Network Working Group                                          R. Barnes
Internet-Draft                                                   Mozilla
Intended status: Standards Track                      J. Hoffman-Andrews
Expires: September 22, 2016 January 9, 2017                                             EFF
                                                               J. Kasten
                                                  University of Michigan
                                                          March 21,
                                                           July 08, 2016

          Automatic Certificate Management Environment (ACME)
                        draft-ietf-acme-acme-02
                        draft-ietf-acme-acme-03

Abstract

   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
   domain names.  Thus, certificate authorities in the Web PKI are
   trusted to verify that an applicant for a certificate legitimately
   represents the domain name(s) in the certificate.  Today, this
   verification is done through a collection of ad hoc mechanisms.  This
   document describes a protocol that a certificate authority (CA) and
   an applicant can use to automate the process of verification and
   certificate issuance.  The protocol also provides facilities for
   other certificate management functions, such as certificate
   revocation.

   DISCLAIMER: This is a work in progress draft of ACME and has not yet
   had a thorough security analysis.

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for
   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

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 22, 2016. January 9, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Deployment Model and Operator Experience  . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Message Transport . . . . . . . . . . . . . . . . . . . . . .   9   8
     5.1.  HTTPS Requests  . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Request Authentication  . . . . . . . . . . . . . . . . .   9
     5.3.  Request URI Type Integrity . . . . . . . . . . . . . . . . . .  10
       5.3.1.  "url" (URL) JWS header parameter  . . . . . . . . . .  10
     5.4.  Replay protection . . . . . . . . . . . . . . . . . . . .  11
       5.4.1.  Replay-Nonce  . . . . . . . . . . . . . . . . . . . .  12  11
       5.4.2.  "nonce" (Nonce) JWS header parameter  . . . . . . . .  12
     5.5.  Rate limits . . . . . . . . . . . . . . . . . . . . . . .  12
     5.6.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Certificate Management  . . . . . . . . . . . . . . . . . . .  14
     6.1.  Resources . . . . . . . . . . . . . . . . . . . . . . . .  14
       6.1.1.  Registration Objects  Directory . . . . . . . . . . . . . . . . . . . . . .  16
       6.1.2.  Authorization  Registration Objects  . . . . . . . . . . . . . . . .  17
     6.2.  Directory
       6.1.3.  Application Objects . . . . . . . . . . . . . . . . .  19
       6.1.4.  Authorization Objects . . . . . . . .  18
     6.3. . . . . . . . .  21
     6.2.  Registration  . . . . . . . . . . . . . . . . . . . . . .  20
       6.3.1.  23
       6.2.1.  Account Key Roll-over . . . . . . . . . . . . . . . .  22
       6.3.2.  Deleting an  25
       6.2.2.  Account deactivation  . . . . . . . . . . . . . . . .  27
     6.3.  Applying for Certificate Issuance . . . . .  23
     6.4.  Identifier Authorization . . . . . . .  28
       6.3.1.  Downloading the Certificate . . . . . . . . .  24
       6.4.1.  Responding to Challenges . . . .  30
     6.4.  Identifier Authorization  . . . . . . . . . .  26
       6.4.2.  Deleting an Authorization . . . . . .  31
       6.4.1.  Responding to Challenges  . . . . . . . .  28
     6.5.  Certificate Issuance . . . . . .  33
       6.4.2.  Deactivating an Authorization . . . . . . . . . . . .  29
     6.6.  35
     6.5.  Certificate Revocation  . . . . . . . . . . . . . . . . .  32  36
   7.  Identifier Validation Challenges  . . . . . . . . . . . . . .  33  38
     7.1.  Key Authorizations  . . . . . . . . . . . . . . . . . . .  35  39
     7.2.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  35  40
     7.3.  TLS with Server Name Indication (TLS SNI) . . . . . . . .  38  42
     7.4.  DNS . . . . . . . . . . . . . . . . . . . . . . . . . . .  39  44
     7.5.  Out-of-Band . . . . . . . . . . . . . . . . . . . . . . .  45
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
   9.  46
     8.1.  Well-Known URI for the HTTP Challenge . . . . . . . . . . . .  41
     9.1.  46
     8.2.  Replay-Nonce HTTP Header  . . . . . . . . . . . . . . . .  41
     9.2.  47
     8.3.  "url" JWS Header Parameter  . . . . . . . . . . . . . . .  47
     8.4.  "nonce" JWS Header Parameter  . . . . . . . . . . . . . .  41
     9.3.  47
     8.5.  URN Sub-namespace for ACME (urn:ietf:params:acme) . . . .  42
     9.4.  48
     8.6.  New Registries  . . . . . . . . . . . . . . . . . . . . .  42
       9.4.1.  48
       8.6.1.  Error Codes . . . . . . . . . . . . . . . . . . . . .  42
       9.4.2.  48
       8.6.2.  Resource Types  . . . . . . . . . . . . . . . . . . .  49
       8.6.3.  Identifier Types  . . . . . . . . . . . . . . . . . .  43
       9.4.3.  49
       8.6.4.  Challenge Types . . . . . . . . . . . . . . . . . . .  43
   10.  50
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
     10.1.  50
     9.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  44
     10.2.  51
     9.2.  Integrity of Authorizations . . . . . . . . . . . . . .  45
     10.3. .  52
     9.3.  Denial-of-Service Considerations  . . . . . . . . . . . .  48
     10.4.  CA Policy Considerations .  54
     9.4.  Server-Side Request Forgery . . . . . . . . . . . . . . .  49
   11. Operational  55
     9.5.  CA Policy Considerations  . . . . . . . . . . . . . . . .  55
   10. Operational Considerations  .  49
     11.1.  Default Virtual Hosts . . . . . . . . . . . . . . . .  56
     10.1.  DNS over TCP .  49
     11.2.  Use of DNSSEC Resolvers . . . . . . . . . . . . . . . .  50
   12. . . . . .  56
     10.2.  Default Virtual Hosts  . . . . . . . . . . . . . . . . .  56
     10.3.  Use of DNSSEC Resolvers  . . . . . . . . . . . . . . . .  57
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  50
   13.  57
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     13.1.  58
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     13.2.  58
     12.2.  Informative References . . . . . . . . . . . . . . . . .  53  60
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54  61

1.  Introduction

   Certificates in the Web PKI [RFC5280] are most commonly used to
   authenticate domain names.  Thus, certificate authorities in the Web
   PKI are trusted to verify that an applicant for a certificate
   legitimately represents the domain name(s) in the certificate.

   Existing Web PKI certificate authorities tend to run on a set of ad
   hoc protocols for certificate issuance and identity verification.  A
   typical user experience is something like:

   o  Generate a PKCS#10 [RFC2314] [RFC2986] Certificate Signing Request (CSR).

   o  Cut-and-paste the CSR into a CA web page.

   o  Prove ownership of the domain by one of the following methods:

      *  Put a CA-provided challenge at a specific place on the web
         server.

      *  Put a CA-provided challenge at a DNS location corresponding to
         the target domain.

