . . . . . . . . . . . . . . . 18 6.2. Directory . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. Registration . . . . . . . . . . . . . . . . . . . . . . 18 6.3. Registration . . . . . . . . . . . . . . . . . . . . . . 20
6.3.1. Recovery Keys . . . . . . . . . . . . . . . . . . . . 20 6.3.1. Account Key Roll-over . . . . . . . . . . . . . . . . 22
6.4. Account Recovery . . . . . . . . . . . . . . . . . . . . 22 6.3.2. Deleting an Account . . . . . . . . . . . . . . . . . 23
6.4.1. MAC-Based Recovery . . . . . . . . . . . . . . . . . 23 6.4. Identifier Authorization . . . . . . . . . . . . . . . . 24
6.4.2. Contact-Based Recovery . . . . . . . . . . . . . . . 25 6.4.1. Responding to Challenges . . . . . . . . . . . . . . 26
6.5. Identifier Authorization . . . . . . . . . . . . . . . . 27 6.4.2. Deleting an Authorization . . . . . . . . . . . . . . 28
6.6. Certificate Issuance . . . . . . . . . . . . . . . . . . 31 6.5. Certificate Issuance . . . . . . . . . . . . . . . . . . 29
6.7. Certificate Revocation . . . . . . . . . . . . . . . . . 34 6.6. Certificate Revocation . . . . . . . . . . . . . . . . . 32
7. Identifier Validation Challenges . . . . . . . . . . . . . . 35 7. Identifier Validation Challenges . . . . . . . . . . . . . . 33
7.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 37 7.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 35
7.2. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7.2. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.3. TLS with Server Name Indication (TLS SNI) . . . . . . . . 40 7.3. TLS with Server Name Indication (TLS SNI) . . . . . . . . 38
7.4. Proof of Possession of a Prior Key . . . . . . . . . . . 42 7.4. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.5. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 9. Well-Known URI for the HTTP Challenge . . . . . . . . . . . . 41
9. Security Considerations . . . . . . . . . . . . . . . . . . . 46 9.1. Replay-Nonce HTTP Header . . . . . . . . . . . . . . . . 41
9.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 46 9.2. "nonce" JWS Header Parameter . . . . . . . . . . . . . . 41
9.2. Integrity of Authorizations . . . . . . . . . . . . . . . 47 9.3. URN Sub-namespace for ACME (urn:ietf:params:acme) . . . . 42
9.3. Preventing Authorization Hijacking . . . . . . . . . . . 50 9.4. New Registries . . . . . . . . . . . . . . . . . . . . . 42
9.4. Denial-of-Service Considerations . . . . . . . . . . . . 52 9.4.1. Error Codes . . . . . . . . . . . . . . . . . . . . . 42
9.5. CA Policy Considerations . . . . . . . . . . . . . . . . 52 9.4.2. Identifier Types . . . . . . . . . . . . . . . . . . 43
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 53 9.4.3. Challenge Types . . . . . . . . . . . . . . . . . . . 43
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 53 10. Security Considerations . . . . . . . . . . . . . . . . . . . 44
11.1. Normative References . . . . . . . . . . . . . . . . . . 53 10.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 44
11.2. Informative References . . . . . . . . . . . . . . . . . 55 10.2. Integrity of Authorizations . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 56 10.3. Denial-of-Service Considerations . . . . . . . . . . . . 48
10.4. CA Policy Considerations . . . . . . . . . . . . . . . . 49
11. Operational Considerations . . . . . . . . . . . . . . . . . 49
11.1. Default Virtual Hosts . . . . . . . . . . . . . . . . . 49
11.2. Use of DNSSEC Resolvers . . . . . . . . . . . . . . . . 50
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 51
13.1. Normative References . . . . . . . . . . . . . . . . . . 51
13.2. Informative References . . . . . . . . . . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
1. Introduction 1. Introduction
Certificates in the Web PKI are most commonly used to authenticate Certificates in the Web PKI [RFC5280] are most commonly used to
domain names. Thus, certificate authorities in the Web PKI are authenticate domain names. Thus, certificate authorities in the Web
trusted to verify that an applicant for a certificate legitimately PKI are trusted to verify that an applicant for a certificate
represents the domain name(s) in the certificate. legitimately represents the domain name(s) in the certificate.
Existing Web PKI certificate authorities tend to run on a set of ad Existing Web PKI certificate authorities tend to run on a set of ad
hoc protocols for certificate issuance and identity verification. A hoc protocols for certificate issuance and identity verification. A
typical user experience is something like: typical user experience is something like:
o Generate a PKCS#10 [RFC2314] Certificate Signing Request (CSR). o Generate a PKCS#10 [RFC2314] Certificate Signing Request (CSR).
o Cut-and-paste the CSR into a CA web page. o Cut-and-paste the CSR into a CA web page.
o Prove ownership of the domain by one of the following methods: o Prove ownership of the domain by one of the following methods:
skipping to change at page 4, line 26 skipping to change at page 4, line 41
interaction. Use of this protocol should radically simplify the interaction. Use of this protocol should radically simplify the
deployment of HTTPS and the practicality of PKIX authentication for deployment of HTTPS and the practicality of PKIX authentication for
other protocols based on TLS [RFC5246]. other protocols based on TLS [RFC5246].
2. Deployment Model and Operator Experience 2. Deployment Model and Operator Experience
The major guiding use case for ACME is obtaining certificates for Web The major guiding use case for ACME is obtaining certificates for Web
sites (HTTPS [RFC2818]). In that case, the server is intended to sites (HTTPS [RFC2818]). In that case, the server is intended to
speak for one or more domains, and the process of certificate speak for one or more domains, and the process of certificate
issuance is intended to verify that the server actually speaks for issuance is intended to verify that the server actually speaks for
the domain. the domain(s).
Different types of certificates reflect different kinds of CA Different types of certificates reflect different kinds of CA
verification of information about the certificate subject. "Domain verification of information about the certificate subject. "Domain
Validation" (DV) certificates are by far the most common type. For Validation" (DV) certificates are by far the most common type. For
DV validation, the CA merely verifies that the requester has DV validation, the CA merely verifies that the requester has
effective control of the web server and/or DNS server for the domain, effective control of the web server and/or DNS server for the domain,
but does not explicitly attempt to verify their real-world identity. but does not explicitly attempt to verify their real-world identity.
(This is as opposed to "Organization Validation" (OV) and "Extended (This is as opposed to "Organization Validation" (OV) and "Extended
Validation" (EV) certificates, where the process is intended to also Validation" (EV) certificates, where the process is intended to also
verify the real-world identity of the requester.) verify the real-world identity of the requester.)
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Different types of certificates reflect different kinds of CA Different types of certificates reflect different kinds of CA
verification of information about the certificate subject. "Domain verification of information about the certificate subject. "Domain
Validation" (DV) certificates are by far the most common type. For Validation" (DV) certificates are by far the most common type. For
DV validation, the CA merely verifies that the requester has DV validation, the CA merely verifies that the requester has
effective control of the web server and/or DNS server for the domain, effective control of the web server and/or DNS server for the domain,
but does not explicitly attempt to verify their real-world identity. but does not explicitly attempt to verify their real-world identity.
(This is as opposed to "Organization Validation" (OV) and "Extended (This is as opposed to "Organization Validation" (OV) and "Extended
Validation" (EV) certificates, where the process is intended to also Validation" (EV) certificates, where the process is intended to also
verify the real-world identity of the requester.) verify the real-world identity of the requester.)
DV certificate validation commonly checks claims about properties DV certificate validation commonly checks claims about properties
related to control of a domain name - properties that can be observed related to control of a domain name - properties that can be observed
by the issuing authority in an interactive process that can be by the issuing authority in an interactive process that can be
conducted purely online. That means that under typical conducted purely online. That means that under typical
circumstances, all steps in the request, verification, and issuance circumstances, all steps in the request, verification, and issuance
process can be represented and performed by Internet protocols with process can be represented and performed by Internet protocols with
no out-of-band human intervention. no out-of-band human intervention.
When an operator deploys a current HTTPS server, it generally prompts When deploying a current HTTPS server, an operator generally gets a
him to generate a self-signed certificate. When an operator deploys prompt to generate a self-signed certificate. When an operator
an ACME-compatible web server, the experience would be something like deploys an ACME-compatible web server, the experience would be
this: something like this:
o The ACME client prompts the operator for the intended domain o The ACME client prompts the operator for the intended domain
name(s) that the web server is to stand for. name(s) that the web server is to stand for.
o The ACME client presents the operator with a list of CAs from o The ACME client presents the operator with a list of CAs from
which it could get a certificate. (This list will change over which it could get a certificate. (This list will change over
time based on the capabilities of CAs and updates to ACME time based on the capabilities of CAs and updates to ACME
configuration.) The ACME client might prompt the operator for configuration.) The ACME client might prompt the operator for
payment information at this point. payment information at this point.
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document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
The two main roles in ACME are "client" and "server". The ACME The two main roles in ACME are "client" and "server". The ACME
client uses the protocol to request certificate management actions, client uses the protocol to request certificate management actions,
such as issuance or revocation. An ACME client therefore typically such as issuance or revocation. An ACME client therefore typically
runs on a web server, mail server, or some other server system which runs on a web server, mail server, or some other server system which
requires valid TLS certificates. The ACME server runs at a requires valid TLS certificates. The ACME server runs at a
certificate authority, and responds to client requests, performing certificate authority, and responds to client requests, performing
the requested actions if the client is authorized. the requested actions if the client is authorized.
For simplicity, in all HTTPS transactions used by ACME, the ACME An ACME client is represented by an "account key pair". The client
client is the HTTPS client and the ACME server is the HTTPS server. uses the private key of this key pair to sign all messages sent to
the server. The server uses the public key to verify the
In the discussion below, we will refer to three different types of authenticity and integrity of messages from the client.
keys / key pairs:
Subject Public Key: A public key to be included in a certificate.
Account Key Pair: A key pair for which the ACME server considers the
holder of the private key authorized to manage certificates for a
given identifier. The same key pair may be authorized for
multiple identifiers.
Recovery Key: A MAC key that a client can use to demonstrate that it
participated in a prior registration transaction.
ACME messaging is based on HTTPS [RFC2818] and JSON [RFC7159]. Since
JSON is a text-based format, binary fields are Base64-encoded. For
Base64 encoding, we use the variant defined in [RFC7515]. The
important features of this encoding are (1) that it uses the URL-safe
character set, and (2) that "=" padding characters are stripped.
Some HTTPS bodies in ACME are authenticated and integrity-protected
by being encapsulated in a JSON Web Signature (JWS) object [RFC7515].
ACME uses a profile of JWS, with the following restrictions:
o The JWS MUST use the Flattened JSON Serialization
o The JWS MUST be encoded using UTF-8
o The JWS Header or Protected Header MUST include "alg" and "jwk"
fields
o The JWS MUST NOT have the value "none" in its "alg" field
Additionally, JWS objects used in ACME MUST include the "nonce"
header parameter, defined below.
4. Protocol Overview 4. Protocol Overview
ACME allows a client to request certificate management actions using ACME allows a client to request certificate management actions using
a set of JSON messages carried over HTTPS. In some ways, ACME a set of JSON messages carried over HTTPS. In some ways, ACME
functions much like a traditional CA, in which a user creates an functions much like a traditional CA, in which a user creates an
account, adds identifiers to that account (proving control of the account, adds identifiers to that account (proving control of the
domains), and requests certificate issuance for those domains while domains), and requests certificate issuance for those domains while
logged in to the account. logged in to the account.
skipping to change at page 8, line 30 skipping to change at page 8, line 9
<------- Authorization <------- Authorization
Once the client has authorized an account key pair for an identifier, Once the client has authorized an account key pair for an identifier,
it can use the key pair to authorize the issuance of certificates for it can use the key pair to authorize the issuance of certificates for
the identifier. To do this, the client sends a PKCS#10 Certificate the identifier. To do this, the client sends a PKCS#10 Certificate
Signing Request (CSR) to the server (indicating the identifier(s) to Signing Request (CSR) to the server (indicating the identifier(s) to
be included in the issued certificate) and a signature over the CSR be included in the issued certificate) and a signature over the CSR
by the private key of the account key pair. by the private key of the account key pair.
Note that as a result, the CSR is signed twice: One by the private
key corresponding to the public key in the CSR, and once by the
private key of the account key pair. The former signature indicates
that the holder of the key in the CSR is willing to act for the
indicated identifiers, and the latter signature indicates to the
server that the issuance of the certificate is authorized by the
client (i.e., the domain holder).
If the server agrees to issue the certificate, then it creates the If the server agrees to issue the certificate, then it creates the
certificate and provides it in its response. The certificate is certificate and provides it in its response. The certificate is
assigned a URI, which the client can use to fetch updated versions of assigned a URI, which the client can use to fetch updated versions of
the certificate. the certificate.
Client Server Client Server
CSR CSR
Signature --------> Signature -------->
<-------- Certificate <-------- Certificate
To revoke a certificate, the client simply sends a revocation To revoke a certificate, the client simply sends a revocation request
request, signed with an authorized key pair, and the server indicates indicating the certificate to be revoked, signed with an authorized
whether the request has succeeded. key pair. The server indicates whether the request has succeeded.
Client Server Client Server
Revocation request Revocation request
Signature --------> Signature -------->
<-------- Result <-------- Result
Note that while ACME is defined with enough flexibility to handle Note that while ACME is defined with enough flexibility to handle
different types of identifiers in principle, the primary use case different types of identifiers in principle, the primary use case
addressed by this document is the case where domain names are used as addressed by this document is the case where domain names are used as
identifiers. For example, all of the identifier validation identifiers. For example, all of the identifier validation
challenges described in Section 7 below address validation of domain challenges described in Section 7 below address validation of domain
names. The use of ACME for other protocols will require further names. The use of ACME for other protocols will require further
specification, in order to describe how these identifiers are encoded specification, in order to describe how these identifiers are encoded
in the protocol, and what types of validation challenges the server in the protocol, and what types of validation challenges the server
might require. might require.
5. Protocol Elements 5. Message Transport
This section describes several components that are used by ACME, and ACME uses a combination of HTTPS and JWS to create a messaging layer
general rules that apply to ACME transactions. with a few important security properties.
Communications between an ACME client and an ACME server are done
over HTTPS, using JWS to provide som additional security properties
for messages sent from the client to the server. HTTPS provides
server authentication and confidentiality. With some ACME-specific
extensions, JWS provides authentication of the client's request
payloads, anti-replay protection, and a degree of integrity for the
HTTPS request URI.
5.1. HTTPS Requests 5.1. HTTPS Requests
Each ACME function is accomplished by the client sending a sequence Each ACME function is accomplished by the client sending a sequence
of HTTPS requests to the server, carrying JSON messages. Use of of HTTPS requests to the server, carrying JSON messages
HTTPS is REQUIRED. Clients SHOULD support HTTP public key pinning [RFC2818][RFC7159]. Use of HTTPS is REQUIRED. Clients SHOULD
[RFC7469], and servers SHOULD emit pinning headers. Each subsection support HTTP public key pinning [RFC7469], and servers SHOULD emit
of Section 6 below describes the message formats used by the pinning headers. Each subsection of Section 6 below describes the
function, and the order in which messages are sent. message formats used by the function, and the order in which messages
are sent.
All ACME requests with a non-empty body MUST encapsulate the body in
a JWS object, signed using the account key pair. The server MUST
verify the JWS before processing the request. (For readability,
however, the examples below omit this encapsulation.) Encapsulating
request bodies in JWS provides a simple authentication of requests by
way of key continuity.
Note that this implies that GET requests are not authenticated.
Servers MUST NOT respond to GET requests for resources that might be
considered sensitive.
An ACME request carries a JSON dictionary that provides the details
of the client's request to the server. In order to avoid attacks
that might arise from sending a request object to a resource of the
wrong type, each request object MUST have a "resource" field that
indicates what type of resource the request is addressed to, as
defined in the below table:
+----------------------+------------------+
| Resource type | "resource" value |
+----------------------+------------------+
| New registration | new-reg |
| | |
| Recover registration | recover-reg |
| | |
| New authorization | new-authz |
| | |
| New certificate | new-cert |
| | |
| Revoke certificate | revoke-cert |
| | |
| Registration | reg |
| | |
| Authorization | authz |
| | |
| Challenge | challenge |
| | |
| Certificate | cert |
+----------------------+------------------+
Other fields in ACME request bodies are described below. In all HTTPS transactions used by ACME, the ACME client is the HTTPS
client and the ACME server is the HTTPS server.
ACME servers that are intended to be generally accessible need to use ACME servers that are intended to be generally accessible need to use
Cross-Origin Resource Sharing (CORS) in order to be accessible from Cross-Origin Resource Sharing (CORS) in order to be accessible from
browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD
set the Access-Control-Allow-Origin header field to the value "*". set the Access-Control-Allow-Origin header field to the value "*".
5.2. Registration Objects Binary fields in the JSON objects used by ACME are encoded using
base64url encoding described in [RFC4648] Section 5, according to the
An ACME registration resource represents a set of metadata associated profile specified in JSON Web Signature [RFC7515] Section 2. This
to an account key pair. Registration resources have the following encoding uses a URL safe character set. Trailing '=' characters MUST
structure: be stripped.
key (required, dictionary): The public key of the account key pair,
encoded as a JSON Web Key object [RFC7517].
contact (optional, array of string): An array of URIs that the
server can use to contact the client for issues related to this
authorization. For example, the server may wish to notify the
client about server-initiated revocation.
agreement (optional, string): A URI referring to a subscriber
agreement or terms of service provided by the server (see below).
Including this field indicates the client's agreement with the
referenced terms.
authorizations (optional, string): A URI from which a list of
authorizations granted to this account can be fetched via a GET
request. The result of the GET request MUST be a JSON object
whose "authorizations" field is an array of strings, where each
string is the URI of an authorization belonging to this
registration. The server SHOULD include pending authorizations,
and SHOULD NOT include authorizations that are invalid or expired.
certificates (optional, string): A URI from which a list of
certificates issued for this account can be fetched via a GET
request. The result of the GET request MUST be a JSON object
whose "certificates" field is an array of strings, where each
string is the URI of a certificate. The server SHOULD NOT include
expired certificates.
{
"resource": "new-reg",
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
],
"agreement": "https://example.com/acme/terms",
"authorizations": "https://example.com/acme/reg/1/authz",
"certificates": "https://example.com/acme/reg/1/cert",
}
5.3. Authorization Objects
An ACME authorization object represents server's authorization for an
account to represent an identifier. In addition to the identifier,
an authorization includes several metadata fields, such as the status
of the authorization (e.g., "pending", "valid", or "revoked") and
which challenges were used to validate possession of the identifier.
The structure of an ACME authorization resource is as follows:
identifier (required, dictionary of string): The identifier that the
account is authorized to represent
type (required, string): The type of identifier.
value (required, string): The identifier itself. 5.2. Request Authentication
status (optional, string): The status of this authorization. All ACME requests with a non-empty body MUST encapsulate the body in
Possible values are: "unknown", "pending", "processing", "valid", a JWS object, signed using the account key pair. The server MUST
"invalid" and "revoked". If this field is missing, then the verify the JWS before processing the request. (For readability,
default value is "pending". however, the examples below omit this encapsulation.) Encapsulating
request bodies in JWS provides a simple authentication of requests by
way of key continuity.
expires (optional, string): The date after which the server will JWS objects sent in ACME requests MUST meet the following additional
consider this authorization invalid, encoded in the format criteria:
specified in RFC 3339 [RFC3339].
challenges (required, array): The challenges that the client needs o The JWS MUST use the Flattened JSON Serialization
to fulfill in order to prove possession of the identifier (for
pending authorizations). For final authorizations, the challenges
that were used. Each array entry is a dictionary with parameters
required to validate the challenge, as specified in Section 7.
combinations (optional, array of arrays of integers): A collection o The JWS MUST be encoded using UTF-8
of sets of challenges, each of which would be sufficient to prove
possession of the identifier. Clients complete a set of
challenges that that covers at least one set in this array.
Challenges are identified by their indices in the challenges
array. If no "combinations" element is included in an
authorization object, the client completes all challenges.
The only type of identifier defined by this specification is a fully- o The JWS Header or Protected Header MUST include "alg" and "jwk"
qualified domain name (type: "dns"). The value of the identifier fields
MUST be the ASCII representation of the domain name. Wildcard domain
names (with "*" as the first label) MUST NOT be included in
authorization requests. See Section 6.6 below for more information
about wildcard domains.
{ o The JWS MUST NOT have the value "none" in its "alg" field
"status": "valid",
"expires": "2015-03-01",
"identifier": { o The JWS Protected Header MUST include the "nonce" field (defined
"type": "dns", below)
"value": "example.org"
},
"challenges": [ Note that this implies that GET requests are not authenticated.
{ Servers MUST NOT respond to GET requests for resources that might be
"type": "http-01", considered sensitive.
"status": "valid",
"validated": "2014-12-01T12:05Z",
"keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE"
}
],
}
5.4. Errors 5.3. Request URI Type Integrity
Errors can be reported in ACME both at the HTTP layer and within ACME It is common in deployment the entity terminating TLS for HTTPS to be
payloads. ACME servers can return responses with an HTTP error different from the entity operating the logical HTTPS server, with a
response code (4XX or 5XX). For example: If the client submits a "request routing" layer in the middle. For example, an ACME CA might
request using a method not allowed in this document, then the server have a content delivery network terminate TLS connections from
MAY return status code 405 (Method Not Allowed). clients so that it can inspect client requests for denial-of-service
protection.
When the server responds with an error status, it SHOULD provide These intermediaries can also change values in the request that are
additional information using problem document not signed in the HTTPS request, e.g., the request URI and headers.
[I-D.ietf-appsawg-http-problem]. The "type" and "detail" fields MUST ACME uses JWS to provides a limited integrity mechanism, which
be populated. To facilitate automatic response to errors, this protects against an intermediary changing the request URI to anothe
document defines the following standard tokens for use in the "type" ACME URI of a different type. (It does not protect against changing
field (within the "urn:acme:" namespace): between URIs of the same type, e.g., from one authorization URI to
another).
+----------------+--------------------------------------------------+ An ACME request carries a JSON dictionary that provides the details
| Code | Semantic | of the client's request to the server. Each request object MUST have
+----------------+--------------------------------------------------+ a "resource" field that indicates what type of resource the request
| badCSR | The CSR is unacceptable (e.g., due to a short | is addressed to, as defined in the below table:
| | key) |
| | |
| badNonce | The client sent an unacceptable anti-replay |
| | nonce |
| | |
| connection | The server could not connect to the client for |
| | DV |
| | |
| dnssec | The server could not validate a DNSSEC signed |
| | domain |
| | |
| malformed | The request message was malformed |
| | |
| serverInternal | The server experienced an internal error |
| | |
| tls | The server experienced a TLS error during DV |
| | |
| unauthorized | The client lacks sufficient authorization |
| | |
| unknownHost | The server could not resolve a domain name |
+----------------+--------------------------------------------------+
Authorization and challenge objects can also contain error +--------------------+------------------+
information to indicate why the server was unable to validate | Resource type | "resource" value |
authorization. +--------------------+------------------+
| New registration | new-reg |
| | |
| New authorization | new-authz |
| | |
| New certificate | new-cert |
| | |
| Revoke certificate | revoke-cert |
| | |
| Registration | reg |
| | |
| Authorization | authz |
| | |
| Challenge | challenge |
| | |
| Certificate | cert |
+--------------------+------------------+
TODO: Flesh out errors and syntax for them Other fields in ACME request bodies are described below.
5.5. Replay protection 5.4. Replay protection
In order to protect ACME resources from any possible replay attacks, In order to protect ACME resources from any possible replay attacks,
ACME requests have a mandatory anti-replay mechanism. This mechanism ACME requests have a mandatory anti-replay mechanism. This mechanism
is based on the server maintaining a list of nonces that it has is based on the server maintaining a list of nonces that it has
issued to clients, and requiring any signed request from the client issued to clients, and requiring any signed request from the client
to carry such a nonce. to carry such a nonce.
An ACME server MUST include a Replay-Nonce header field in each An ACME server MUST include a Replay-Nonce header field in each
successful response it provides to a client, with contents as successful response it provides to a client, with contents as
specified below. In particular, the ACME server MUST provide a specified below. In particular, the ACME server MUST provide a
skipping to change at page 14, line 30 skipping to change at page 12, line 4
Every JWS sent by an ACME client MUST include, in its protected Every JWS sent by an ACME client MUST include, in its protected
header, the "nonce" header parameter, with contents as defined below. header, the "nonce" header parameter, with contents as defined below.
As part of JWS verification, the ACME server MUST verify that the As part of JWS verification, the ACME server MUST verify that the
value of the "nonce" header is a value that the server previously value of the "nonce" header is a value that the server previously
provided in a Replay-Nonce header field. Once a nonce value has provided in a Replay-Nonce header field. Once a nonce value has
appeared in an ACME request, the server MUST consider it invalid, in appeared in an ACME request, the server MUST consider it invalid, in
the same way as a value it had never issued. the same way as a value it had never issued.
When a server rejects a request because its nonce value was When a server rejects a request because its nonce value was
unacceptable (or not present), it SHOULD provide HTTP status code 400 unacceptable (or not present), it SHOULD provide HTTP status code 400
(Bad Request), and indicate the ACME error code "urn:acme:badNonce". (Bad Request), and indicate the ACME error code
"urn:ietf:params:acme:error:badNonce".
The precise method used to generate and track nonces is up to the The precise method used to generate and track nonces is up to the
server. For example, the server could generate a random 128-bit server. For example, the server could generate a random 128-bit
value for each response, keep a list of issued nonces, and strike value for each response, keep a list of issued nonces, and strike
nonces from this list as they are used. nonces from this list as they are used.
5.5.1. Replay-Nonce 5.4.1. Replay-Nonce
The "Replay-Nonce" header field includes a server-generated value The "Replay-Nonce" header field includes a server-generated value
that the server can use to detect unauthorized replay in future that the server can use to detect unauthorized replay in future
client requests. The server should generate the value provided in client requests. The server should generate the value provided in
Replay-Nonce in such a way that they are unique to each message, with Replay-Nonce in such a way that they are unique to each message, with
high probability. high probability.
The value of the Replay-Nonce field MUST be an octet string encoded The value of the Replay-Nonce field MUST be an octet string encoded
according to the base64url encoding described in Section 2 of according to the base64url encoding described in Section 2 of
[RFC7515]. Clients MUST ignore invalid Replay-Nonce values. [RFC7515]. Clients MUST ignore invalid Replay-Nonce values.
base64url = [A-Z] / [a-z] / [0-9] / "-" / "_" base64url = [A-Z] / [a-z] / [0-9] / "-" / "_"
Replay-Nonce = *base64url Replay-Nonce = *base64url
The Replay-Nonce header field SHOULD NOT be included in HTTP request The Replay-Nonce header field SHOULD NOT be included in HTTP request
messages. messages.
5.5.2. "nonce" (Nonce) JWS header parameter 5.4.2. "nonce" (Nonce) JWS header parameter
The "nonce" header parameter provides a unique value that enables the The "nonce" header parameter provides a unique value that enables the
verifier of a JWS to recognize when replay has occurred. The "nonce" verifier of a JWS to recognize when replay has occurred. The "nonce"
header parameter MUST be carried in the protected header of the JWS. header parameter MUST be carried in the protected header of the JWS.
The value of the "nonce" header parameter MUST be an octet string, The value of the "nonce" header parameter MUST be an octet string,
encoded according to the base64url encoding described in Section 2 of encoded according to the base64url encoding described in Section 2 of
[RFC7515]. If the value of a "nonce" header parameter is not valid [RFC7515]. If the value of a "nonce" header parameter is not valid
according to this encoding, then the verifier MUST reject the JWS as according to this encoding, then the verifier MUST reject the JWS as
malformed. malformed.
5.6. Key Agreement 5.5. Errors
Certain elements of the protocol will require the establishment of a
shared secret between the client and the server, in such a way that
an entity observing the ACME protocol cannot derive the secret. In
these cases, we use a simple ECDH key exchange, based on the system
used by CMS [RFC5753]:
o Inputs:
* Client-generated key pair
* Server-generated key pair
* Length of the shared secret to be derived
* Label
o Perform the ECDH primitive operation to obtain Z (Section 3.3.1 of
[SEC1])
o Select a hash algorithm according to the curve being used:
* For "P-256", use SHA-256
* For "P-384", use SHA-384 Errors can be reported in ACME both at the HTTP layer and within ACME
payloads. ACME servers can return responses with an HTTP error
response code (4XX or 5XX). For example: If the client submits a
request using a method not allowed in this document, then the server
MAY return status code 405 (Method Not Allowed).
* For "P-521", use SHA-512 When the server responds with an error status, it SHOULD provide
additional information using problem document
[I-D.ietf-appsawg-http-problem]. To facilitate automatic response to
errors, this document defines the following standard tokens for use
in the "type" field (within the "urn:ietf:params:acme:error:"
namespace):
o Derive the shared secret value using the KDF in Section 3.6.1 of +----------------+--------------------------------------------------+
[SEC1] using Z and the selected hash algorithm, and with the UTF-8 | Code | Description |
encoding of the label as the SharedInfo value +----------------+--------------------------------------------------+
| badCSR | The CSR is unacceptable (e.g., due to a short |
| | key) |
| | |
| badNonce | The client sent an unacceptable anti-replay |
| | nonce |
| | |
| connection | The server could not connect to the client for |
| | validation |
| | |
| dnssec | The server could not validate a DNSSEC signed |
| | domain |
| | |
| malformed | The request message was malformed |
| | |
| serverInternal | The server experienced an internal error |
| | |
| tls | The server experienced a TLS error during |
| | validation |
| | |
| unauthorized | The client lacks sufficient authorization |
| | |
| unknownHost | The server could not resolve a domain name |
| | |
| rateLimited | The request exceeds a rate limit |
| | |
| invalidContact | The provided contact URI for a registration was |
| | invalid |
+----------------+--------------------------------------------------+
In cases where the length of the derived secret is shorter than the This list is not exhaustive. The server MAY return errors whose
output length of the chosen hash algorithm, the KDF referenced above "type" field is set to a URI other than those defined above. Servers
reduces to a single hash invocation. The shared secret is equal to MUST NOT use the ACME URN namespace for errors other than the
the leftmost octets of the following: standard types. Clients SHOULD display the "detail" field of such
errors.
H( Z || 00000001 || label ) Authorization and challenge objects can also contain error
information to indicate why the server was unable to validate
authorization.
6. Certificate Management 6. Certificate Management
In this section, we describe the certificate management functions In this section, we describe the certificate management functions
that ACME enables: that ACME enables:
o Account Key Registration o Account Key Registration
o Account Recovery
o Account Key Authorization o Account Key Authorization
o Certificate Issuance o Certificate Issuance
o Certificate Renewal o Certificate Renewal
o Certificate Revocation o Certificate Revocation
6.1. Resources 6.1. Resources
skipping to change at page 17, line 4 skipping to change at page 14, line 44
o A "directory" resource o A "directory" resource
o A "new-registration" resource o A "new-registration" resource
o A "new-authorization" resource o A "new-authorization" resource
o A "new-certificate" resource o A "new-certificate" resource
o A "revoke-certificate" resource o A "revoke-certificate" resource
For the "new-X" resources above, the server MUST have exactly one For the "new-X" resources above, the server MUST have exactly one
resource for each function. This resource may be addressed by resource for each function. This resource may be addressed by
multiple URIs, but all must provide equivalent functionality. multiple URIs, but all must provide equivalent functionality.