      *  Receive CA challenge at a (hopefully) administrator-controlled
         e-mail address corresponding to the domain and then respond to
         it on the CA's web page.

   o  Download the issued certificate and install it on their Web
      Server.

   With the exception of the CSR itself and the certificates that are
   issued, these are all completely ad hoc procedures and are
   accomplished by getting the human user to follow interactive natural-
   language instructions from the CA rather than by machine-implemented
   published protocols.  In many cases, the instructions are difficult
   to follow and cause significant confusion.  Informal usability tests
   by the authors indicate that webmasters often need 1-3 hours to
   obtain and install a certificate for a domain.  Even in the best
   case, the lack of published, standardized mechanisms presents an
   obstacle to the wide deployment of HTTPS and other PKIX-dependent
   systems because it inhibits mechanization of tasks related to
   certificate issuance, deployment, and revocation.

   This document describes an extensible framework for automating the
   issuance and domain validation procedure, thereby allowing servers
   and infrastructural software to obtain certificates without user
   interaction.  Use of this protocol should radically simplify the
   deployment of HTTPS and the practicality of PKIX authentication for
   other protocols based on TLS [RFC5246].

2.  Deployment Model and Operator Experience

   The major guiding use case for ACME is obtaining certificates for Web
   sites (HTTPS [RFC2818]).  In that case, the server is intended to
   speak for one or more domains, and the process of certificate
   issuance is intended to verify that the server actually speaks for
   the domain(s).

   Different types of certificates reflect different kinds of CA
   verification of information about the certificate subject.  "Domain
   Validation" (DV) certificates are by far the most common type.  For
   DV validation, the CA merely verifies that the requester has
   effective control of the web server and/or DNS server for the domain,
   but does not explicitly attempt to verify their real-world identity.
   (This is as opposed to "Organization Validation" (OV) and "Extended
   Validation" (EV) certificates, where the process is intended to also
   verify the real-world identity of the requester.)

   DV certificate validation commonly checks claims about properties
   related to control of a domain name - properties that can be observed
   by the issuing authority in an interactive process that can be
   conducted purely online.  That means that under typical
   circumstances, all steps in the request, verification, and issuance
   process can be represented and performed by Internet protocols with
   no out-of-band human intervention.

   When deploying a current HTTPS server, an operator generally gets a
   prompt to generate a self-signed certificate.  When an operator
   deploys an ACME-compatible web server, the experience would be
   something like this:

   o  The ACME client prompts the operator for the intended domain
      name(s) that the web server is to stand for.

   o  The ACME client presents the operator with a list of CAs from
      which it could get a certificate.  (This list will change over
      time based on the capabilities of CAs and updates to ACME
      configuration.)  The ACME client might prompt the operator for
      payment information at this point.

   o  The operator selects a CA.

   o  In the background, the ACME client contacts the CA and requests
      that a certificate be issued for the intended domain name(s).

   o  Once the CA is satisfied, the certificate is issued and the ACME
      client automatically downloads and installs it, potentially
      notifying the operator via e-mail, SMS, etc.

   o  The ACME client periodically contacts the CA to get updated
      certificates, stapled OCSP responses, or whatever else would be
      required to keep the server functional and its credentials up-to-
      date.

   The overall idea is that it's nearly as easy to deploy with a CA-
   issued certificate as a self-signed certificate, and that once the
   operator has done so, the process is self-sustaining with minimal
   manual intervention.  Close integration of ACME with HTTPS servers,
   for example, can allow the immediate and automated deployment of
   certificates as they are issued, optionally sparing the human
   administrator from additional configuration work.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The two main roles in ACME are "client" and "server".  The ACME
   client uses the protocol to request certificate management actions,
   such as issuance or revocation.  An ACME client therefore typically
   runs on a web server, mail server, or some other server system which
   requires valid TLS certificates.  The ACME server runs at a
   certificate authority, and responds to client requests, performing
   the requested actions if the client is authorized.

   An ACME client is represented by an "account key pair".  The client
   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
   authenticity and integrity of messages from the client.

4.  Protocol Overview

   ACME allows a client to request certificate management actions using
   a set of JSON messages carried over HTTPS.  In some ways, ACME
   functions much like a traditional CA, in which a user creates an
   account, adds identifiers to that account (proving control of the
   domains), and requests certificate issuance for those domains while
   logged in to the account.

   In ACME, the account is represented by an account key pair.  The "add
   a domain" function is accomplished by authorizing the key pair for a
   given domain.  Certificate issuance and revocation are authorized by
   a signature with the key pair.

   The first phase of ACME is for the client to register with the ACME
   server.  The client generates an asymmetric key pair and associates
   this key pair with a set of contact information by signing the
   contact information.  The server acknowledges the registration by
   replying with a registration object echoing the client's input.  The
   server can also provide terms of service at this stage, which the
   client can present to a human user.

         Client                                                  Server

         Contact Information
         Signature                     ------->

                                       <-------            Registration

   Before a
                                                       Terms of Service

   Once the client can issue certificates, is registered, there are three major steps it must establish an
   authorization with the server needs
   to take to get a certificate:

   1.  Apply for an account key pair a certificate to act be issued

   2.  Fulfill the server's requirements for issuance

   3.  Finalize the
   identifier(s) application and request issuance

   The client's application for a certificate describes the desired
   certificate using a PKCS#10 Certificate Signing Request (CSR) plus a
   few additional fields that it wishes to include capture semantics that are not supported
   in the certificate.  To do
   this, CSR format.  If the server is willing to consider issuing such
   a certificate, it responds with a list of requirements that the
   client must demonstrate to satisfy before the certificate will be issued.

   For example, in most cases, the server both (1) will require the client to
   demonstrate that it
   holds controls the private key identifiers in the requested
   certificate.  Because there are many different ways to validate
   possession of different types of identifiers, the account key pair, and (2) server will choose
   from an extensible set of challenges that it has
   authority over are appropriate for the
   identifier being claimed.

   Proof of possession  The client responds with a set of
   responses that tell the account key is built into the ACME
   protocol.  All messages from the client to the server are signed by
   the client, and the server verifies them using the public key of the
   account key pair.

   To verify that the client controls the identifier being claimed, the
   server issues the client a set of challenges.  Because there are many
   different ways to validate possession of different types of
   identifiers, the server will choose from an extensible set of
   challenges that are appropriate for the identifier being claimed.
   The client responds with a set of responses that tell the server
   which challenges server which challenges the client has
   completed.  The server then validates the challenges to check that
   the client has accomplished the challenge.

   For example, if

   Once the client requests a domain name, validation process is complete and the server might
   challenge is satisfied
   that the client to provision a record in has met its requirements, the DNS under that
   name, or to provision a file on a web server referenced by an A or
   AAAA record under that name.  The server would then query the DNS for can either
   proactively issue the record in question, requested certificate or send an HTTP request wait for the file.  If the client provisioned the DNS or the web server as expected, then to
   request that the
   server considers application be "finalized", at which point the client authorized for
   certificate will be issued and provided to the domain name.

         Client                                                  Server

         Identifier client.

         Application
         Signature                     ------->
                                       <-------              Challenges            Requirements
                                                     (e.g., Challenges)

         Responses
         Signature                     ------->

                                       <-------       Updated Challenge

                             <~~~~~~~~Validation~~~~~~~~>

         Poll

         Finalize application
         Signature                     ------->
                                       <-------           Authorization

   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
   the identifier.             Certificate

   To do this, revoke a certificate, the client simply sends a PKCS#10 Certificate
   Signing Request (CSR) to the server (indicating revocation request
   indicating the identifier(s) to
   be included in the issued certificate) and a signature over the CSR
   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
   certificate and provides it in its response.  The certificate is
   assigned a URI, which the client can use to fetch updated versions of
   the certificate.

         Client                                                 Server

         CSR
         Signature                    -------->

                                      <--------            Certificate

   To revoke a certificate, the client simply sends a revocation request
   indicating the certificate certificate to be revoked, signed with an authorized
   key pair.  The server indicates whether the request has succeeded.

         Client                                                 Server

         Revocation request
         Signature                    -------->

                                      <--------                 Result

   Note that while ACME is defined with enough flexibility to handle
   different types of identifiers in principle, the primary use case
   addressed by this document is the case where domain names are used as
   identifiers.  For example, all of the identifier validation
   challenges described in Section 7 below address validation of domain
   names.  The use of ACME for other protocols will require further
   specification, in order to describe how these identifiers are encoded
   in the protocol, and what types of validation challenges the server
   might require.

5.  Message Transport

   ACME uses a combination of HTTPS and JWS to create a messaging layer
   with a few important security properties.

   Communications between an ACME client and an ACME server are done
   over HTTPS, using JWS to provide som some 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

   Each ACME function is accomplished by the client sending a sequence
   of HTTPS requests to the server, carrying JSON messages
   [RFC2818][RFC7159].  Use of HTTPS is REQUIRED.  Clients SHOULD
   support HTTP public key pinning [RFC7469], and servers SHOULD emit
   pinning headers.  Each subsection of Section 6 below describes the
   message formats used by the function, and the order in which messages
   are sent.

   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
   Cross-Origin Resource Sharing (CORS) in order to be accessible from
   browser-based clients [W3C.CR-cors-20130129].  Such servers SHOULD
   set the Access-Control-Allow-Origin header field to the value "*".

   Binary fields in the JSON objects used by ACME are encoded using
   base64url encoding described in [RFC4648] Section 5, according to the
   profile specified in JSON Web Signature [RFC7515] Section 2.  This
   encoding uses a URL safe character set.  Trailing '=' characters MUST
   be stripped.

5.2.  Request Authentication

   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.

   JWS objects sent in ACME requests MUST meet the following additional
   criteria:

   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

   o  The JWS Protected Header MUST include the following fields:

      *  "alg"

      *  "jwk"

      *  "nonce" field (defined below)
      *  "url" (defined below)

   Note that this implies that GET requests are not authenticated.
   Servers MUST NOT respond to GET requests for resources that might be
   considered sensitive.

   In the examples below, JWS objects are shown in the JSON or flattened
   JSON serialization, with the protected header and payload expressed
   as base64url(content) instead of the actual base64-encoded value, so
   that the content is readable.  Some fields are omitted for brevity,
   marked with "...".

5.3.  Request URI Type Integrity

   It is common in deployment the entity terminating TLS for HTTPS to be
   different from the entity operating the logical HTTPS server, with a
   "request routing" layer in the middle.  For example, an ACME CA might
   have a content delivery network terminate TLS connections from
   clients so that it can inspect client requests for denial-of-service
   protection.

   These intermediaries can also change values in the request that are
   not signed in the HTTPS request, e.g., the request URI and headers.
   ACME uses JWS to provides provide a limited integrity mechanism, which
   protects against an intermediary changing the request URI to anothe another
   ACME URI of a different type.  (It does not protect against changing
   between URIs of the same type, e.g., from one authorization URI to
   another).

   An

   As noted above, all ACME request carries object carry a JSON dictionary that provides "url" parameter in
   their protected header.  This header parameter encodes the details
   of URL to
   which the client is directing the request.  On receiving such an
   object in an HTTP request, the server MUST compare the "url"
   parameter to the request URI.  If the two do not match, then the
   server MUST reject the client's request as unauthorized.

   Except for the directory resource, all ACME resources are addressed
   with URLs provided to the client by the server.  Each request object  In such cases, the
   client MUST have
   a "resource" set the "url" field that indicates what type of resource to the request exact string provided by the
   server (rather than performing any re-encoding on the URL).

5.3.1.  "url" (URL) JWS header parameter

   The "url" header parameter specifies the URL to which this JWS object
   is addressed to, as defined directed [RFC3986].  The "url" parameter MUST be carried in the below table:

                 +--------------------+------------------+
                 | Resource type      | "resource"
   protected header of the JWS.  The value |
                 +--------------------+------------------+
                 | New registration   | new-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. of the "nonce" header MUST be
   a JSON string representing the URL.

5.4.  Replay protection

   In order to protect ACME resources from any possible replay attacks,
   ACME requests have a mandatory anti-replay mechanism.  This mechanism
   is based on the server maintaining a list of nonces that it has
   issued to clients, and requiring any signed request from the client
   to carry such a nonce.

   An ACME server MUST include a Replay-Nonce header field in each
   successful response it provides to a client, with contents as
   specified below.  In particular, the ACME server MUST provide a
   Replay-Nonce header field in response to a HEAD request for any valid
   resource.  (This allows clients to easily obtain a fresh nonce.)  It
   MAY also provide nonces in error responses.

   Every JWS sent by an ACME client MUST include, in its protected
   header, the "nonce" header parameter, with contents as defined below.
   As part of JWS verification, the ACME server MUST verify that the
   value of the "nonce" header is a value that the server previously
   provided in a Replay-Nonce header field.  Once a nonce value has
   appeared in an ACME request, the server MUST consider it invalid, in
   the same way as a value it had never issued.

   When a server rejects a request because its nonce value was
   unacceptable (or not present), it SHOULD provide HTTP status code 400
   (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
   server.  For example, the server could generate a random 128-bit
   value for each response, keep a list of issued nonces, and strike
   nonces from this list as they are used.

5.4.1.  Replay-Nonce

   The "Replay-Nonce" header field includes a server-generated value
   that the server can use to detect unauthorized replay in future
   client requests.  The server should generate the value provided in
   Replay-Nonce in such a way that they are unique to each message, with
   high probability.

   The value of the Replay-Nonce field MUST be an octet string encoded
   according to the base64url encoding described in Section 2 of
   [RFC7515].  Clients MUST ignore invalid Replay-Nonce values.

     base64url = [A-Z] / [a-z] / [0-9] / "-" / "_"

     Replay-Nonce = *base64url

   The Replay-Nonce header field SHOULD NOT be included in HTTP request
   messages.