In general, the intent is for authorization and certificate resources
to contain only public information, so that CAs may publish these
resources to document what certificates have been issued and how they
were authorized. Non-public information, such as contact
information, is stored in registration resources.
ACME uses different URIs for different management functions. Each ACME uses different URIs for different management functions. Each
function is listed in a directory along with its corresponding URI, function is listed in a directory along with its corresponding URI,
so clients only need to be configured with the directory URI. so clients only need to be configured with the directory URI. These
URIs are connected by a few different link relations [RFC5988].
The "up" link relation is used with challenge resources to indicate The "up" link relation is used with challenge resources to indicate
the authorization resource to which a challenge belongs. It is also the authorization resource to which a challenge belongs. It is also
used from certificate resources to indicate a resource from which the used from certificate resources to indicate a resource from which the
client may fetch a chain of CA certificates that could be used to client may fetch a chain of CA certificates that could be used to
validate the certificate in the original resource. validate the certificate in the original resource.
The "directory" link relation is present on all resources other than
the directory and indicates the directory URL.
The following diagram illustrates the relations between resources on The following diagram illustrates the relations between resources on
an ACME server. The solid lines indicate link relations, and the an ACME server. The solid lines indicate link relations, and the
dotted lines correspond to relationships expressed in other ways, dotted lines correspond to relationships expressed in other ways,
e.g., the Location header in a 201 (Created) response. e.g., the Location header in a 201 (Created) response.
directory directory
. .
. .
.................................................... ....................................................
. . . . . . . .
skipping to change at page 18, line 24 skipping to change at page 16, line 24
| Poll for status | GET authz | 200 | | Poll for status | GET authz | 200 |
| | | | | | | |
| Request issuance | POST new-cert | 201 -> cert | | Request issuance | POST new-cert | 201 -> cert |
| | | | | | | |
| Check for new cert | GET cert | 200 | | Check for new cert | GET cert | 200 |
+--------------------+----------------+--------------+ +--------------------+----------------+--------------+
The remainder of this section provides the details of how these The remainder of this section provides the details of how these
resources are structured and how the ACME protocol makes use of them. resources are structured and how the ACME protocol makes use of them.
6.1.1. Registration Objects
An ACME registration resource represents a set of metadata associated
to an account key pair. Registration resources have the following
structure:
key (required, dictionary): The public key of the account key pair,
encoded as a JSON Web Key object [RFC7517].
contact (optional, array of string): An array of URIs that the
server can use to contact the client for issues related to this
authorization. For example, the server may wish to notify the
client about server-initiated revocation.
agreement (optional, string): A URI referring to a subscriber
agreement or terms of service provided by the server (see below).
Including this field indicates the client's agreement with the
referenced terms.
authorizations (required, string): A URI from which a list of
authorizations granted to this account can be fetched via a GET
request. The result of the GET request MUST be a JSON object
whose "authorizations" field is an array of strings, where each
string is the URI of an authorization belonging to this
registration. The server SHOULD include pending authorizations,
and SHOULD NOT include authorizations that are invalid or expired.
The server MAY return an incomplete list, along with a Link header
with link relation "next" indicating a URL to retrieve further
entries.
certificates (required, string): A URI from which a list of
certificates issued for this account can be fetched via a GET
request. The result of the GET request MUST be a JSON object
whose "certificates" field is an array of strings, where each
string is the URI of a certificate. The server SHOULD NOT include
expired or revoked certificates. The server MAY return an
incomplete list, along with a Link header with link relation
"next" indicating a URL to retrieve further entries.
{
"resource": "new-reg",
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
],
"agreement": "https://example.com/acme/terms",
"authorizations": "https://example.com/acme/reg/1/authz",
"certificates": "https://example.com/acme/reg/1/cert",
}
6.1.2. Authorization Objects
An ACME authorization object represents server's authorization for an
account to represent an identifier. In addition to the identifier,
an authorization includes several metadata fields, such as the status
of the authorization (e.g., "pending", "valid", or "revoked") and
which challenges were used to validate possession of the identifier.
The structure of an ACME authorization resource is as follows:
identifier (required, dictionary of string): The identifier that the
account is authorized to represent
type (required, string): The type of identifier.
value (required, string): The identifier itself.
status (required, string): The status of this authorization.
Possible values are: "unknown", "pending", "processing", "valid",
"invalid" and "revoked". If this field is missing, then the
default value is "pending".
expires (optional, string): The timestamp after which the server
will consider this authorization invalid, encoded in the format
specified in RFC 3339 [RFC3339]. This field is REQUIRED for
objects with "valid" in the "status field.
challenges (required, array): The challenges that the client needs
to fulfill in order to prove possession of the identifier (for
pending authorizations). For final authorizations, the challenges
that were used. Each array entry is a dictionary with parameters
required to validate the challenge, as specified in Section 7.
combinations (optional, array of arrays of integers): A collection
of sets of challenges, each of which would be sufficient to prove
possession of the identifier. Clients complete a set of
challenges that covers at least one set in this array. Challenges
are identified by their indices in the challenges array. If no
"combinations" element is included in an authorization object, the
client completes all challenges.
The only type of identifier defined by this specification is a fully-
qualified domain name (type: "dns"). The value of the identifier
MUST be the ASCII representation of the domain name. Wildcard domain
names (with "*" as the first label) MUST NOT be included in
authorization requests. See Section 6.5 below for more information
about wildcard domains.
{
"status": "valid",
"expires": "2015-03-01T14:09:00Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"type": "http-01",
"status": "valid",
"validated": "2014-12-01T12:05:00Z",
"keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE"
}
],
}
6.2. Directory 6.2. Directory
In order to help clients configure themselves with the right URIs for In order to help clients configure themselves with the right URIs for
each ACME operation, ACME servers provide a directory object. This each ACME operation, ACME servers provide a directory object. This
should be the root URL with which clients are configured. It is a should be the only URL needed to configure clients. It is a JSON
JSON dictionary, whose keys are the "resource" values listed in dictionary, whose keys are the "resource" values listed in
Section 5.1, and whose values are the URIs used to accomplish the Section 5.1, and whose values are the URIs used to accomplish the
corresponding function. corresponding function.
There is no constraint on the actual URI of the directory except that
it should be different from the other ACME server resources' URIs,
and that it should not clash with other services. For instance:
o a host which function as both an ACME and Web server may want to
keep the root path "/" for an HTML "front page", and and place the
ACME directory under path "/acme".
o a host which only functions as an ACME server could place the
directory under path "/".
The dictionary MAY additionally contain a key "meta". If present, it
MUST be a JSON dictionary; each item in the dictionary is an item of
metadata relating to the service provided by the ACME server.
The following metadata items are defined, all of which are OPTIONAL:
"terms-of-service" (optional, string): A URI identifying the current
terms of service.
"website" (optional, string)): An HTTP or HTTPS URL locating a
website providing more information about the ACME server.
"caa-identities" (optional, array of string): Each string MUST be a
lowercase hostname which the ACME server recognises as referring
to itself for the purposes of CAA record validation as defined in
[RFC6844]. This allows clients to determine the correct issuer
domain name to use when configuring CAA record.
Clients access the directory by sending a GET request to the Clients access the directory by sending a GET request to the
directory URI. directory URI.
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/json Content-Type: application/json
{ {
"new-reg": "https://example.com/acme/new-reg", "new-reg": "https://example.com/acme/new-reg",
"recover-reg": "https://example.com/acme/recover-reg",
"new-authz": "https://example.com/acme/new-authz", "new-authz": "https://example.com/acme/new-authz",
"new-cert": "https://example.com/acme/new-cert", "new-cert": "https://example.com/acme/new-cert",
"revoke-cert": "https://example.com/acme/revoke-cert" "revoke-cert": "https://example.com/acme/revoke-cert",
"meta": {
"terms-of-service": "https://example.com/acme/terms",
"website": "https://www.example.com/",
"caa-identities": ["example.com"]
}
} }
6.3. Registration 6.3. Registration
A client creates a new account with the server by sending a POST A client creates a new account with the server by sending a POST
request to the server's new-registration URI. The body of the request to the server's new-registration URI. The body of the
request is a stub registration object containing only the "contact" request is a stub registration object containing only the "contact"
field (along with the required "resource" field). field (along with the required "resource" field).
POST /acme/new-registration HTTP/1.1 POST /acme/new-registration HTTP/1.1
skipping to change at page 19, line 29 skipping to change at page 20, line 36
sent by the client, as well as any other fields that it does not sent by the client, as well as any other fields that it does not
recognize. If new fields are specified in the future, the recognize. If new fields are specified in the future, the
specification of those fields MUST describe whether they may be specification of those fields MUST describe whether they may be
provided by the client. provided by the client.
The server creates a registration object with the included contact The server creates a registration object with the included contact
information. The "key" element of the registration is set to the information. The "key" element of the registration is set to the
public key used to verify the JWS (i.e., the "jwk" element of the JWS public key used to verify the JWS (i.e., the "jwk" element of the JWS
header). The server returns this registration object in a 201 header). The server returns this registration object in a 201
(Created) response, with the registration URI in a Location header (Created) response, with the registration URI in a Location header
field. The server MUST also indicate its new-authorization URI using field. The server SHOULD also indicate its new-authorization URI
the "next" link relation. using the "next" link relation.
If the server already has a registration object with the provided If the server already has a registration object with the provided
account key, then it MUST return a 409 (Conflict) response and account key, then it MUST return a 409 (Conflict) response and
provide the URI of that registration in a Location header field. provide the URI of that registration in a Location header field.
This allows a client that has an account key but not the This allows a client that has an account key but not the
corresponding registration URI to recover the registration URI. corresponding registration URI to recover the registration URI.
If the server wishes to present the client with terms under which the If the server wishes to present the client with terms under which the
ACME service is to be used, it MUST indicate the URI where such terms ACME service is to be used, it MUST indicate the URI where such terms
can be accessed in a Link header with link relation "terms-of- can be accessed in a Link header with link relation "terms-of-
service". As noted above, the client may indicate its agreement with service". As noted above, the client may indicate its agreement with
these terms by updating its registration to include the "agreement" these terms by updating its registration to include the "agreement"
field, with the terms URI as its value. field, with the terms URI as its value. When these terms change in a
way that requires an agreement update, the server MUST use a
different URI in the Link header.
HTTP/1.1 201 Created HTTP/1.1 201 Created
Content-Type: application/json Content-Type: application/json
Location: https://example.com/acme/reg/asdf Location: https://example.com/acme/reg/asdf
Link: <https://example.com/acme/new-authz>;rel="next" Link: <https://example.com/acme/new-authz>;rel="next"
Link: <https://example.com/acme/recover-reg>;rel="recover"
Link: <https://example.com/acme/terms>;rel="terms-of-service" Link: <https://example.com/acme/terms>;rel="terms-of-service"
Link: <https://example.com/acme/some-directory>;rel="directory"
{ {
"key": { /* JWK from JWS header */ }, "key": { /* JWK from JWS header */ },
"contact": [ "contact": [
"mailto:cert-admin@example.com", "mailto:cert-admin@example.com",
"tel:+12025551212" "tel:+12025551212"
] ]
} }
If the client wishes to update this information in the future, it If the client wishes to update this information in the future, it
sends a POST request with updated information to the registration sends a POST request with updated information to the registration
URI. The server MUST ignore any updates to the "key", URI. The server MUST ignore any updates to the "key",
"authorizations, or "certificates" fields, and MUST verify that the "authorizations, or "certificates" fields, and MUST verify that the
request is signed with the private key corresponding to the "key" request is signed with the private key corresponding to the "key"
field of the request before updating the registration. field of the request before updating the registration.
Servers SHOULD NOT respond to GET requests for registration resources For example, to update the contact information in the above
as these requests are not authenticated. If a client wishes to query registration, the client could send the following request:
the server for information about its account (e.g., to examine the
"contact" or "certificates" fields), then it SHOULD do so by sending
a POST request with an empty update. That is, it should send a JWS
whose payload is trivial ({"resource":"reg"}). In this case the
server reply MUST contain the same link headers sent for a new
registration, to allow a client to retreive the "new-authorization"
and "terms-of-service" URI
6.3.1. Recovery Keys
If the client wishes to establish a secret key with the server that
it can use to recover this account later (a "recovery key"), then it
must perform a simple key agreement protocol as part of the new-
registration transaction. The client and server perform an ECDH
exchange through the new-registration transaction (using the
technique in Section 5.6), and the result is the recovery key.
To request a recovery key, the client includes a "recoveryKey" field
in its new-registration request. The value of this field is a JSON
object.
client (required, JWK): The client's ECDH public key
length (required, number): The length of the derived secret, in
octets.
In the client's request, this object contains a JWK for a random ECDH
public key generated by the client and the client-selected length
value. Clients need to choose length values that balance security
and usability. On the one hand, a longer secret makes it more
difficult for an attacker to recover the secret when it is used for
recovery (see Section 6.4.1). On the other hand, clients may wish to
make the recovery key short enough for a user to easily write it
down.
POST /acme/new-registration HTTP/1.1 POST /acme/reg/asdf HTTP/1.1
Host: example.com Host: example.com
{ {
"resource": "new-reg", "resource": "reg",
"contact": [ "contact": [
"mailto:cert-admin@example.com", "mailto:certificates@example.com",
"tel:+12025551212" "tel:+12125551212"
], ],
"recoveryKey": {
"client": { "kty": "EC", ... },
"length": 128
}
} }
/* Signed as JWS */ /* Signed as JWS */
The server MUST validate that the elliptic curve ("crv") and length Servers SHOULD NOT respond to GET requests for registration resources
value chosen by the client are acceptable, and that it is otherwise as these requests are not authenticated. If a client wishes to query
willing to create a recovery key. If not, then it MUST reject the the server for information about its account (e.g., to examine the
new-registration request. "contact" or "certificates" fields), then it SHOULD do so by sending
a POST request with an empty update. That is, it should send a JWS
If the server agrees to create a recovery key, then it generates its whose payload is trivial ({"resource":"reg"}). In this case the
own random ECDH key pair and combines it with with the client's server reply MUST contain the same link headers sent for a new
public key as described in Section 5.6 above, using the label registration, to allow a client to retrieve the "new-authorization"
"recovery". The derived secret value is the recovery key. The and "terms-of-service" URI
server then returns to the client the ECDH key that it generated.
The server MUST generate a fresh key pair for every transaction.
server (required, JWK): The server's ECDH public key
HTTP/1.1 201 Created
Content-Type: application/json
Location: https://example.com/acme/reg/asdf
{
"key": { /* JWK from JWS header */ },
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
],
"recoveryKey": { 6.3.1. Account Key Roll-over
"server": { "kty": "EC", ... }
}
}
On receiving the server's response, the client can compute the A client may wish to change the public key that is associated with a
recovery key by combining the server's public key together with the registration, e.g., in order to mitigate the risk of key compromise.
private key corresponding to the public key that it sent to the To do this, the client first constructs a JSON object representing a
server. request to update the registration:
Clients may refresh the recovery key associated with a registration resource (required, string): The string "reg", indicating an update
by sending a POST request with a new recoveryKey object. If the to the registration.
server agrees to refresh the recovery key, then it responds in the
same way as to a new registration request that asks for a recovery
key.
POST /acme/reg/asdf HTTP/1.1 oldKey (required, string): The JWK thumbprint of the old key
Host: example.com [RFC7638], base64url-encoded
{ {
"resource": "reg", "resource": "reg",
"recoveryKey": { "oldKey": "D7J9RL1f-RWUl68JP-gW1KSl2TkIrJB7hK6rLFFeYMU"
"client": { "kty": "EC", ... }
}
} }
/* Signed as JWS */
6.4. Account Recovery
Once a client has created an account with an ACME server, it is
possible that the private key for the account will be lost. The
recovery contacts included in the registration allows the client to
recover from this situation, as long as it still has access to these
contacts.