5.4.2.  "nonce" (Nonce) JWS header parameter

   The "nonce" header parameter provides a unique value that enables the
   verifier of a JWS to recognize when replay has occurred.  The "nonce"
   header parameter MUST be carried in the protected header of the JWS.

   The value of the "nonce" header parameter MUST be an octet string,
   encoded according to the base64url encoding described in Section 2 of
   [RFC7515].  If the value of a "nonce" header parameter is not valid
   according to this encoding, then the verifier MUST reject the JWS as
   malformed.

5.5.  Errors

   Errors  Rate limits

   Creation of resources can be reported in ACME both at the HTTP layer rate limited to ensure fair usage and within ACME
   payloads.  ACME servers can return responses
   prevent abuse.  Once the rate limit is exceeded, the server MUST
   respond with an HTTP error
   response with the code (4XX or 5XX).  For example: If "rateLimited".  Additionally, the client submits
   server SHOULD send a "Retry-After" header indicating when the current
   request using may succeed again.  If multiple rate limits are in place,
   that is the time where all rate limits allow access again for the
   current request with exactly the same parameters.

   In addition to the human readable "detail" field of the error
   response, the server MAY send one or multiple tokens in the "Link"
   header pointing to documentation about the specific hit rate limits
   using the "rate-limit" relation.

5.6.  Errors

   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).

   When the server responds with an error status, it SHOULD provide
   additional information using problem document
   [I-D.ietf-appsawg-http-problem]. [RFC7807].  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):

   +----------------+--------------------------------------------------+

   +-----------------------+-------------------------------------------+
   | Code                  | Description                               |
   +----------------+--------------------------------------------------+
   +-----------------------+-------------------------------------------+
   | badCSR                | The CSR is unacceptable (e.g., due to a short   |
   |                       | short key)                                |
   |                       |                                           |
   | badNonce              | The client sent an unacceptable anti-replay anti-     |
   |                       | replay nonce                              |
   |                       |                                           |
   | connection            | The server could not connect to the client for           |
   |                       | validation target                         |
   |                       |                                           |
   | dnssec                | The server could not validate a DNSSEC signed validation failed                  |
   |                       | domain                                           |
   | caa                   | CAA records forbid the CA from issuing    |
   |                       |                                           |
   | malformed             | The request message was malformed         |
   |                       |                                           |
   | serverInternal        | The server experienced an internal error  |
   |                       |                                           |
   | tls                   | The server experienced received 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                                   |
   +----------------+--------------------------------------------------+
   |                       |                                           |
   | rejectedIdentifier    | The server will not issue for the         |
   |                       | identifier                                |
   |                       |                                           |
   | unsupportedIdentifier | Identifier is not supported, but may be   |
   |                       | in future                                 |
   +-----------------------+-------------------------------------------+

   This list is not exhaustive.  The server MAY return errors whose
   "type" field is set to a URI other than those defined above.  Servers
   MUST NOT use the ACME URN namespace for errors other than the
   standard types.  Clients SHOULD display the "detail" field of such
   errors.

   Authorization and challenge objects can also contain error
   information to indicate why the server was unable to validate
   authorization.

6.  Certificate Management

   In this section, we describe the certificate management functions
   that ACME enables:

   o  Account Key Registration

   o  Application for a Certificate

   o  Account Key Authorization

   o  Certificate Issuance

   o  Certificate Renewal

   o  Certificate Revocation

6.1.  Resources

   ACME is structured as a REST application with a few types of
   resources:

   o  Registration resources, representing information about an account

   o  Application resources, represnting an account's requests to issue
      certificates

   o  Authorization resources, representing an account's authorization
      to act for an identifier

   o  Challenge resources, representing a challenge to prove control of
      an identifier

   o  Certificate resources, representing issued certificates

   o  A "directory" resource

   o  A "new-registration" resource

   o  A "new-authorization" "new-application" resource

   o  A "new-certificate" "revoke-certificate" resource

   o  A "revoke-certificate" "key-change" resource
   For the "new-X" resources above, the server MUST have exactly one
   resource for each function.  This resource may be addressed by
   multiple URIs, but all must provide equivalent functionality.

   ACME uses different URIs for different management functions.  Each
   function is listed in a directory along with its corresponding 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 authorization resource to which a challenge belongs.  It is also
   used from certificate resources to indicate a resource from which the
   client may fetch a chain of CA certificates that could be used to
   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
   an ACME server.  The solid lines indicate link relations, and  For the
   dotted lines correspond to relationships most part, these relations are expressed by
   URLs provided as strings in other ways,
   e.g., the Location header resources' JSON representations.
   Lines with labels in a 201 (Created) response. quotes indicate HTTP link relations

                                  directory
                                      .
                                      .
          ....................................................
          .                  .                  .            .
          .                  .                  .            .
          V     "next"
                                      |
                                      |
          ----------------------------------------------------
          |                  |                               |
          |                  |                               |
          V      "next"                  V                               V
       new-reg ---+----> new-authz ---+----> new-cert            new-app                       revoke-cert
          .
          |          .                  |         .                               ^
          .
          |          .                  |         .                               | "revoke"
          V       |                  V                               |         V
         reg -------------> app -------------> cert ---------+
                            |
         reg* ----+        authz -----+       cert-----------+
                            . ^                  |
                            .
                            | | "up"             | "up"
                            V |                  V
                          challenge
                           authz             cert-chain
                            | ^
                            | | "up"
                            V |
                          challenge

   The following table illustrates a typical sequence of requests
   required to establish a new account with the server, prove control of
   an identifier, issue a certificate, and fetch an updated certificate
   some time after issuance.  The "->" is a mnemonic for a Location
   header pointing to a created resource.

          +--------------------+----------------+--------------+

           +--------------------+----------------+------------+
           | Action             | Request        | Response   |
          +--------------------+----------------+--------------+
           +--------------------+----------------+------------+
           | Register           | POST new-reg   | 201 -> reg |
           |                    |                |            |
           | Request challenges Apply for a cert   | POST new-authz new-app   | 201 -> app |
           |                    |                |            |
           | Fetch challenges   | GET  authz     | 200        |
           |                    |                |            |
           | Answer challenges  | POST challenge | 200        |
           |                    |                |            |
           | Poll for status    | GET  authz     | 200        |
           |                    |                |            |
           | Request issuance   | POST new-cert app       | 201 -> cert 200        |
           |                    |                |            |
           | Check for new cert | GET  cert      | 200        |
          +--------------------+----------------+--------------+
           +--------------------+----------------+------------+

   The remainder of this section provides the details of how these
   resources are structured and how the ACME protocol makes use of them.

6.1.1.  Registration Objects

   An  Directory

   In order to help clients configure themselves with the right URIs for
   each ACME registration resource represents operation, ACME servers provide a set of metadata associated directory object.  This
   should be the only URL needed to an account key pair.  Registration resources have configure clients.  It is a JSON
   dictionary, whose keys are drawn from the following
   structure:

   key (required, dictionary):  The public key of table and whose
   values are the account key pair,
      encoded as a JSON Web corresponding URLs.

                   +-------------+--------------------+
                   | Key object [RFC7517].

   contact (optional, array of string):  An array         | URL in value       |
                   +-------------+--------------------+
                   | new-reg     | New registration   |
                   |             |                    |
                   | new-app     | New application    |
                   |             |                    |
                   | revoke-cert | Revoke certificate |
                   |             |                    |
                   | key-change  | Key change         |
                   +-------------+--------------------+

   There is no constraint on the actual URI of URIs the directory except that
   it should be different from the other ACME server can use to contact the client for issues related to this
      authorization. resources' URIs,
   and that it should not clash with other services.  For example, the instance:

   o  a host which function as both an ACME and Web server may wish want to notify
      keep the
      client about server-initiated revocation.

   agreement (optional, string):  A URI referring to a subscriber
      agreement or terms of service provided by 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 (see below).
      Including this field indicates could place 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.
      directory under path "/".

   The result of the GET request dictionary MAY additionally contain a key "meta".  If present, it
   MUST be a JSON object
      whose "authorizations" field is an array of strings, where dictionary; each
      string is item in the URI of dictionary is an authorization belonging item of
   metadata relating to this
      registration. the service provided by the ACME server.

   The server SHOULD include pending authorizations,
      and SHOULD NOT include authorizations that following metadata items 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, defined, all of which are OPTIONAL:

   "terms-of-service" (optional, string):  A URI from which a list identifying the current
      terms of
      certificates issued for this account can be fetched via service.

   "website" (optional, string)):  An HTTP or HTTPS URL locating a GET
      request.  The result of
      website providing more information about the GET request MUST be a JSON object
      whose "certificates" field is an ACME server.

   "caa-identities" (optional, array of strings, where each string):  Each string is the URI of MUST be a certificate.  The server SHOULD NOT include
      expired or revoked certificates.  The
      lowercase hostname which the ACME server MAY return an
      incomplete list, along with a Link header with link relation
      "next" indicating a URL recognises as referring
      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 itself for an
   account to represent an identifier.  In addition to the identifier,
   an authorization includes several metadata fields, such purposes of CAA record validation as defined in
      [RFC6844].  This allows clients to determine the status
   of correct issuer
      domain name to use when configuring CAA record.

   Clients access the authorization (e.g., "pending", "valid", or "revoked") and
   which challenges were used directory by sending a GET request to validate possession of the identifier.

   The structure of an
   directory URI.

   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "new-reg": "https://example.com/acme/new-reg",
     "new-app": "https://example.com/acme/new-app",
     "revoke-cert": "https://example.com/acme/revoke-cert",
     "key-change": "https://example.com/acme/key-change",
     "meta": {
       "terms-of-service": "https://example.com/acme/terms",
       "website": "https://www.example.com/",
       "caa-identities": ["example.com"]
     }
   }

6.1.2.  Registration Objects

   An ACME authorization registration resource is as follows:

   identifier (required, dictionary represents a set of string):  The identifier that the
      account is authorized metadata associated
   to represent

      type an account key pair.  Registration resources have the following
   structure:

   key (required, string): dictionary):  The type public key of identifier.

      value (required, string):  The identifier itself. the account key pair,
      encoded as a JSON Web Key object [RFC7517].

   status (required, string):  The status  "good" or "deactivated"

   contact (optional, array of string):  An array of URIs that the
      server can use to contact the client for issues related to this
      authorization.
      Possible values are: "unknown", "pending", "processing", "valid",
      "invalid" and "revoked".  If this field is missing, then  For example, the
      default value is "pending".

   expires server may wish to notify the
      client about server-initiated revocation.

   agreement (optional, string):  The timestamp after which  A URI referring to a subscriber
      agreement or terms of service provided by the server
      will consider (see below).
      Including this authorization invalid, encoded in the format
      specified in RFC 3339 [RFC3339].  This field is REQUIRED for
      objects indicates the client's agreement with "valid" in the "status field.

   challenges
      referenced terms.

   applications (required, array): string):  A URI from which a list of
      authorizations submitted by this account can be fetched via a GET
      request.  The challenges that the client needs
      to fulfill in order to prove possession result of the identifier (for
      pending authorizations).  For final authorizations, the challenges
      that were used.  Each array entry is GET request MUST be a dictionary with parameters
      required to validate the challenge, as specified in Section 7.

   combinations (optional, JSON object
      whose "applications" field is an array of arrays of integers):  A collection
      of sets of challenges, strings, where each of which would be sufficient to prove
      possession of
      string is the identifier.  Clients complete a set URI 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. belonging to this
      registration.  The only type server SHOULD include pending applications, and
      SHOULD NOT include applications that are invalid.  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 identifier defined by
      certificates issued for this specification is account can be fetched via a fully-
   qualified domain name (type: "dns"). GET
      request.  The value result of the identifier GET request MUST be the ASCII representation a JSON object
      whose "certificates" field is an array of strings, where each
      string is the domain name.  Wildcard domain
   names (with "*" as the first label) MUST URI of a certificate.  The server SHOULD 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": 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.

   {
       "type": "dns",
       "value": "example.org"
     },

     "challenges":
     "contact": [
       {
         "type": "http-01",
         "status": "valid",
         "validated": "2014-12-01T12:05:00Z",
         "keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE"
       }
       "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"
   }

6.2.  Directory

   In order to help clients configure themselves with the right URIs for
   each ACME operation,

6.1.3.  Application Objects

   An ACME servers provide registration resource represents a directory object.  This
   should be the only URL needed to configure clients.  It is client's request for a JSON
   dictionary, whose keys are the "resource" values listed in
   Section 5.1,
   certificate, and whose values are the URIs is used to accomplish the
   corresponding function.

   There is no constraint on track the actual URI progress of the directory except that
   it should be different from application
   through to issuance.  Thus, the other ACME server resources' URIs, object contains information about the
   requested certificate, the server's requirements, and any
   certificates that it should not clash with other services.  For instance:

   o  a host which function as both an ACME have resulted from this application.

   status (required, string):  The status of this authorization.
      Possible values are: "unknown", "pending", "processing", "valid",
      and Web "invalid".

   expires (optional, string):  The timestamp after which the server may want to
      keep
      will consider this application invalid, encoded in the root path "/" format
      specified in RFC 3339 [RFC3339].  This field is REQUIRED for an HTML "front page", and and place
      objects with "pending" or "valid" in the
      ACME directory under path "/acme".

   o  a host which only functions as an ACME server could place status field.

   csr (required, string):  A CSR encoding the
      directory under path "/". parameters for the
      certificate being requested [RFC2986].  The dictionary MAY additionally contain a key "meta".  If present, it
   MUST be a JSON dictionary; each item CSR is sent in the dictionary is an item
      Base64url-encoded version of
   metadata relating to the service provided by DER format.  (Note: This field
      uses the ACME server.