By "recovery", we mean that the information associated with an old
account key is bound to a new account key. When a recovery process
succeeds, the server provides the client with a new registration
whose contents are the same as base registration object - except for
the "key" field, which is set to the new account key. The server
reassigns resources associated with the base registration to the new
registration (e.g., authorizations and certificates). The server
SHOULD delete the old registration resource after it has been used as
a base for recovery.
In addition to the recovery mechanisms defined by ACME, individual
client implementations may also offer implementation-specific
recovery mechanisms. For example, if a client creates account keys
deterministically from a seed value, then this seed could be used to
recover the account key by re-generating it. Or an implementation
could escrow an encrypted copy of the account key with a cloud
storage provider, and give the encryption key to the user as a
recovery value.
6.4.1. MAC-Based Recovery
With MAC-based recovery, the client proves to the server that it
holds a secret value established in the initial registration
transaction. The client requests MAC-based recovery by sending a MAC
over the new account key, using the recovery key from the initial
registration.
method (required, string): The string "mac"
base (required, string): The URI for the registration to be
recovered.
mac (required, string): A JSON-formatted JWS object using an HMAC The client signs this object with the new key pair and encodes the
algorithm, whose payload is the JWK representation of the public object and signature as a JWS. The client then sends this JWS to the
key of the new account key pair. server in the "newKey" field of a request to update the registration.
POST /acme/recover-reg HTTP/1.1 POST /acme/reg/asdf HTTP/1.1
Host: example.com Host: example.com
{ {
"resource": "recover-reg", "resource": "reg",
"method": "mac", "newKey": /* JSON object signed as JWS with new key */
"base": "https://example.com/acme/reg/asdf",
"mac": {
"header": { "alg": "HS256" },
"payload": base64(JWK(newAccountKey)),
"signature": "5wUrDI3eAaV4wl2Rfj3aC0Pp--XB3t4YYuNgacv_D3U"
}
} }
/* Signed as JWS, with new account key */ /* Signed as JWS with original key */
On receiving such a request the server MUST verify that:
o The base registration has a recovery key associated with it
o The "alg" value in the "mac" JWS represents a MAC algorithm
o The "mac" JWS is valid according to the validation rules in On receiving a request to the registration URL with the "newKey"
[RFC7515], using the recovery key as the MAC key attribute set, the server MUST perform the following steps:
o The JWK in the payload represents the new account key (i.e. the 1. Check that the contents of the "newKey" attribute are a valid JWS
key used to verify the ACME message)
If those conditions are met, and the recovery request is otherwise 2. Check that the "newKey" JWS verifies using the key in the "jwk"
acceptable to the server, then the recovery process has succeeded. header parameter of the JWS
The server creates a new registration resource based on the base
registration and the new account key, and returns it on a 201
(Created) response, together with a Location header indicating a URI
for the new registration. If the recovery request is unsuccessful,
the server returns an error response, such as 403 (Forbidden).
HTTP/1.1 201 Created 3. Check that the payload of the JWS is a valid JSON object
Content-Type: application/json
Location: https://example.com/acme/reg/asdf
Link: <https://example.com/acme/new-authz>;rel="next"
Link: <https://example.com/acme/recover-reg>;rel="recover"
Link: <https://example.com/acme/terms>;rel="terms-of-service"
{ 4. Check that the "resource" field of the object has the value "reg"
"key": { /* JWK from JWS header */ },
"contact": [ 5. Check that the "oldKey" field of the object contains the JWK
"mailto:cert-admin@example.com", thumbprint of the account key for this registration
"tel:+12025551212"
],
"authorizations": "...", If all of these checks pass, then the server updates the registration
"certificate": "..." by replacing the old account key with the public key carried in the
} "jwk" header parameter of the "newKey" JWS object.
6.4.2. Contact-Based Recovery If the update was successful, then the server sends a response with
status code 200 (OK) and the updated registration object as its body.
If the update was not successful, then the server responds with an
error status code and a problem document describing the error.
In the contact-based recovery process, the client requests that the 6.3.2. Deleting an Account
server send a message to one of the contact URIs registered for the
account. That message indicates some action that the server requires
the client's user to perform, e.g., clicking a link in an email. If
the user successfully completes the server's required actions, then
the server will bind the account to the new account key.
(Note that this process is almost entirely out of band with respect If a client no longer wishes to have an account key registered with
to ACME. ACME only allows the client to initiate the process, and the server, it may request that the server delete its account by
the server to indicate the result.) sending a POST request to the account URI containing the "delete"
field.
To initiate contact-based recovery, the client sends a POST request delete (required, boolean): The boolean value "true".
to the server's recover-registration URI, with a body specifying
which registration is to be recovered. The body of the request MUST
be signed by the client's new account key pair.
method (required, string): The string "contact" The request object MUST contain the "resource" field as required
above (with the value "reg"). It MUST NOT contain any fields besides
"resource" and "delete".
base (required, string): The URI for the registration to be Note that although this object is very simple, the risk of replay or
recovered. fraudulent generation via signing oracles is mitigated by the need
for an anti-replay token in the protected header of the JWS.
POST /acme/recover-reg HTTP/1.1 POST /acme/reg/asdf HTTP/1.1
Host: example.com Host: example.com
{ {
"resource": "recover-reg", "resource": "reg",
"method": "contact", "delete": true,
"base": "https://example.com/acme/reg/asdf",
"contact": [
"mailto:forgetful@example.net"
]
}
/* Signed as JWS, with new account key */
If the server agrees to attempt contact-based recovery, then it
creates a new registration resource containing a stub registration
object. The stub registration has the client's new account key and
contacts, but no authorizations or certificates associated. The
server returns the stub contact in a 201 (Created) response, along
with a Location header field indicating the URI for the new
registration resource (which will be the registration URI if the
recovery succeeds).
HTTP/1.1 201 Created
Content-Type: application/json
Location: https://example.com/acme/reg/qwer
{
"key": { /* new account key from JWS header */ },
"contact": [
"mailto:forgetful@example.net"
]
} }
/* Signed as JWS */
After recovery has been initiated, the server follows its chosen On receiving a POST to an account URI containing a "delete" field,
recovery process, out-of-band to ACME. While the recovery process is the server MUST verify that no other fields were sent in the object
ongoing, the client may poll the registration resource's URI for (other than "resource"), and it MUST verify that the value of the
status, by sending a POST request with a trivial body "delete" field is "true" (as a boolean, not a string). If either of
({"resource":"reg"}). If the recovery process is still pending, the these checks fails, then the server MUST reject the request with
server sends a 202 (Accepted) status code, and a Retry-After header status code 400 (Bad Request).
field. If the recovery process has failed, the server sends an error
code (e.g., 404), and SHOULD delete the stub registration resource.
If the recovery process has succeeded, then the server will send a If the server accepts the deletion request, then it MUST delete the
200 (OK) response, containing the full registration object, with any account and all related objects and send a response with a 200 (OK)
necessary information copied from the old registration). The client status code and an empty body. The server SHOULD delete any
may now use this in the same way as if he had gotten it from a new- authorization objects related to the deleted account, since they can
registration transaction. no longer be used. The server SHOULD NOT delete certificate objects
related to the account, since certificates issued under the account
continue to be valid until they expire or are revoked.
6.5. Identifier Authorization 6.4. Identifier Authorization
The identifier authorization process establishes the authorization of The identifier authorization process establishes the authorization of
an account to manage certificates for a given identifier. This an account to manage certificates for a given identifier. This
process must assure the server of two things: First, that the client process must assure the server of two things: First, that the client
controls the private key of the account key pair, and second, that controls the private key of the account key pair, and second, that
the client holds the identifier in question. This process may be the client holds the identifier in question. This process may be
repeated to associate multiple identifiers to a key pair (e.g., to repeated to associate multiple identifiers to a key pair (e.g., to
request certificates with multiple identifiers), or to associate request certificates with multiple identifiers), or to associate
multiple accounts with an identifier (e.g., to allow multiple multiple accounts with an identifier (e.g., to allow multiple
entities to manage certificates). entities to manage certificates).
skipping to change at page 28, line 11 skipping to change at page 25, line 14
is of a supported type. Servers might also check names against a is of a supported type. Servers might also check names against a
blacklist of known high-value identifiers. If the server is blacklist of known high-value identifiers. If the server is
unwilling to issue for the identifier, it SHOULD return a 403 unwilling to issue for the identifier, it SHOULD return a 403
(Forbidden) error, with a problem document describing the reason for (Forbidden) error, with a problem document describing the reason for
the rejection. the rejection.
If the server is willing to proceed, it builds a pending If the server is willing to proceed, it builds a pending
authorization object from the initial authorization object submitted authorization object from the initial authorization object submitted
by the client. by the client.
o "identifier" the identifier submitted by the client. o "identifier" the identifier submitted by the client
o "status": MUST be "pending" o "status": MUST be "pending" unless the server has out-of-band
information about the client's authorization status
o "challenges" and "combinations": As selected by the server's o "challenges" and "combinations": As selected by the server's
policy for this identifier policy for this identifier
o The "expires" field MUST be absent.
The server allocates a new URI for this authorization, and returns a The server allocates a new URI for this authorization, and returns a
201 (Created) response, with the authorization URI in a Location 201 (Created) response, with the authorization URI in a Location
header field, and the JSON authorization object in the body. header field, and the JSON authorization object in the body.
HTTP/1.1 201 Created HTTP/1.1 201 Created
Content-Type: application/json Content-Type: application/json
Location: https://example.com/authz/asdf Location: https://example.com/authz/asdf
Link: <https://example.com/acme/new-cert>;rel="next" Link: <https://example.com/acme/new-cert>;rel="next"
Link: <https://example.com/acme/some-directory>;rel="directory"
{ {
"status": "pending", "status": "pending",
"identifier": { "identifier": {
"type": "dns", "type": "dns",
"value": "example.org" "value": "example.org"
}, },
"challenges": [ "challenges": [
skipping to change at page 29, line 31 skipping to change at page 26, line 32
"uri": "https://example.com/authz/asdf/0", "uri": "https://example.com/authz/asdf/0",
"token": "IlirfxKKXAsHtmzK29Pj8A" "token": "IlirfxKKXAsHtmzK29Pj8A"
}, },
{ {
"type": "dns-01", "type": "dns-01",
"uri": "https://example.com/authz/asdf/1", "uri": "https://example.com/authz/asdf/1",
"token": "DGyRejmCefe7v4NfDGDKfA" "token": "DGyRejmCefe7v4NfDGDKfA"
} }
}, },
"combinations": [ "combinations": [[0], [1]]
[0, 2],
[1, 2]
]
} }
The client needs to respond with information to complete the 6.4.1. Responding to Challenges
challenges. To do this, the client updates the authorization object
received from the server by filling in any required information in To prove control of the identifer and receive authorization, the
the elements of the "challenges" dictionary. (This is also the stage client needs to respond with information to complete the challenges.
where the client should perform any actions required by the To do this, the client updates the authorization object received from
challenge.) the server by filling in any required information in the elements of
the "challenges" dictionary. (This is also the stage where the
client should perform any actions required by the challenge.)
The client sends these updates back to the server in the form of a The client sends these updates back to the server in the form of a
JSON object with the response fields required by the challenge type, JSON object with the response fields required by the challenge type,
carried in a POST request to the challenge URI (not authorization URI carried in a POST request to the challenge URI (not authorization URI
or the new-authorization URI). This allows the client to send or the new-authorization URI). This allows the client to send
information only for challenges it is responding to. information only for challenges it is responding to.
For example, if the client were to respond to the "http-01" challenge For example, if the client were to respond to the "http-01" challenge
in the above authorization, it would send the following request: in the above authorization, it would send the following request:
POST /acme/authz/asdf/0 HTTP/1.1 POST /acme/authz/asdf/0 HTTP/1.1
Host: example.com Host: example.com
{ {
"resource": "challenge", "resource": "challenge",
"type": "http-01", "type": "http-01",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
} }
/* Signed as JWS */ /* Signed as JWS */
The server updates the authorization document by updating its The server updates the authorization document by updating its
representation of the challenge with the response fields provided by representation of the challenge with the response fields provided by
the client. The server MUST ignore any fields in the response object the client. The server MUST ignore any fields in the response object
that are not specified as response fields for this type of challenge. that are not specified as response fields for this type of challenge.
The server provides a 200 (OK) response with the updated challenge The server provides a 200 (OK) response with the updated challenge
object as its body. object as its body.
If the client's response is invalid for some reason, or does not If the client's response is invalid for some reason, or does not
provide the server with appropriate information to validate the provide the server with appropriate information to validate the
challenge, then the server MUST return an HTTP error. On receiving challenge, then the server MUST return an HTTP error. On receiving
such an error, the client MUST undo any actions that have been taken such an error, the client SHOULD undo any actions that have been
to fulfil the challenge, e.g., removing files that have been taken to fulfill the challenge, e.g., removing files that have been
provisioned to a web server. provisioned to a web server.
Presumably, the client's responses provide the server with enough Presumably, the client's responses provide the server with enough
information to validate one or more challenges. The server is said information to validate one or more challenges. The server is said
to "finalize" the authorization when it has completed all the to "finalize" the authorization when it has completed all the
validations it is going to complete, and assigns the authorization a validations it is going to complete, and assigns the authorization a
status of "valid" or "invalid", corresponding to whether it considers status of "valid" or "invalid", corresponding to whether it considers
the account authorized for the identifier. If the final state is the account authorized for the identifier. If the final state is
"valid", the server MUST add an "expires" field to the authorization. "valid", the server MUST add an "expires" field to the authorization.
When finalizing an authorization, the server MAY remove the When finalizing an authorization, the server MAY remove the
skipping to change at page 30, line 52 skipping to change at page 28, line 4
will need to poll the authorization resource to see when it is will need to poll the authorization resource to see when it is
finalized. For challenges where the client can tell when the server finalized. For challenges where the client can tell when the server
has validated the challenge (e.g., by seeing an HTTP or DNS request has validated the challenge (e.g., by seeing an HTTP or DNS request
from the server), the client SHOULD NOT begin polling until it has from the server), the client SHOULD NOT begin polling until it has
seen the validation request from the server. seen the validation request from the server.
To check on the status of an authorization, the client sends a GET To check on the status of an authorization, the client sends a GET
request to the authorization URI, and the server responds with the request to the authorization URI, and the server responds with the
current authorization object. In responding to poll requests while current authorization object. In responding to poll requests while
the validation is still in progress, the server MUST return a 202 the validation is still in progress, the server MUST return a 202
(Accepted) response with a Retry-After header field. (Accepted) response, and MAY include a Retry-After header field to
suggest a polling interval to the client.
GET /acme/authz/asdf HTTP/1.1 GET /acme/authz/asdf HTTP/1.1
Host: example.com Host: example.com
HTTP/1.1 200 OK HTTP/1.1 200 OK
{ {
"status": "valid", "status": "valid",
"expires": "2015-03-01", "expires": "2015-03-01T14:09:00Z",
"identifier": { "identifier": {
"type": "dns", "type": "dns",
"value": "example.org" "value": "example.org"
}, },
"challenges": [ "challenges": [
{ {
"type": "http-01" "type": "http-01"
"status": "valid", "status": "valid",
"validated": "2014-12-01T12:05Z", "validated": "2014-12-01T12:05:00Z",
"keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE" "token": "IlirfxKKXAsHtmzK29Pj8A",
"keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
} }
] ]
} }
6.6. Certificate Issuance 6.4.2. Deleting an Authorization
The holder of an authorized key pair for an identifier may use ACME If a client wishes to relinquish its authorization to issue
to request that a certificate be issued for that identifier. The certificates for an identifier, then it may request that the server
client makes this request by sending a POST request to the server's delete the authorization. The client makes this request by sending a
new-certificate resource. The body of the POST is a JWS object whose POST request to the authorization URI containing a payload in the
JSON payload contains a Certificate Signing Request (CSR) [RFC2986]. same format as in Section 6.3.2. The only difference is that the
The CSR encodes the parameters of the requested certificate; value of the "resource" field is "authz".
authority to issue is demonstrated by the JWS signature by an account
key, from which the server can look up related authorizations. POST /acme/authz/asdf HTTP/1.1
Host: example.com
{
"resource": "authz",
"delete": true,
}
/* Signed as JWS */
The server MUST perform the same validity checks as in Section 6.3.2
and reject the request if they fail. If the server deletes the
account then it MUST send a response with a 200 (OK) status code and
an empty body.
6.5. Certificate Issuance
The holder of an account key pair authorized for one or more
identifiers may use ACME to request that a certificate be issued for
any subset of those identifiers. The client makes this request by
sending a POST request to the server's new-certificate resource. The
body of the POST is a JWS object whose JSON payload contains a
Certificate Signing Request (CSR) [RFC2986]. The CSR encodes the
parameters of the requested certificate; authority to issue is
demonstrated by the JWS signature by an account key, from which the
server can look up related authorizations. Some attributes which
cannot be reflected in a CSR are placed directly in the certificate
request.
csr (required, string): A CSR encoding the parameters for the csr (required, string): A CSR encoding the parameters for the
certificate being requested. The CSR is sent in the certificate being requested. The CSR is sent in the Base64url-
Base64-encoded version of the DER format. (Note: This field uses encoded version of the DER format. (Note: This field uses the
the same modified Base64-encoding rules used elsewhere in this same modified Base64 encoding rules used elsewhere in this
document, so it is different from PEM.) document, so it is different from PEM.)
notBefore (optional, string): The requested value of the notBefore
field in the certificate, in the date format defined in [RFC3339]
notAfter (optional, string): The requested value of the notAfter
field in the certificate, in the date format defined in [RFC3339]
POST /acme/new-cert HTTP/1.1 POST /acme/new-cert HTTP/1.1
Host: example.com Host: example.com
Accept: application/pkix-cert Accept: application/pkix-cert
{ {
"resource": "new-cert", "resource": "new-cert",
"csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P", "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z"
} }
/* Signed as JWS */ /* Signed as JWS */
The CSR encodes the client's requests with regard to the content of The CSR encodes the client's requests with regard to the content of
the certificate to be issued. The CSR MUST indicate the requested the certificate to be issued. The CSR MUST indicate the requested
identifiers, either in the commonName portion of the requested identifiers, either in the commonName portion of the requested
subject name, or in an extensionRequest attribute [RFC2985] subject name, or in an extensionRequest attribute [RFC2985]
requesting a subjectAltName extension. requesting a subjectAltName extension.
The values provided in the CSR are only a request, and are not The values provided in the CSR are only a request, and are not
guaranteed. The server or CA may alter any fields in the certificate guaranteed. The server SHOULD return an error if it cannot fulfil
before issuance. For example, the CA may remove identifiers that are the request as specified, but MAY issue a certificate with contents
not authorized for the account key that signed the request. other than those requested, according to its local policy (e.g.,
removing identifiers for which the client is not authorized).
It is up to the server's local policy to decide which names are It is up to the server's local policy to decide which names are
acceptable in a certificate, given the authorizations that the server acceptable in a certificate, given the authorizations that the server
associates with the client's account key. A server MAY consider a associates with the client's account key. A server MAY consider a
client authorized for a wildcard domain if it is authorized for the client authorized for a wildcard domain if it is authorized for the
underlying domain name (without the "*" label). Servers SHOULD NOT underlying domain name (without the "*" label). Servers SHOULD NOT
extend authorization across identifier types. For example, if a extend authorization across identifier types. For example, if a
client is authorized for "example.com", then the server should not client is authorized for "example.com", then the server should not
allow the client to issue a certificate with an iPAddress allow the client to issue a certificate with an iPAddress
subjectAltName, even if it contains an IP address to which subjectAltName, even if it contains an IP address to which
skipping to change at page 33, line 20 skipping to change at page 31, line 5
Retry-After: 120 Retry-After: 120
The default format of the certificate is DER (application/pkix-cert). The default format of the certificate is DER (application/pkix-cert).
The client may request other formats by including an Accept header in The client may request other formats by including an Accept header in
its request. its request.
The server provides metadata about the certificate in HTTP headers. The server provides metadata about the certificate in HTTP headers.
In particular, the server MUST include a Link relation header field In particular, the server MUST include a Link relation header field
[RFC5988] with relation "up" to provide a certificate under which [RFC5988] with relation "up" to provide a certificate under which
this certificate was issued, and one with relation "author" to this certificate was issued, and one with relation "author" to
indicate the registration under which this certicate was issued. The indicate the registration under which this certificate was issued.
server MAY also include an Expires header as a hint to the client
The server MAY include an Expires header as a hint to the client
about when to renew the certificate. (Of course, the real expiration about when to renew the certificate. (Of course, the real expiration
of the certificate is controlled by the notAfter time in the of the certificate is controlled by the notAfter time in the
certificate itself.) certificate itself.)
If the CA participates in Certificate Transparency (CT) [RFC6962],
then they may want to provide the client with a Signed Certificate
Timestamp (SCT) that can be used to prove that a certificate was
submitted to a CT log. An SCT can be included as an extension in the
certificate or as an extension to OCSP responses for the certificate.
The server can also provide the client with direct access to an SCT
for a certificate using a Link relation header field with relation
"ct-sct".
GET /acme/cert/asdf HTTP/1.1 GET /acme/cert/asdf HTTP/1.1
Host: example.com Host: example.com
Accept: application/pkix-cert Accept: application/pkix-cert
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/pkix-cert Content-Type: application/pkix-cert
Link: <https://example.com/acme/ca-cert>;rel="up";title="issuer" Link: <https://example.com/acme/ca-cert>;rel="up";title="issuer"
Link: <https://example.com/acme/revoke-cert>;rel="revoke" Link: <https://example.com/acme/revoke-cert>;rel="revoke"
Link: <https://example.com/acme/reg/asdf>;rel="author" Link: <https://example.com/acme/reg/asdf>;rel="author"
Link: <https://example.com/acme/sct/asdf>;rel="ct-sct"
Link: <https://example.com/acme/some-directory>;rel="directory"
Location: https://example.com/acme/cert/asdf Location: https://example.com/acme/cert/asdf
Content-Location: https://example.com/acme/cert-seq/12345 Content-Location: https://example.com/acme/cert-seq/12345
[DER-encoded certificate] [DER-encoded certificate]
A certificate resource always represents the most recent certificate A certificate resource always represents the most recent certificate
issued for the name/key binding expressed in the CSR. If the CA issued for the name/key binding expressed in the CSR. If the CA
allows a certificate to be renewed, then it publishes renewed allows a certificate to be renewed, then it publishes renewed
versions of the certificate through the same certificate URI. versions of the certificate through the same certificate URI.
skipping to change at page 34, line 23 skipping to change at page 32, line 19
Likewise, in order to prevent unnecessary renewal due to queries by Likewise, in order to prevent unnecessary renewal due to queries by
parties other than the account key holder, certificate URIs should be parties other than the account key holder, certificate URIs should be
structured as capability URLs [W3C.WD-capability-urls-20140218]. structured as capability URLs [W3C.WD-capability-urls-20140218].
From the client's perspective, there is no difference between a From the client's perspective, there is no difference between a
certificate URI that allows renewal and one that does not. If the certificate URI that allows renewal and one that does not. If the
client wishes to obtain a renewed certificate, and a GET request to client wishes to obtain a renewed certificate, and a GET request to
the certificate URI does not yield one, then the client may initiate the certificate URI does not yield one, then the client may initiate
a new-certificate transaction to request one. a new-certificate transaction to request one.
6.7. Certificate Revocation 6.6. Certificate Revocation
To request that a certificate be revoked, the client sends a POST To request that a certificate be revoked, the client sends a POST
request to the ACME server's revoke-cert URI. The body of the POST request to the ACME server's revoke-cert URI. The body of the POST
is a JWS object whose JSON payload contains the certificate to be is a JWS object whose JSON payload contains the certificate to be
revoked: revoked:
certificate (required, string): The certificate to be revoked, in certificate (required, string): The certificate to be revoked, in
the Base64-encoded version of the DER format. (Note: This field the base64url-encoded version of the DER format. (Note: This
uses the same modified Base64-encoding rules used elsewhere in field uses the same modified Base64 encoding rules used elsewhere
this document, so it is different from PEM.) in this document, so it is different from PEM.)
POST /acme/revoke-cert HTTP/1.1 POST /acme/revoke-cert HTTP/1.1
Host: example.com Host: example.com
{ {
"resource": "revoke-cert", "resource": "revoke-cert",
"certificate": "MIIEDTCCAvegAwIBAgIRAP8..." "certificate": "MIIEDTCCAvegAwIBAgIRAP8..."
} }
/* Signed as JWS */ /* Signed as JWS */
Revocation requests are different from other ACME request in that Revocation requests are different from other ACME request in that
they can be signed either with an account key pair or the key pair in they can be signed either with an account key pair or the key pair in
the certificate. Before revoking a certificate, the server MUST the certificate. Before revoking a certificate, the server MUST
verify at least one of these conditions applies: verify that the key used to sign the request is authorized to revoke
the certificate. The server SHOULD consider at least the following
keys authorized for a given certificate:
o the public key of the key pair signing the request matches the o the public key in the certificate.
public key in the certificate.
o the key pair signing the request is an account key, and the o an account key that is authorized to act for all of the
corresponding account is authorized to act for all of the
identifier(s) in the certificate. identifier(s) in the certificate.
If the revocation succeeds, the server responds with status code 200 If the revocation succeeds, the server responds with status code 200
(OK). If the revocation fails, the server returns an error. (OK). If the revocation fails, the server returns an error.
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Length: 0 Content-Length: 0
--- or --- --- or ---
HTTP/1.1 403 Forbidden HTTP/1.1 403 Forbidden
Content-Type: application/problem+json Content-Type: application/problem+json
Content-Language: en Content-Language: en
{ {
"type": "urn:acme:error:unauthorized" "type": "urn:ietf:params:acme:error:unauthorized"
"detail": "No authorization provided for name example.net" "detail": "No authorization provided for name example.net"
"instance": "http://example.com/doc/unauthorized" "instance": "http://example.com/doc/unauthorized"
} }
7. Identifier Validation Challenges 7. Identifier Validation Challenges
There are few types of identifier in the world for which there is a There are few types of identifiers in the world for which there is a
standardized mechanism to prove possession of a given identifier. In standardized mechanism to prove possession of a given identifier. In
all practical cases, CAs rely on a variety of means to test whether all practical cases, CAs rely on a variety of means to test whether
an entity applying for a certificate with a given identifier actually an entity applying for a certificate with a given identifier actually
controls that identifier. controls that identifier.
Challenges provide the server with assurance that an account key Challenges provide the server with assurance that an account key
holder is also the entity that controls an identifier. For each type holder is also the entity that controls an identifier. For each type
of challenge, it must be the case that in order for an entity to of challenge, it must be the case that in order for an entity to
successfully complete the challenge the entity must both: successfully complete the challenge the entity must both:
o Hold the private key of the account key pair used to respond to o Hold the private key of the account key pair used to respond to
the challenge the challenge
o Control the identifier in question o Control the identifier in question
Section 9 documents how the challenges defined in this document meet Section 10 documents how the challenges defined in this document meet
these requirements. New challenges will need to document how they these requirements. New challenges will need to document how they
do. do.
To accommodate this reality, ACME includes an extensible challenge/ ACME uses an extensible challenge/response framework for identifier
response framework for identifier validation. This section describes validation. The server presents a set of challenge in the
an initial set of Challenge types. Each challenge must describe: authorization object it sends to a client (as objects in the
"challenges" array), and the client responds by sending a response
object in a POST request to a challenge URI.
o Content of Challenge payloads (in Challenge messages) This section describes an initial set of challenge types. Each
challenge must describe:
o Content of Response payloads (in authorizationRequest messages) 1. Content of challenge objects
o How the server uses the Challenge and Response to verify control 2. Content of response objects
of an identifier
The general structure of Challenge and Response payloads is as 3. How the server uses the challenge and response to verify control
follows: of an identifier
type (required, string): The type of Challenge or Response encoded Challenge objects all contain the following basic fields:
in the object.
type (required, string): The type of challenge encoded in the
object.
uri (required, string): The URI to which a response can be posted. uri (required, string): The URI to which a response can be posted.
status (optional, string): : The status of this authorization. status (required, string): The status of this authorization.
Possible values are: "unknown", "pending", "processing", "valid", Possible values are: "pending", "valid", and "invalid". If this
"invalid" and "revoked". If this field is missing, then the default field is missing, then the default value is "pending".
value is "pending".
validated (optional, string): : The time at which this challenge was validated (optional, string): The time at which this challenge was
completed by the server, encoded in the format specified in RFC 3339 completed by the server, encoded in the format specified in RFC
[RFC3339]. 3339 [RFC3339]. This field is REQUIRED if the "status" field is
"valid".
error (optional, dictionary of string): : The error that occurred error (optional, dictionary of string): The error that occurred
while the server was validating the challenge, if any. This field is while the server was validating the challenge, if any. This field
structured as a problem document [I-D.ietf-appsawg-http-problem]. is structured as a problem document
[I-D.ietf-appsawg-http-problem].
All additional fields are specified by the Challenge type. The All additional fields are specified by the challenge type. If the
server MUST ignore any values provided in the "uri", "status", server sets a challenge's "status" to "invalid", it SHOULD also
"validated", and "error" fields of a Response payload. If the server include the "error" field to help the client diagnose why they failed
sets a Challenge's "status" to "invalid", it SHOULD also include the the challenge.
"error" field to help the client diagnose why they failed the
challenge.
Different challenges allow the server to obtain proof of different Different challenges allow the server to obtain proof of different
aspects of control over an identifier. In some challenges, like HTTP aspects of control over an identifier. In some challenges, like HTTP
and TLS SNI, the client directly proves its ability to do certain and TLS SNI, the client directly proves its ability to do certain
things related to the identifier. In the Proof of Possession things related to the identifier. The choice of which challenges to
challenge, the client proves historical control of the identifier, by offer to a client under which circumstances is a matter of server
reference to a prior authorization transaction or certificate. policy.
The choice of which Challenges to offer to a client under which
circumstances is a matter of server policy. A CA may choose
different sets of challenges depending on whether it has interacted
with a domain before, and how. For example:
o New domain with no known certificates: Domain Validation (HTTP or
TLS SNI)
o Domain for which known certs exist from other CAs: DV + Proof of
Possession of previous CA-signed key
o Domain with a cert from this CA, lost account key: DV + PoP of
ACME-certified Subject key
o Domain with a cert from this CA, all keys and recovery mechanisms
lost: Out of band proof of authority for the domain
The identifier validation challenges described in this section all The identifier validation challenges described in this section all
relate to validation of domain names. If ACME is extended in the relate to validation of domain names. If ACME is extended in the
future to support other types of identifier, there will need to be future to support other types of identifier, there will need to be
new Challenge types, and they will need to specify which types of new challenge types, and they will need to specify which types of
identifier they apply to. identifier they apply to.