   The following metadata items are defined, all of which are OPTIONAL:

   "terms-of-service" same modified Base64 encoding rules used elsewhere in
      this document, so it is different from PEM.)

   notBefore (optional, string):  A URI identifying the current
      terms  The requested value of service.

   "website" (optional, string)):  An HTTP or HTTPS URL locating a
      website providing more information about the ACME server.

   "caa-identities" notBefore
      field in the certificate, in the date format defined in [RFC3339]

   notAfter (optional, array of string):  Each string MUST be a
      lowercase hostname which  The requested value of the ACME server recognises as referring
      to itself for notAfter
      field in the purposes of CAA record validation as certificate, in the date format defined in
      [RFC6844].  This allows clients to determine [RFC3339]

   requirements (required, array):  The requirements that the correct issuer
      domain name client
      needs to use when configuring CAA record.

   Clients access fulfill before the directory by sending a GET request to requested certificate can be granted
      (for pending applications).  For final applications, the
   directory URI.

   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "new-reg": "https://example.com/acme/new-reg",
     "new-authz": "https://example.com/acme/new-authz",
     "new-cert": "https://example.com/acme/new-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

   A client creates
      requirements that were met.  Each entry is a new account dictionary with
      parameters describing the server by sending a POST
   request to the server's new-registration URI.  The body of the
   request is a stub registration object containing only the "contact"
   field (along with requirement (see below).

   certificate (optional, string):  A URL for the required "resource" field).

   POST /acme/new-registration HTTP/1.1
   Host: example.com certificate that has
      been issued in response to this application.

   {
     "resource": "new-reg",
     "contact":
     "status": "pending",
     "expires": "2015-03-01T14:09:00Z",

     "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "requirements": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ],
       {
         "type": "authorization",
         "status": "valid",
         "url": "https://example.com/acme/authz/1234"
       },
       {
         "type": "out-of-band",
         "status": "pending",
         "url": "https://example.com/acme/payment/1234"
       }
   /* Signed as JWS */

   The server MUST ignore any values provided in
     ]

     "certificate": "https://example.com/acme/cert/1234"
   }

   [[ Open issue: There are two possible behaviors for the "key",
   "authorizations", and "certificates" fields in registration bodies
   sent by CA here.
   Either (a) the client, as well as any other fields CA automatically issues once all the requirements are
   fulfilled, or (b) the CA waits for confirmation from the client that
   it does not
   recognize. should issue.  If new fields are specified we allow both, we will need a signal in the future, the
   specification
   application object of those fields MUST describe whether they may confirmation is required.  I would
   prefer that auto-issue be
   provided by the client.

   The server creates default, which would imply a registration object with syntax
   like "confirm": true ]]

   [[ Open issue: Should this syntax allow multiple certificates?  That
   would support reissuance / renewal in a straightforward way,
   especially if the included contact
   information. CSR / notBefore / notAfter could be updated. ]]

   The "key" element elements of the registration is set to "requirements" array are immutable once set,
   except for their "status" fields.  If any other part of the
   public key used to verify object
   changes after the JWS (i.e., object is created, the "jwk" element of client MUST consider the JWS
   header).
   application invalid.

   The server returns this registration object "requirements" array in a 201
   (Created) response, with the registration URI in a Location header
   field.  The server challenge SHOULD also indicate its new-authorization URI
   using reflect everything
   that the "next" link relation.

   If the server already has a registration object with the provided
   account key, then it MUST return a 409 (Conflict) response and
   provide CA required the URI of that registration client to do before issuance, even if some
   requirements were fulfilled in earlier applications.  For example, if
   a Location header field.
   This CA allows multiple applications to be fufilled based on a client single
   authorization transaction, then it must reflect that has an account key but not authorization in
   all of the
   corresponding registration URI to recover applications.

   Each entry in the registration URI.

   If "requirements" array expresses a requirement from
   the server wishes to present CA for the client to takek a particular action.  All requirements
   objects have the following basic fields:

   type (required, string):  The type of requirement (see below for
      defined types)

   status (required, string):  The status of this requirement.  Possible
      values are: "pending", "valid", and "invalid".

   All additional fields are specified by the requirement type.

6.1.3.1.  Authorization Requirement

   A requirement with terms under which type "authorization" requests that the ACME service is to be used, it MUST indicate client
   complete an authorization transaction.  The server specifies the
   authorization by pre-provisioning a pending authorization resource
   and providing the URI where such terms
   can be accessed for this resource in a Link header with link relation "terms-of-
   service".  As noted above, the requirement.

   url (required, string):  The URL for the authorization resource

   To fulfill this requirement, the ACME client may indicate its agreement with
   these terms by updating its registration to include should fetch the "agreement"
   field, with
   authorization object from the terms URI indicated URL, then follow the process
   for obtaining authorization as its value.  When these terms change specified in a
   way Section 6.4.

6.1.3.2.  Out-of-Band Requirement

   A requirement with type "out-of-band" requests that requires an agreement update, the server MUST use ACME client
   have a
   different URI in the Link header.

   HTTP/1.1 201 Created
   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/terms>;rel="terms-of-service"
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "key": { /* JWK from JWS header */ },

     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ]
   }

   If the client wishes to update this information human user visit a web page in order to receive further
   instructions for how to fulfill the future, it
   sends requirement.  The requirement
   object provides a POST request with updated information to URI for the registration
   URI. web page to be visited.

   url (required, string):  The server MUST ignore any updates URL to the "key",
   "authorizations, or "certificates" fields, and be visited.  The scheme of this
      URL MUST verify that be "http" or "https"

   To fulfill this requirement, the
   request is signed with ACME client should direct the private key corresponding user
   to the "key"
   field of the request before updating the registration.

   For example, indicated web page.

6.1.4.  Authorization Objects

   An ACME authorization object represents server's authorization for an
   account to represent an identifier.  In addition to update the contact information in identifier,
   an authorization includes several metadata fields, such as the above
   registration, status
   of the client could send authorization (e.g., "pending", "valid", or "revoked") and
   which challenges were used to validate possession of the following request:

   POST /acme/reg/asdf HTTP/1.1
   Host: example.com

   {
     "resource": "reg",
     "contact": [
       "mailto:certificates@example.com",
       "tel:+12125551212"
     ],
   }
   /* Signed identifier.