[[ Editor's Note: In pre-RFC versions of this specification, [[ Editor's Note: In pre-RFC versions of this specification,
challenges are labeled by type, and with the version of the draft in challenges are labeled by type, and with the version of the draft in
which they were introduced. For example, if an HTTP challenge were which they were introduced. For example, if an HTTP challenge were
introduced in version -03 and a breaking change made in version -05, introduced in version -03 and a breaking change made in version -05,
then there would be a challenge labeled "http-03" and one labeled then there would be a challenge labeled "http-03" and one labeled
"http-05" - but not one labeled "http-04", since challenge in version "http-05" - but not one labeled "http-04", since challenge in version
-04 was compatible with one in version -04. ]] -04 was compatible with one in version -04. ]]
[[ Editor's Note: Operators SHOULD NOT issue "combinations" arrays in [[ Editor's Note: Operators SHOULD NOT issue "combinations" arrays in
authorization objects that require the client to perform multiple authorization objects that require the client to perform multiple
challenges over the same type, e.g., ["http-03", "http-05"]. challenges over the same type, e.g., ["http-03", "http-05"].
Challenges within a type are testing the same capability of the Challenges within a type are testing the same capability of the
domain owner, and it may not be possible to satisfy both at once. ]] domain owner, and it may not be possible to satisfy both at once. ]]
7.1. Key Authorizations 7.1. Key Authorizations
Several of the challenges in this document makes use of a key Several of the challenges in this document makes use of a key
authorization string. A key authorization expresses a domain authorization string. A key authorization is a string that expresses
holder's authorization for a specified key to satisfy a specified a domain holder's authorization for a specified key to satisfy a
challenge, by concatenating the token for the challenge with a key specified challenge, by concatenating the token for the challenge
fingerprint, separated by a "." character: with a key fingerprint, separated by a "." character:
key-authz = token || '.' || base64(JWK_Thumbprint(accountKey)) key-authz = token || '.' || base64url(JWK\_Thumbprint(accountKey))
The "JWK_Thumbprint" step indicates the computation specified in The "JWK_Thumbprint" step indicates the computation specified in
[RFC7638], using the SHA-256 digest. As specified in the individual [RFC7638], using the SHA-256 digest. As specified in the individual
challenges below, the token for a challenge is a JSON string challenges below, the token for a challenge is a JSON string
comprised entirely of characters in the base64 alphabet. The "||" comprised entirely of characters in the URL-safe Base64 alphabet.
operator indicates concatenation of strings. The "||" operator indicates concatenation of strings.
In computations involving key authorizations, such as the digest In computations involving key authorizations, such as the digest
computations required for the DNS and TLS SNI challenges, the key computations required for the DNS and TLS SNI challenges, the key
authorization string MUST be represented in UTF-8 form (or, authorization string MUST be represented in UTF-8 form (or,
equivalently, ASCII). equivalently, ASCII).
An example of how to compute a JWK thumbprint can be found in
Section 3.1 of [RFC7638]. Note that some cryptographic libraries
prepend a zero octet to the representation of the RSA public key
parameters N and E, in order to avoid ambiguity with regard to the
sign of the number. As noted in JWA [RFC7518], a JWK object MUST NOT
include this zero octet. That is, any initial zero octets MUST be
stripped before the values are base64url-encoded.
7.2. HTTP 7.2. HTTP
With Simple HTTP validation, the client in an ACME transaction proves With HTTP validation, the client in an ACME transaction proves its
its control over a domain name by proving that it can provision control over a domain name by proving that it can provision resources
resources on an HTTP server that responds for that domain name. The on an HTTP server that responds for that domain name. The ACME
ACME server challenges the client to provision a file with a specific server challenges the client to provision a file at a specific path,
JWS as its contents. with a specific string as its content.
As a domain may resolve to multiple IPv4 and IPv6 addresses, the As a domain may resolve to multiple IPv4 and IPv6 addresses, the
server will connect to at least one of the hosts found in A and AAAA server will connect to at least one of the hosts found in A and AAAA
records, at its discretion. Because many webservers allocate a records. Because many web servers allocate a default HTTPS virtual
default HTTPS virtual host to a particular low-privilege tenant user host to a particular low-privilege tenant user in a subtle and non-
in a subtle and non-intuitive manner, the challenge must be completed intuitive manner, the challenge must be completed over HTTP, not
over HTTP, not HTTPS. HTTPS.
type (required, string): The string "http-01" type (required, string): The string "http-01"
token (required, string): A random value that uniquely identifies token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy, the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the URL-safe Base64 alphabet. contain any characters outside the URL-safe Base64 alphabet and
MUST NOT contain any padding characters ("=").
{ {
"type": "http-01", "type": "http-01",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA", "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA",
} }
A client responds to this challenge by constructing a key A client responds to this challenge by constructing a key
authorization from the "token" value provided in the challenge and authorization from the "token" value provided in the challenge and
the client's account key. The client then provisions the key the client's account key. The client then provisions the key
authorization as a resource on the HTTP server for the domain in authorization as a resource on the HTTP server for the domain in
question. question.
evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA
.nP1qzpXGymHBrUEepNY9HCsQk7K8KhOypzEt62jcerQ
The path at which the resource is provisioned is comprised of the The path at which the resource is provisioned is comprised of the
fixed prefix ".well-known/acme-challenge/", followed by the "token" fixed prefix ".well-known/acme-challenge/", followed by the "token"
value in the challenge. value in the challenge. The value of the resource MUST be the ASCII
representation of the key authorization.
.well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA .well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA
The client's response to this challenge indicates its agreement to The client's response to this challenge indicates its agreement to
this challenge by sending the server the key authorization covering this challenge by sending the server the key authorization covering
the challenge's token and the client's account key: the challenge's token and the client's account key. In addition, the
client MAY advise the server at which IP the challenge is
provisioned.
keyAuthorization (required, string): The key authorization for this keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and challenge. This value MUST match the token from the challenge and
the client's account key. the client's account key.
address (optional, string): An IPv4 or IPv6 address, in dotted
decimal form or [RFC4291] form, respectively. If given, this
address MUST be included in the set of IP addresses to which the
domain name resolves when the server attempts validation. If
given, the server SHOULD connect to that specific IP address
instead of arbitrarily choosing an IP from the set of A and AAAA
records to which the domain name resolves.
{ {
"keyAuthorization": "evaGxfADs...62jcerQ" "keyAuthorization": "evaGxfADs...62jcerQ"
} }
/* Signed as JWS */ /* Signed as JWS */
On receiving a response, the server MUST verify that the key On receiving a response, the server MUST verify that the key
authorization in the response matches the "token" value in the authorization in the response matches the "token" value in the
challenge and the client's account key. If they do not match, then challenge and the client's account key. If they do not match, then
the server MUST return an HTTP error in response to the POST request the server MUST return an HTTP error in response to the POST request
in which the client sent the challenge. in which the client sent the challenge.
Given a Challenge/Response pair, the server verifies the client's Given a challenge/response pair, the server verifies the client's
control of the domain by verifying that the resource was provisioned control of the domain by verifying that the resource was provisioned
as expected. as expected.
1. Form a URI by populating the URI template [RFC6570] 1. Form a URI by populating the URI template [RFC6570]
"http://{domain}/.well-known/acme-challenge/{token}", where: "http://{domain}/.well-known/acme-challenge/{token}", where:
* the domain field is set to the domain name being verified; and * the domain field is set to the domain name being verified; and
* the token field is set to the token in the challenge. * the token field is set to the token in the challenge.
2. Verify that the resulting URI is well-formed. 2. Verify that the resulting URI is well-formed.
3. Dereference the URI using an HTTP or HTTPS GET request. If using 3. If the client has supplied an address to use, verify that the
HTTPS, the ACME server MUST ignore the certificate provided by address is included in the A or AAAA records to which the domain
the HTTPS server. name resolves. If the address is not included in the result, the
validation fails.
4. Verify that the Content-Type header of the response is either 4. Dereference the URI using an HTTP GET request. If an address was
absent, or has the value "text/plain". supplied by the client, use that address to establish the HTTP
connection.
5. Verify that the body of the response is well-formed key 5. Verify that the body of the response is well-formed key
authorization. authorization. The server SHOULD ignore whitespace characters at
the end of the body.
6. Verify that key authorization provided by the server matches the 6. Verify that key authorization provided by the server matches the
token for this challenge and the client's account key. token for this challenge and the client's account key.
If all of the above verifications succeed, then the validation is If all of the above verifications succeed, then the validation is
successful. If the request fails, or the body does not pass these successful. If the request fails, or the body does not pass these
checks, then it has failed. checks, then it has failed.
7.3. TLS with Server Name Indication (TLS SNI) 7.3. TLS with Server Name Indication (TLS SNI)
The TLS with Server Name Indication (TLS SNI) validation method The TLS with Server Name Indication (TLS SNI) validation method
proves control over a domain name by requiring the client to proves control over a domain name by requiring the client to
configure a TLS server referenced by an A/AAAA record under the configure a TLS server referenced by an A/AAAA record under the
domain name to respond to specific connection attempts utilizing the domain name to respond to specific connection attempts utilizing the
Server Name Indication extension [RFC6066]. The server verifies the Server Name Indication extension [RFC6066]. The server verifies the
client's challenge by accessing the reconfigured server and verifying client's challenge by accessing the reconfigured server and verifying
a particular challenge certificate is presented. a particular challenge certificate is presented.
type (required, string): The string "tls-sni-01" type (required, string): The string "tls-sni-02"
token (required, string): A random value that uniquely identifies token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy, the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the URL-safe Base64 alphabet. contain any characters outside the URL-safe Base64 alphabet and
MUST NOT contain any padding characters ("=").
n (required, number): Number of tls-sni-01 iterations
{ {
"type": "tls-sni-01", "type": "tls-sni-02",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA", "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
"n": 25
} }
A client responds to this challenge by constructing a key A client responds to this challenge by constructing a self-signed
authorization from the "token" value provided in the challenge and certificate which the client MUST provision at the domain name
the client's account key. The client first computes the SHA-256 concerned in order to pass the challenge.
digest Z0 of the UTF8-encoded key authorization, and encodes Z0 in
UTF-8 lower-case hexadecimal form. The client then generates
iterated hash values Z1...Z(n-1) as follows:
Z(i) = lowercase_hexadecimal(SHA256(Z(i-1))). The certificate may be constructed arbitrarily, except that each
certificate MUST have exactly two subjectAlternativeNames, SAN A and
SAN B. Both MUST be dNSNames.
The client generates a self-signed certificate for each iteration of SAN A MUST be constructed as follows: compute the SHA-256 digest of
Zi with a single subjectAlternativeName extension dNSName that is the UTF-8-encoded challenge token and encode it in lowercase
"<Zi[0:32]>.<Zi[32:64]>.acme.invalid", where "Zi[0:32]" and hexadecimal form. The dNSName is "x.y.token.acme.invalid", where x
"Zi[32:64]" represent the first 32 and last 32 characters of the hex- is the first half of the hexadecimal representation and y is the
encoded value, respectively (following the notation used in Python). second half.
The client then configures the TLS server at the domain such that
when a handshake is initiated with the Server Name Indication
extension set to "<Zi[0:32]>.<Zi[32:64]>.acme.invalid", the
corresponding generated certificate is presented.
The response to the TLS SNI challenge simply acknowledges that the SAN B MUST be constructed as follows: compute the SHA-256 digest of
the UTF-8 encoded key authorization and encode it in lowercase
hexadecimal form. The dNSName is "x.y.ka.acme.invalid" where x is
the first half of the hexadecimal representation and y is the second
half.
The client MUST ensure that the certificate is served to TLS
connections specifying a Server Name Indication (SNI) value of SAN A.
The response to the TLS-SNI challenge simply acknowledges that the
client is ready to fulfill this challenge. client is ready to fulfill this challenge.
keyAuthorization (required, string): The key authorization for this keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and challenge. This value MUST match the token from the challenge and
the client's account key. the client's account key.
{ {
"keyAuthorization": "evaGxfADs...62jcerQ", "keyAuthorization": "evaGxfADs...62jcerQ",
} }
/* Signed as JWS */ /* Signed as JWS */
On receiving a response, the server MUST verify that the key On receiving a response, the server MUST verify that the key
authorization in the response matches the "token" value in the authorization in the response matches the "token" value in the
challenge and the client's account key. If they do not match, then challenge and the client's account key. If they do not match, then
the server MUST return an HTTP error in response to the POST request the server MUST return an HTTP error in response to the POST request
in which the client sent the challenge. in which the client sent the challenge.
Given a Challenge/Response pair, the ACME server verifies the Given a challenge/response pair, the ACME server verifies the
client's control of the domain by verifying that the TLS server was client's control of the domain by verifying that the TLS server was
configured appropriately. configured appropriately, using these steps:
1. Choose a subset of the N iterations to check, according to local
policy.
2. For each iteration, compute the Zi-value from the key
authorization in the same way as the client.
3. Open a TLS connection to the domain name being validated on the 1. Compute SAN A and SAN B in the same way as the client.
requested port, presenting the value
"<Zi[0:32]>.<Zi[32:64]>.acme.invalid" in the SNI field (where the
comparison is case-insensitive).
4. Verify that the certificate contains a subjectAltName extension 2. Open a TLS connection to the domain name being validated on the
with the dNSName of "<Z[0:32]>.<Z[32:64]>.acme.invalid", and that requested port, presenting SAN A in the SNI field. In the
no other dNSName entries of the form "*.acme.invalid" are present ClientHello initiating the TLS handshake, the server MUST include
in the subjectAltName extension. a server_name extension (i.e., SNI) containing SAN A. The server
SHOULD ensure that it does not reveal SAN B in any way when
making the TLS connection, such that the presentation of SAN B in
the returned certificate proves association with the client.
It is RECOMMENDED that the ACME server verify a random subset of the 3. Verify that the certificate contains a subjectAltName extension
N iterations with an appropriate sized to ensure that an attacker who containing dNSName entries of SAN A and SAN B and no other
can provision certs for a default virtual host, but not for arbitrary entries. The comparison MUST be insensitive to case and ordering
simultaneous virtual hosts, cannot pass the challenge. For instance, of names.
testing a subset of 5 of N=25 domains ensures that such an attacker
has only a one in 25/5 chance of success if they post certs Zn in
random succession. (This probability is enforced by the requirement
that each certificate have only one Zi value.)
It is RECOMMENDED that the ACME server validation TLS connections It is RECOMMENDED that the ACME server validation TLS connections
from multiple vantage points to reduce the risk of DNS hijacking from multiple vantage points to reduce the risk of DNS hijacking
attacks. attacks.
If all of the above verifications succeed, then the validation is If all of the above verifications succeed, then the validation is
successful. Otherwise, the validation fails. successful. Otherwise, the validation fails.
7.4. Proof of Possession of a Prior Key 7.4. DNS
The Proof of Possession challenge verifies that a client possesses a
private key corresponding to a server-specified public key, as
demonstrated by its ability to sign with that key. This challenge is
meant to be used when the server knows of a public key that is
already associated with the identifier being claimed, and wishes for
new authorizations to be authorized by the holder of the
corresponding private key. For DNS identifiers, for example, this
can help guard against domain hijacking.
This method is useful if a server policy calls for issuing a
certificate only to an entity that already possesses the subject
private key of a particular prior related certificate (perhaps issued
by a different CA). It may also help enable other kinds of server
policy that are related to authenticating a client's identity using
digital signatures.
This challenge proceeds in much the same way as the proof of
possession of the authorized key pair in the main ACME flow
(challenge + authorizationRequest). The server provides a nonce and
the client signs over the nonce. The main difference is that rather
than signing with the private key of the key pair being authorized,
the client signs with a private key specified by the server. The
server can specify which key pair(s) are acceptable directly (by
indicating a public key), or by asking for the key corresponding to a
certificate.
The server provides the following fields as part of the challenge:
type (required, string): The string "proofOfPossession-01"
certs (optional, array of string): An array of certificates, in
Base64-encoded DER format, that contain acceptable public keys.