   The structure of an ACME authorization resource is as JWS */

   Servers SHOULD NOT respond follows:

   identifier (required, dictionary of string):  The identifier that the
      account is authorized to GET requests for registration resources
   as these requests are not authenticated. 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 a client wishes to query 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 information about its account (e.g., to examine
      objects with "valid" in the
   "contact" or "certificates" fields), "status field.

   scope (optional, string):  If this field is present, then it SHOULD do so by sending
   a POST request with an empty update.  That is, it should send MUST
      contain a JWS
   whose payload URI for an application resource, such that this
      authorization is trivial ({"resource":"reg"}).  In only valid for that resource.  If this case field is
      absent, then the
   server reply CA MUST contain the same link headers sent consider this authorization valid for a new
   registration, to allow a client to retrieve all
      applications until the authorization expires. [[ Open issue: More
      flexible scoping? ]]

   challenges (required, array):  The challenges that the "new-authorization"
   and "terms-of-service" URI

6.3.1.  Account Key Roll-over

   A client may wish needs
      to change fulfill in order to prove possession of the public key identifier (for
      pending authorizations).  For final authorizations, the challenges
      that were used.  Each array entry is associated with a
   registration, e.g., in order dictionary with parameters
      required to mitigate validate the risk 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 key compromise.
   To do this, the client first constructs a JSON object representing identifier.  Clients complete a
   request to update set of
      challenges that covers at least one set in this array.  Challenges
      are identified by their indices in the registration:

   resource (required, string):  The string "reg", indicating challenges array.  If no
      "combinations" element is included in an update
      to authorization object, the registration.

   oldKey (required, string):
      client completes all challenges.

   The JWK thumbprint only type of identifier defined by this specification is a fully-
   qualified domain name (type: "dns").  The value of the old key
      [RFC7638], base64url-encoded 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.

   {
     "status": "valid",
     "expires": "2015-03-01T14:09:00Z",

     "identifier": {
       "type": "dns",
       "value": "example.org"
     },

     "challenges": [
       {
     "resource": "reg",
     "oldKey": "D7J9RL1f-RWUl68JP-gW1KSl2TkIrJB7hK6rLFFeYMU"
         "type": "http-01",
         "status": "valid",
         "validated": "2014-12-01T12:05:00Z",
         "keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE"
       }

   The
     ]
   }

6.2.  Registration

   A client signs this object with the creates a new key pair and encodes account with the
   object and signature as server by sending a JWS.  The client then sends this JWS POST
   request to the
   server in the "newKey" field server's new-registration URI.  The body of a the
   request to update is a stub registration object containing only the registration. "contact"
   field.

   POST /acme/reg/asdf /acme/new-reg HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "resource": "reg",
     "newKey": /* JSON object signed as JWS with new key */
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "6S8IqOGY7eL2lsGoTZYifg",
       "url": "https://example.com/acme/new-reg"
     })
     "payload": base64url({
       "contact": [
         "mailto:cert-admin@example.com",
         "tel:+12025551212"
       ]
     }),
     "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I"
   }
   /* Signed as JWS with original key */

   On receiving a request to the registration URL with the "newKey"
   attribute set, the

   The server MUST perform the following steps:

   1.  Check that ignore any values provided in the contents of "key",
   "authorizations", and "certificates" fields in registration bodies
   sent by the "newKey" attribute are a valid JWS

   2.  Check client, as well as any other fields that the "newKey" JWS verifies using the key it does not
   recognize.  If new fields are specified in the "jwk"
       header parameter of the JWS

   3.  Check that future, the payload
   specification of those fields MUST describe whether they may be
   provided by the JWS is client.

   The server creates a valid JSON object

   4.  Check that the "resource" field of the object has the value "reg"

   5.  Check that the "oldKey" field of the registration object contains the JWK
       thumbprint of with the account key for this registration

   If all included contact
   information.  The "key" element of these checks pass, then the server updates the registration
   by replacing the old account key with is set to the
   public key carried in used to verify the JWS (i.e., the "jwk" header parameter element of the "newKey" JWS object.

   If the update was successful, then the
   header).  The server sends returns this registration object in a response 201
   (Created) response, with
   status code 200 (OK) and the updated registration object as its body. URI in a Location header
   field.

   If the update was not successful, then the server responds already has a registration object with an
   error status code and the provided
   account key, then it MUST return a problem document describing 409 (Conflict) response and
   provide the error.

6.3.2.  Deleting an Account

   If URI of that registration in a Location header field.
   This allows a client no longer wishes to have that has an account key registered with
   the server, it may request that the server delete its account by
   sending a POST request to but not the account
   corresponding registration URI containing to recover the "delete"
   field.

   delete (required, boolean): The boolean value "true".

   The request object MUST contain registration URI.

   If the "resource" field as required
   above (with server wishes to present the value "reg").  It MUST NOT contain any fields besides
   "resource" and "delete".

   Note that although this object is very simple, client with terms under which the risk of replay or
   fraudulent generation via signing oracles
   ACME service is mitigated by to be used, it MUST indicate the need
   for an anti-replay token URI where such terms
   can be accessed in the protected a Link header of with link relation "terms-of-
   service".  As noted above, the JWS.

   POST /acme/reg/asdf HTTP/1.1
   Host: example.com

   {
     "resource": "reg",
     "delete": true,
   }
   /* Signed as JWS */

   On receiving a POST client may indicate its agreement with
   these terms by updating its registration to an account URI containing a "delete" field,
   the server MUST verify that no other fields were sent in the object
   (other than "resource"), and it MUST verify that include the value of "agreement"
   field, with the
   "delete" field is "true" (as a boolean, not a string).  If either of terms URI as its value.  When these checks fails, then terms change in a
   way that requires an agreement update, the server MUST reject use a
   different URI in the request with
   status code 400 (Bad Request). Link header.

   HTTP/1.1 201 Created
   Content-Type: application/json
   Location: https://example.com/acme/reg/asdf
   Link: <https://example.com/acme/terms>;rel="terms-of-service"
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "key": { /* JWK from JWS header */ },
     "status": "good",

     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ]
   }

   If the server accepts client wishes to update this information in the deletion request, then future, it MUST delete the
   account and all related objects and send
   sends a response POST request with a 200 (OK)
   status code and an empty body.  The server SHOULD delete any
   authorization objects related updated information to the deleted account, since they can
   no longer be used. registration
   URI.  The server SHOULD NOT delete certificate objects
   related MUST ignore any updates to the account, since certificates issued under the account
   continue to be valid until they expire "key",
   "authorizations, or are revoked.

6.4.  Identifier Authorization

   The identifier authorization process establishes the authorization of
   an account to manage certificates for a given identifier.  This
   process must assure the server of two things: First, "certificates" fields, and MUST verify that the client
   controls
   request is signed with the private key of the account key pair, and second, that corresponding to the client holds "key"
   field of the identifier in question.  This process may be
   repeated to associate multiple identifiers to a key pair (e.g., to request certificates with multiple identifiers), or to associate
   multiple accounts with an identifier (e.g., to allow multiple
   entities before updating the registration.

   For example, to manage certificates).

   As illustrated by update the figure contact information in the overview section above, above
   registration, the
   authorization process proceeds in two phases.  The client first could send the following request:

   POST /acme/reg/asdf HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "ax5RnthDqp_Yf4_HZnFLmA",
       "url": "https://example.com/acme/reg/asdf"
     })
     "payload": base64url({
       "contact": [
         "mailto:certificates@example.com",
         "tel:+12125551212"
       ]
     }),
     "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o"
   }

   Servers SHOULD NOT respond to GET requests for registration resources
   as these requests are not authenticated.  If a new authorization, and client wishes to query
   the server issues challenges, then 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 ({}).