{
"type": "proofOfPossession-01",
"certs": ["MIIF7z...bYVQLY"]
}
In response to this challenge, the client uses the private key
corresponding to one of the acceptable public keys to sign a JWS
object including data related to the challenge. The validation
object covered by the signature has the following fields:
type (required, string): The string "proofOfPossession"
identifiers (required, identifier): A list of identifiers for which
the holder of the prior key authorizes the new key
accountKey (required, JWK): The client's account public key
{
"type": "proofOfPossession-01",
"identifiers: [{"type": "dns", "value": "example.com"}],
"accountKey": { "kty": "RSA", ... }
}
This JWS is NOT REQUIRED to have a "nonce" header parameter (as with
the JWS objects that carry ACME request objects). This allows proof-
of-possession response objects to be computed off-line. For example,
as part of a domain transfer, the new domain owner might require the
old domain owner to sign a proof-of-possession validation object, so
that the new domain owner can present that in an ACME transaction
later.
The validation JWS MUST contain a "jwk" header parameter indicating
the public key under which the server should verify the JWS.
The client's response includes the server-provided nonce, together
with a signature over that nonce by one of the private keys requested
by the server.
type (required, string): The string "proofOfPossession"
authorization (required, JWS): The validation JWS
{
"type": "proofOfPossession-01",
"authorization": {
"header": {
"alg": "RS256",
"jwk": {
"kty": "RSA",
"e": "AQAB",
"n": "AMswMT...3aVtjE"
}
},
"payload": "SfiR1...gSAl7A",
"signature": "XcQLfL...cW5beg"
}
}
To validate a proof-of-possession challenge, the server performs the
following steps:
1. Verify that the public key in the "jwk" header of the
"authorization" JWS corresponds to one of the certificates in the
"certs" field of the challenge
2. Verify the "authorization" JWS using the key indicated in its
"jwk" header
3. Decode the payload of the JWS as UTF-8 encoded JSON
4. Verify that there are exactly three fields in the decoded object,
and that:
* The "type" field is set to "proofOfPossession"
* The "identifier" field contains the identifier for which
authorization is being validated
* The "accountKey" field matches the account key for which the
challenge was issued
If all of the above verifications succeed, then the validation is
successful. Otherwise, the validation fails.
7.5. DNS
When the identifier being validated is a domain name, the client can When the identifier being validated is a domain name, the client can
prove control of that domain by provisioning resource records under prove control of that domain by provisioning a resource record under
it. The DNS challenge requires the client to provision a TXT record it. The DNS challenge requires the client to provision a TXT record
containing a designated value under a specific validation domain containing a designated value under a specific validation domain
name. name.
type (required, string): The string "dns-01" type (required, string): The string "dns-01"
token (required, string): A random value that uniquely identifies token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy, the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the URL-safe Base64 alphabet. contain any characters outside the URL-safe Base64 alphabet and
MUST NOT contain any padding characters ("=").
{ {
"type": "dns-01", "type": "dns-01",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
} }
A client responds to this challenge by constructing a key A client responds to this challenge by constructing a key
authorization from the "token" value provided in the challenge and authorization from the "token" value provided in the challenge and
the client's account key. The client then computes the SHA-256 the client's account key. The client then computes the SHA-256
digest of the key authorization. digest of the key authorization.
The record provisioned to the DNS is the base64 encoding of this The record provisioned to the DNS is the base64url encoding of this
digest. The client constructs the validation domain name by digest. The client constructs the validation domain name by
prepending the label "_acme-challenge" to the domain name being prepending the label "_acme-challenge" to the domain name being
validated, then provisions a TXT record with the digest value under validated, then provisions a TXT record with the digest value under
that name. For example, if the domain name being validated is that name. For example, if the domain name being validated is
"example.com", then the client would provision the following DNS "example.com", then the client would provision the following DNS
record: record:
_acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM" _acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM"
The response to the DNS challenge simply acknowledges that the client The response to the DNS challenge provides the computed key
is ready to fulfill this challenge. authorization to acknowledge that the client is ready to fulfill this
challenge.
keyAuthorization (required, string): The key authorization for this keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and challenge. This value MUST match the token from the challenge and
the client's account key. the client's account key.
{ {
"keyAuthorization": "evaGxfADs...62jcerQ", "keyAuthorization": "evaGxfADs...62jcerQ",
} }
/* Signed as JWS */ /* Signed as JWS */
skipping to change at page 46, line 7 skipping to change at page 41, line 18
3. Verify that the contents of one of the TXT records matches the 3. Verify that the contents of one of the TXT records matches the
digest value digest value
If all of the above verifications succeed, then the validation is If all of the above verifications succeed, then the validation is
successful. If no DNS record is found, or DNS record and response successful. If no DNS record is found, or DNS record and response
payload do not pass these checks, then the validation fails. payload do not pass these checks, then the validation fails.
8. IANA Considerations 8. IANA Considerations
TODO [[ Editor's Note: Should we create a registry for tokens that go into
the various JSON objects used by this protocol, i.e., the field names
in the JSON objects? ]]
o Register .well-known path 9. Well-Known URI for the HTTP Challenge
o Register Replay-Nonce HTTP header The "Well-Known URIs" registry should be updated with the following
additional value (using the template from [RFC5785]):
o Register "nonce" JWS header parameter URI suffix: acme-challenge
o Register "urn:acme" namespace Change controller: IETF
o Create identifier validation method registry Specification document(s): This document, Section Section 7.2
o Registries of syntax tokens, e.g., message types / error types? Related information: N/A
9. Security Considerations 9.1. Replay-Nonce HTTP Header
The "Message Headers" registry should be updated with the following
additional value:
| Header Field Name | Protocol | Status | Reference |
+:------------+:------+:------+:-----------+ | Replay-Nonce | http |
standard | Section 5.4.1 |
9.2. "nonce" JWS Header Parameter
The "JSON Web Signature and Encryption Header Parameters" registry
should be updated with the following additional value:
o Header Parameter Name: "nonce"
o Header Parameter Description: Nonce
o Header Parameter Usage Location(s): JWE, JWS
o Change Controller: IESG
o Specification Document(s): Section 5.4.2 of RFC XXXX
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
9.3. URN Sub-namespace for ACME (urn:ietf:params:acme)
The "IETF URN Sub-namespace for Registered Protocol Parameter
Identifiers" registry should be updated with the following additional
value, following the template in [RFC3553]:
Registry name: acme
Specification: RFC XXXX
Repository: URL-TBD
Index value: No transformation needed. The
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document, and replace URL-TBD with the URL assigned by IANA
for registries of ACME parameters. ]]
9.4. New Registries
This document requests that IANA create three new registries:
1. ACME Error Codes
2. ACME Identifier Types
3. ACME Challenge Types
All of these registries should be administered under a Specification
Required policy [RFC5226].
9.4.1. Error Codes
This registry lists values that are used within URN values that are
provided in the "type" field of problem documents in ACME.
Template:
o Code: The label to be included in the URN for this error,
following "urn:ietf:params:acme:"
o Description: A human-readable description of the error
o Reference: Where the error is defined
Initial contents: The codes and descriptions in the table in
Section 5.5 above, with the Reference field set to point to this
specification.
9.4.2. Identifier Types
This registry lists the types of identifiers that ACME clients may
request authorization to issue in certificates.
Template:
o Label: The value to be put in the "type" field of the identifier
object
o Reference: Where the identifier type is defined
Initial contents:
+-------+-----------+
| Label | Reference |
+-------+-----------+
| dns | RFC XXXX |
+-------+-----------+
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
9.4.3. Challenge Types
This registry lists the ways that ACME servers can offer to validate
control of an identifier. The "Identifier Type" field in template
MUST be contained in the Label column of the ACME Identifier Types
registry.
Template:
o Label: The value to be put in the "type" field of challenge
objects using this validation mechanism
o Identifier Type: The type of identifier that this mechanism
applies to
o Reference: Where the challenge type is defined
Initial Contents
+---------+-----------------+-----------+
| Label | Identifier Type | Reference |
+---------+-----------------+-----------+
| http | dns | RFC XXXX |
| | | |
| tls-sni | dns | RFC XXXX |
| | | |
| dns | dns | RFC XXXX |
+---------+-----------------+-----------+
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
10. Security Considerations
ACME is a protocol for managing certificates that attest to ACME is a protocol for managing certificates that attest to
identifier/key bindings. Thus the foremost security goal of ACME is identifier/key bindings. Thus the foremost security goal of ACME is
to ensure the integrity of this process, i.e., to ensure that the to ensure the integrity of this process, i.e., to ensure that the
bindings attested by certificates are correct, and that only bindings attested by certificates are correct, and that only
authorized entities can manage certificates. ACME identifies clients authorized entities can manage certificates. ACME identifies clients
by their account keys, so this overall goal breaks down into two more by their account keys, so this overall goal breaks down into two more
precise goals: precise goals:
1. Only an entity that controls a identifier can get an account key 1. Only an entity that controls an identifier can get an account key
authorized for that identifier authorized for that identifier
2. Once authorized, an account key's authorizations cannot be 2. Once authorized, an account key's authorizations cannot be
improperly transferred to another account key improperly transferred to another account key
In this section, we discuss the threat model that underlies ACME and In this section, we discuss the threat model that underlies ACME and
the ways that ACME achieves these security goals within that threat the ways that ACME achieves these security goals within that threat
model. We also discuss the denial-of-service risks that ACME servers model. We also discuss the denial-of-service risks that ACME servers
face, and a few other miscellaneous considerations. face, and a few other miscellaneous considerations.
9.1. Threat model 10.1. Threat model
As a service on the Internet, ACME broadly exists within the Internet As a service on the Internet, ACME broadly exists within the Internet
threat model [RFC3552]. In analyzing ACME, it is useful to think of threat model [RFC3552]. In analyzing ACME, it is useful to think of
an ACME server interacting with other Internet hosts along three an ACME server interacting with other Internet hosts along three
"channels": "channels":
o An ACME channel, over which the ACME HTTPS requests are exchanged o An ACME channel, over which the ACME HTTPS requests are exchanged
o A validation channel, over which the ACME server performs o A validation channel, over which the ACME server performs
additional requests to validate a client's control of an additional requests to validate a client's control of an
identifier identifier
skipping to change at page 47, line 47 skipping to change at page 45, line 47
On the ACME channel, in addition to network-layer attackers, we also On the ACME channel, in addition to network-layer attackers, we also
need to account for application-layer man in the middle attacks, and need to account for application-layer man in the middle attacks, and
for abusive use of the protocol itself. Protection against for abusive use of the protocol itself. Protection against
application-layer MitM addresses potential attackers such as Content application-layer MitM addresses potential attackers such as Content
Distribution Networks (CDNs) and middleboxes with a TLS MitM Distribution Networks (CDNs) and middleboxes with a TLS MitM
function. Preventing abusive use of ACME means ensuring that an function. Preventing abusive use of ACME means ensuring that an
attacker with access to the validation or contact channels can't attacker with access to the validation or contact channels can't
obtain illegitimate authorization by acting as an ACME client obtain illegitimate authorization by acting as an ACME client
(legitimately, in terms of the protocol). (legitimately, in terms of the protocol).
9.2. Integrity of Authorizations 10.2. Integrity of Authorizations
ACME allows anyone to request challenges for an identifier by ACME allows anyone to request challenges for an identifier by
registering an account key and sending a new-authorization request registering an account key and sending a new-authorization request
under that account key. The integrity of the authorization process under that account key. The integrity of the authorization process
thus depends on the identifier validation challenges to ensure that thus depends on the identifier validation challenges to ensure that
the challenge can only be completed by someone who both (1) holds the the challenge can only be completed by someone who both (1) holds the
private key of the account key pair, and (2) controls the identifier private key of the account key pair, and (2) controls the identifier
in question. in question.
Validation responses need to be bound to an account key pair in order Validation responses need to be bound to an account key pair in order
skipping to change at page 48, line 37 skipping to change at page 46, line 37
o ACME server performs validation query and sees the response o ACME server performs validation query and sees the response
provisioned by the legitimate domain holder provisioned by the legitimate domain holder
o Because the challenges were issued in response to a message signed o Because the challenges were issued in response to a message signed
account key B, the ACME server grants authorization to account key account key B, the ACME server grants authorization to account key
B (the MitM) instead of account key A (the legitimate domain B (the MitM) instead of account key A (the legitimate domain
holder) holder)
All of the challenges above that require an out-of-band query by the All of the challenges above that require an out-of-band query by the
server have a binding to the account private key, such that the only server have a binding to the account private key, such that only the
the account private key holder can successfully respond to the account private key holder can successfully respond to the validation
validation query: query:
o HTTP: The value provided in the validation request is signed by o HTTP: The value provided in the validation request is signed by
the account private key. the account private key.
o TLS SNI: The validation TLS request uses the account key pair as o TLS SNI: The validation TLS request uses the account key pair as
the server's key pair. the server's key pair.
o DNS: The MAC covers the account key, and the MAC key is derived o DNS: The MAC covers the account key, and the MAC key is derived
from an ECDH public key signed with the account private key. from an ECDH public key signed with the account private key.
o Proof of possession of a prior key: The signature by the prior key
covers the account public key.
The association of challenges to identifiers is typically done by The association of challenges to identifiers is typically done by
requiring the client to perform some action that only someone who requiring the client to perform some action that only someone who
effectively controls the identifier can perform. For the challenges effectively controls the identifier can perform. For the challenges
in this document, the actions are: in this document, the actions are:
o HTTP: Provision files under .well-known on a web server for the o HTTP: Provision files under .well-known on a web server for the
domain domain
o TLS SNI: Configure a TLS server for the domain o TLS SNI: Configure a TLS server for the domain
o DNS: Provision DNS resource records for the domain o DNS: Provision DNS resource records for the domain
o Proof of possession of a prior key: Sign using the private key
specified by the server
There are several ways that these assumptions can be violated, both There are several ways that these assumptions can be violated, both
by misconfiguration and by attack. For example, on a web server that by misconfiguration and by attack. For example, on a web server that
allows non-administrative users to write to .well-known, any user can allows non-administrative users to write to .well-known, any user can
claim to own the server's hostname by responding to a Simple HTTP claim to own the server's hostname by responding to an HTTP
challenge, and likewise for TLS configuration and TLS SNI. challenge, and likewise for TLS configuration and TLS SNI.
The use of hosting providers is a particular risk for ACME The use of hosting providers is a particular risk for ACME
validation. If the owner of the domain has outsourced operation of validation. If the owner of the domain has outsourced operation of
DNS or web services to a hosting provider, there is nothing that can DNS or web services to a hosting provider, there is nothing that can
be done against tampering by the hosting provider. As far as the be done against tampering by the hosting provider. As far as the
outside world is concerned, the zone or web site provided by the outside world is concerned, the zone or web site provided by the
hosting provider is the real thing. hosting provider is the real thing.
More limited forms of delegation can also lead to an unintended party More limited forms of delegation can also lead to an unintended party
gaining the ability to successfully complete a validation gaining the ability to successfully complete a validation
transaction. For example, suppose an ACME server follows HTTP transaction. For example, suppose an ACME server follows HTTP
redirects in Simple HTTP validation and a web site operator redirects in HTTP validation and a web site operator provisions a
provisions a catch-all redirect rule that redirects requests for catch-all redirect rule that redirects requests for unknown resources
unknown resources to different domain. Then the target of the to different domain. Then the target of the redirect could use that
redirect could use that to get a certificate through Simple HTTP to get a certificate through HTTP validation, since the validation
validation, since the validation path will not be known to the path will not be known to the primary server.
primary server.
The DNS is a common point of vulnerability for all of these The DNS is a common point of vulnerability for all of these
challenges. An entity that can provision false DNS records for a challenges. An entity that can provision false DNS records for a
domain can attack the DNS challenge directly, and can provision false domain can attack the DNS challenge directly, and can provision false
A/AAAA records to direct the ACME server to send its TLS SNI or HTTP A/AAAA records to direct the ACME server to send its TLS SNI or HTTP
validation query to a server of the attacker's choosing. There are a validation query to a server of the attacker's choosing. There are a
few different mitigations that ACME servers can apply: few different mitigations that ACME servers can apply:
o Checking the DNSSEC status of DNS records used in ACME validation o Always querying the DNS using a DNSSEC-validating resolver
(for zones that are DNSSEC-enabled) (enhancing security for zones that are DNSSEC-enabled)
o Querying the DNS from multiple vantage points to address local o Querying the DNS from multiple vantage points to address local
attackers attackers
o Applying mitigations against DNS off-path attackers, e.g., adding o Applying mitigations against DNS off-path attackers, e.g., adding
entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP
Given these considerations, the ACME validation process makes it Given these considerations, the ACME validation process makes it
impossible for any attacker on the ACME channel, or a passive impossible for any attacker on the ACME channel, or a passive
attacker on the validation channel to hijack the authorization attacker on the validation channel to hijack the authorization
skipping to change at page 50, line 28 skipping to change at page 48, line 22
attacker's account key, but this is prevented by binding the attacker's account key, but this is prevented by binding the
validation response to the account key used to request challenges. A validation response to the account key used to request challenges. A
passive attacker on the validation channel can observe the correct passive attacker on the validation channel can observe the correct
validation response and even replay it, but that response can only be validation response and even replay it, but that response can only be
used with the account key for which it was generated. used with the account key for which it was generated.
An active attacker on the validation channel can subvert the ACME An active attacker on the validation channel can subvert the ACME
process, by performing normal ACME transactions and providing a process, by performing normal ACME transactions and providing a
validation response for his own account key. The risks due to validation response for his own account key. The risks due to
hosting providers noted above are a particular case. For identifiers hosting providers noted above are a particular case. For identifiers
where the server already has some credential associated with the where the server already has some public key associated with the
domain this attack can be prevented by requiring the client to domain this attack can be prevented by requiring the client to prove
complete a proof-of-possession challenge. control of the corresponding private key.
9.3. Preventing Authorization Hijacking
The account recovery processes described in Section 6.4 allow
authorization to be transferred from one account key to another, in
case the former account key pair's private key is lost. ACME needs
to prevent these processes from being exploited by an attacker to
hijack the authorizations attached to one key and assign them to a
key of the attacker's choosing.
Recovery takes place in two steps: 1. Provisioning recovery
information (contact or recovery key) 2. Using recovery information
to recover an account
The provisioning process needs to ensure that only the account key
holder ends up with information that is useful for recovery. The
recovery process needs to assure that only the (now former) account
key holder can successfully execute recovery, i.e., that this entity
is the only one that can choose the new account key that receives the
capabilities held by the account being recovered.
MAC-based recovery can be performed if the attacker knows the account
key and registration URI for the account being recovered. Both of
these are difficult to obtain for a network attacker, because ACME
uses HTTPS, though if the recovery key and registration URI are
sufficiently predictable, the attacker might be able to guess them.
An ACME MitM can see the registration URI, but still has to guess the
recovery key, since neither the ECDH in the provisioning phase nor
HMAC in the recovery phase will reveal it to him.
ACME clients can thus mitigate problems with MAC-based recovery by
using long recovery keys. ACME servers should enforce a minimum
recovery key length, and impose rate limits on recovery to limit an
attacker's ability to test different guesses about the recovery key.
Contact-based recovery uses both the ACME channel and the contact
channel. The provisioning process is only visible to an ACME MitM,
and even then, the MitM can only observe the contact information
provided. If the ACME attacker does not also have access to the
contact channel, there is no risk.
The security of the contact-based recovery process is entirely
dependent on the security of the contact channel. The details of
this will depend on the specific out-of-band technique used by the
server. For example:
o If the server requires a user to click a link in a message sent to
a contact address, then the contact channel will need to ensure
that the message is only available to the legitimate owner of the
contact address. Otherwise, a passive attacker could see the link
and click it first, or an active attacker could redirect the
message.
o If the server requires a user to respond to a message sent to a
contact address containing a secret value, then the contact
channel will need to ensure that an attacker cannot observe the
secret value and spoof a message from the contact address.
In practice, many contact channels that can be used to reach many
clients do not provide strong assurances of the types noted above.
In designing and deploying contact-based recovery schemes, ACME
servers operators will need to find an appropriate balance between
using contact channels that can reach many clients and using contact-
based recovery schemes that achieve an appropriate level of risk
using those contact channels.
9.4. Denial-of-Service Considerations 10.3. Denial-of-Service Considerations
As a protocol run over HTTPS, standard considerations for TCP-based As a protocol run over HTTPS, standard considerations for TCP-based
and HTTP-based DoS mitigation also apply to ACME. and HTTP-based DoS mitigation also apply to ACME.
At the application layer, ACME requires the server to perform a few At the application layer, ACME requires the server to perform a few
potentially expensive operations. Identifier validation transactions potentially expensive operations. Identifier validation transactions
require the ACME server to make outbound connections to potentially require the ACME server to make outbound connections to potentially
attacker-controlled servers, and certificate issuance can require attacker-controlled servers, and certificate issuance can require
interactions with cryptographic hardware. interactions with cryptographic hardware.
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All of these attacks can be mitigated by the application of All of these attacks can be mitigated by the application of
appropriate rate limits. Issues closer to the front end, like POST appropriate rate limits. Issues closer to the front end, like POST
body validation, can be addressed using HTTP request limiting. For body validation, can be addressed using HTTP request limiting. For
validation and certificate requests, there are other identifiers on validation and certificate requests, there are other identifiers on
which rate limits can be keyed. For example, the server might limit which rate limits can be keyed. For example, the server might limit
the rate at which any individual account key can issue certificates, the rate at which any individual account key can issue certificates,
or the rate at which validation can be requested within a given or the rate at which validation can be requested within a given
subtree of the DNS. subtree of the DNS.
9.5. CA Policy Considerations 10.4. CA Policy Considerations
The controls on issuance enabled by ACME are focused on validating The controls on issuance enabled by ACME are focused on validating
that a certificate applicant controls the identifier he claims. that a certificate applicant controls the identifier he claims.
Before issuing a certificate, however, there are many other checks Before issuing a certificate, however, there are many other checks
that a CA might need to perform, for example: that a CA might need to perform, for example:
o Has the client agreed to a subscriber agreement? o Has the client agreed to a subscriber agreement?
o Is the claimed identifier syntactically valid? o Is the claimed identifier syntactically valid?
skipping to change at page 53, line 4 skipping to change at page 49, line 28
* If the leftmost label is a '*', then have the appropriate * If the leftmost label is a '*', then have the appropriate
checks been applied? checks been applied?
* Is the name on the Public Suffix List? * Is the name on the Public Suffix List?
* Is the name a high-value name? * Is the name a high-value name?
* Is the name a known phishing domain? * Is the name a known phishing domain?
o Is the key in the CSR sufficiently strong? o Is the key in the CSR sufficiently strong?
o Is the CSR signed with an acceptable algorithm? o Is the CSR signed with an acceptable algorithm?
CAs that use ACME to automate issuance will need to ensure that their CAs that use ACME to automate issuance will need to ensure that their
servers perform all necessary checks before issuing. servers perform all necessary checks before issuing.
10. Acknowledgements 11. Operational Considerations
There are certain factors that arise in operational reality that
operators of ACME-based CAs will need to keep in mind when
configuring their services. For example:
o It is advisable to perform DNS queries via TCP to mitigate DNS
forgery attacks over UDP
[[ TODO: Other operational considerations ]]
11.1. Default Virtual Hosts
In many cases, TLS-based services are deployed on hosted platforms
that use the Server Name Indication (SNI) TLS extension to
distinguish between different hosted services or "virtual hosts".
When a client initiates a TLS connection with an SNI value indicating
a provisioned host, the hosting platform routes the connection to
that host.
When a connection come in with an unknown SNI value, one might expect
the hosting platform to terminate the TLS connection. However, some
hosting platforms will choose a virtual host to be the "default", and
route connections with unknown SNI values to that host.
In such cases, the owner of the default virtual host can complete a
TLS-based challenge (e.g., "tls-sni-02") for any domain with an A
record that points to the hosting platform. This could result in
mis-issuance in cases where there are multiple hosts with different
owners resident on the hosting platform.
A CA that accepts TLS-based proof of domain control should attempt to
check whether a domain is hosted on a domain with a default virtual
host before allowing an authorization request for this host to use a
TLS-based challenge. A default virtual host can be detected by
initiating TLS connections to the host with random SNI values within
the namespace used for the TLS-based challenge (the "acme.invalid"
namespace for "tls-sni-02").
11.2. Use of DNSSEC Resolvers
An ACME-based CA will often need to make DNS queries, e.g., to
validate control of DNS names. Because the security of such
validations ultimately depends on the authenticity of DNS data, every
possible precaution should be taken to secure DNS queries done by the
CA. It is therefore RECOMMENDED that ACME-based CAs make all DNS
queries via DNSSEC-validating stub or recursive resolvers. This
provides additional protection to domains which choose to make use of
DNSSEC.
An ACME-based CA must use only a resolver if it trusts the resolver
and every component of the network route by which it is accessed. It
is therefore RECOMMENDED that ACME-based CAs operate their own
DNSSEC-validating resolvers within their trusted network and use
these resolvers both for both CAA record lookups and all record
lookups in furtherance of a challenge scheme (A, AAAA, TXT, etc.).
12. Acknowledgements
In addition to the editors listed on the front page, this document In addition to the editors listed on the front page, this document
has benefited from contributions from a broad set of contributors, has benefited from contributions from a broad set of contributors,
all the way back to its inception. all the way back to its inception.
o Peter Eckersley, EFF o Peter Eckersley, EFF
o Eric Rescorla, Mozilla o Eric Rescorla, Mozilla
o Seth Schoen, EFF o Seth Schoen, EFF
skipping to change at page 53, line 31 skipping to change at page 51, line 14
o Alex Halderman, University of Michigan o Alex Halderman, University of Michigan
o Martin Thomson, Mozilla o Martin Thomson, Mozilla
o Jakub Warmuz, University of Oxford o Jakub Warmuz, University of Oxford
This document draws on many concepts established by Eric Rescorla's This document draws on many concepts established by Eric Rescorla's
"Automated Certificate Issuance Protocol" draft. Martin Thomson "Automated Certificate Issuance Protocol" draft. Martin Thomson
provided helpful guidance in the use of HTTP. provided helpful guidance in the use of HTTP.
11. References 13. References
11.1. Normative References 13.1. Normative References
[I-D.ietf-appsawg-http-problem] [I-D.ietf-appsawg-http-problem]
mnot, m. and E. Wilde, "Problem Details for HTTP APIs", mnot, m. and E. Wilde, "Problem Details for HTTP APIs",
draft-ietf-appsawg-http-problem-01 (work in progress), draft-ietf-appsawg-http-problem-03 (work in progress),
September 2015. January 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax [RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998, Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998,
<http://www.rfc-editor.org/info/rfc2314>. <http://www.rfc-editor.org/info/rfc2314>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985, DOI Classes and Attribute Types Version 2.0", RFC 2985,
10.17487/RFC2985, November 2000, DOI 10.17487/RFC2985, November 2000,
<http://www.rfc-editor.org/info/rfc2985>. <http://www.rfc-editor.org/info/rfc2985>.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986, DOI Request Syntax Specification Version 1.7", RFC 2986,
10.17487/RFC2986, November 2000, DOI 10.17487/RFC2986, November 2000,
<http://www.rfc-editor.org/info/rfc2986>. <http://www.rfc-editor.org/info/rfc2986>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>. <http://www.rfc-editor.org/info/rfc3339>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
Resource Identifier (URI): Generic Syntax", STD 66, RFC IETF URN Sub-namespace for Registered Protocol
3986, DOI 10.17487/RFC3986, January 2005, Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
<http://www.rfc-editor.org/info/rfc3986>. 2003, <http://www.rfc-editor.org/info/rfc3553>.
[RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access Protocol [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
(LDAP): String Representation of Distinguished Names", RFC Architecture", RFC 4291, DOI 10.17487/RFC4291, February
4514, DOI 10.17487/RFC4514, June 2006, 2006, <http://www.rfc-editor.org/info/rfc4291>.
<http://www.rfc-editor.org/info/rfc4514>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>. <http://www.rfc-editor.org/info/rfc4648>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008, DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>. <http://www.rfc-editor.org/info/rfc5226>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/ (TLS) Protocol Version 1.2", RFC 5246,
RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <http://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <http://www.rfc-editor.org/info/rfc5280>.
[RFC5753] Turner, S. and D. Brown, "Use of Elliptic Curve [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Cryptography (ECC) Algorithms in Cryptographic Message Uniform Resource Identifiers (URIs)", RFC 5785,
Syntax (CMS)", RFC 5753, DOI 10.17487/RFC5753, January DOI 10.17487/RFC5785, April 2010,
2010, <http://www.rfc-editor.org/info/rfc5753>. <http://www.rfc-editor.org/info/rfc5785>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/ [RFC5988] Nottingham, M., "Web Linking", RFC 5988,
RFC5988, October 2010, DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>. <http://www.rfc-editor.org/info/rfc5988>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, DOI Extensions: Extension Definitions", RFC 6066,
10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>. <http://www.rfc-editor.org/info/rfc6066>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/ and D. Orchard, "URI Template", RFC 6570,
RFC6570, March 2012, DOI 10.17487/RFC6570, March 2012,
<http://www.rfc-editor.org/info/rfc6570>. <http://www.rfc-editor.org/info/rfc6570>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<http://www.rfc-editor.org/info/rfc6844>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>. 2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>. 2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>. 2015, <http://www.rfc-editor.org/info/rfc7515>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/ [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
RFC7517, May 2015, DOI 10.17487/RFC7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>. <http://www.rfc-editor.org/info/rfc7517>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
10.17487/RFC7518, May 2015, DOI 10.17487/RFC7518, May 2015,
<http://www.rfc-editor.org/info/rfc7518>. <http://www.rfc-editor.org/info/rfc7518>.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK) [RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK)
Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
2015, <http://www.rfc-editor.org/info/rfc7638>. 2015, <http://www.rfc-editor.org/info/rfc7638>.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1: 13.2. Informative References
Elliptic Curve Cryptography", May 2009,
<http://www.secg.org/sec1-v2.pdf>.
11.2. Informative References
[I-D.vixie-dnsext-dns0x20] [I-D.vixie-dnsext-dns0x20]
Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to
Improve Transaction Identity", draft-vixie-dnsext- Improve Transaction Identity", draft-vixie-dnsext-
dns0x20-00 (work in progress), March 2008. dns0x20-00 (work in progress), March 2008.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, DOI Text on Security Considerations", BCP 72, RFC 3552,
10.17487/RFC3552, July 2003, DOI 10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>. <http://www.rfc-editor.org/info/rfc3552>.
[W3C.CR-cors-20130129] [W3C.CR-cors-20130129]
Kesteren, A., "Cross-Origin Resource Sharing", World Wide Kesteren, A., "Cross-Origin Resource Sharing", World Wide
Web Consortium CR CR-cors-20130129, January 2013, Web Consortium CR CR-cors-20130129, January 2013,
<http://www.w3.org/TR/2013/CR-cors-20130129>. <http://www.w3.org/TR/2013/CR-cors-20130129>.
[W3C.WD-capability-urls-20140218] [W3C.WD-capability-urls-20140218]
Tennison, J., "Good Practices for Capability URLs", World Tennison, J., "Good Practices for Capability URLs", World
Wide Web Consortium WD WD-capability-urls-20140218, Wide Web Consortium WD WD-capability-urls-20140218,
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