6.2.1.  Account Key Roll-over

   A client responds may wish to those challenges and change the server validates public key that is associated with a
   registration in order to recover from a key compromise or proactively
   mitigate the
   client's responses. impact of an unnoticed key compromise.

   To begin change the key authorization process, associate with an account, the client sends a POST
   request to containing a key-change object with the server's new-authorization resource. following fields:

   oldKey (required, JWK):  The body JWK representation of the
   POST request MUST contain a JWS object, whose payload is a partial
   authorization object.  This JWS object MUST contain only the
   "identifier" field, so that original key
      (i.e., the server knows what identifier is being
   authorized. client's current account key)

   newKey (requrired, JWK):  The server MUST ignore any other fields present in JWK representation of the
   client's request object. new key
   The authorization object is implicitly tied JWS of this POST must have two signatures: one signature from the
   existing key on the account, and one signature from the new key that
   the client proposes to use.  This demonstrates that the account client
   actually has control of the private key used corresponding to sign the request.  Once created, new
   public key.  The protected header must contain a JWK field containing
   the authorization may only be
   updated by that account. current account key.

   POST /acme/new-authorization /acme/key-change HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "resource": "new-authz",
     "identifier": {
       "type": "dns",
       "value": "example.org"
     }
   }
     "payload": base64url({
       "oldKey": /* Signed as JWS Old key in JWK form */

   Before processing the authorization further,
       "newKey": /* New key in JWK form */
     }),
     "signatures": [{
       "protected": base64url({
         "alg": "ES256",
         "jwk": /* old key */,
         "nonce": "pq00v-D1KB0sReG4jFfqVg",
         "url": "https://example.com/acme/key-change"
       }),
       "signature": "XFvVbo9diBlIBvhE...UI62sNT6MZsCJpQo"
     }, {
       "protected": base64url({
         "alg": "ES256",
         "jwk": /* new key */,
         "nonce": "vYjyueEYhMjpVQHe_unw4g",
         "url": "https://example.com/acme/key-change"
       }),
       "signature": "q20gG1f1r9cD6tBM...a48h0CkP11tl5Doo"
     }]
   }

   On receiving key-change request, the server SHOULD
   determine whether it is willing to issue certificates for MUST perform the
   identifier.  For example,
   following steps in addition to the server should check typical JWS validation:

   1.  Check that the identifier
   is of JWS protected header container a supported type.  Servers might also check names against "jwk" field
       containing a
   blacklist of known high-value identifiers.  If key that matches a currently active account.

   2.  Check that there are exactly two signatures on the server is
   unwilling to issue for JWS.

   3.  Check that one of the identifier, it SHOULD return a 403
   (Forbidden) error, with a problem document describing signatures validates using the reason for account key
       from (1).

   4.  Check that the rejection.

   If "key" field contains a well-formed JWK that meets
       key strength requirements.

   5.  Check that the server "key" field is willing not equivalent to proceed, it builds a pending
   authorization object from the initial authorization object submitted
   by current
       account key or any other currently active account key.

   6.  Check that one of the client.

   o  "identifier" two signatures on the identifier submitted by JWS validates using
       the client

   o  "status": MUST be "pending" unless JWK from the "key" field.

   If all of these checks pass, then the server has out-of-band
      information about updates the client's authorization status

   o  "challenges" and "combinations": As selected
   corresponding registration by replacing the server's
      policy for this identifier

   The server allocates a new URI for this authorization, old account key with the
   new public key and returns a
   201 (Created) response, status code 200.  Otherwise, the server
   responds with an error status code and a problem document describing
   the authorization URI in error.

6.2.2.  Account deactivation

   A client may deactivate an account by posting a Location
   header field, and signed update to the JSON authorization object in
   server with a status field of "deactivated."  Clients may wish to do
   this when the body. account key is compromised.

   POST /acme/reg/asdf HTTP/1.1 201 Created
   Host: example.com
   Content-Type: application/json
   Location: https://example.com/authz/asdf
   Link: <https://example.com/acme/new-cert>;rel="next"
   Link: <https://example.com/acme/some-directory>;rel="directory" application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "ntuJWWSic4WVNSqeUmshgg",
       "url": "https://example.com/acme/reg/asdf"
     })
     "payload": base64url({
       "status": "pending",

     "identifier": {
       "type": "dns",
       "value": "example.org"
     },

     "challenges": [
       {
         "type": "http-01",
         "uri": "https://example.com/authz/asdf/0",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
       },
       {
         "type": "dns-01",
         "uri": "https://example.com/authz/asdf/1",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       }
     },

     "combinations": [[0], [1]] "deactivated"
     }),
     "signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y"
   }

6.4.1.  Responding to Challenges

   To prove control of the identifer and receive authorization, the
   client needs to respond with information to complete the challenges.
   To do this,

   The server MUST verify that the client updates request is signed by the authorization object received from account key.
   If the server by filling in any required information in accepts the elements deactivation request, it should reply with
   a 200 (OK) status code and the current contents of the "challenges" dictionary.  (This registration
   object.

   Once an account is also deactivated, the stage where the
   client should perform any actions required server MUST NOT accept further
   requests authorized by the challenge.) that account's key.  It is up to server policy
   how long to retain data related to that account, whether to revoke
   certificates issued by that account, and whether to send email to
   that account's contacts.  ACME does not provide a way to reactivate a
   deactivated account.

6.3.  Applying for Certificate Issuance

   The holder of an account key pair may use ACME to submit an
   application for a certificate to be issued.  The client sends these updates back makes this
   request by sending a POST request to the server in the form server's new-application
   resource.  The body of the POST is a JWS object whose JSON payload is
   a subset of the application object with defined in Section 6.1.3,
   containing the response fields required by that describe the challenge type,
   carried in a POST request certificate to be issued:

   csr (required, string):  A CSR encoding the challenge URI (not authorization URI
   or the new-authorization URI).  This allows the client to send
   information only parameters for challenges it the
      certificate being requested [RFC2986].  The CSR is responding to.

   For example, if sent in the client were to respond to
      Base64url-encoded version of the "http-01" challenge
   in DER format.  (Note: This field
      uses the above authorization, same modified Base64 encoding rules used elsewhere in
      this document, so it would send the following request:

   POST /acme/authz/asdf/0 HTTP/1.1
   Host: example.com

   {
     "resource": "challenge",
     "type": "http-01",
     "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
   }
   /* Signed as JWS */ is different from PEM.)

   notBefore (optional, string):  The server updates the authorization document by updating its
   representation requested value of the challenge with the response fields provided by notBefore
      field in the client.  The server MUST ignore any fields certificate, in the response object
   that are not specified as response fields for this type of challenge. date format defined in [RFC3339]

   notAfter (optional, string):  The server provides a 200 (OK) response with requested value of the updated challenge
   object as its body.

   If notAfter
      field in the client's response is invalid for some reason, or does not
   provide certificate, in the date format defined in [RFC3339]

   POST /acme/new-app HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "5XJ1L3lEkMG7tR6pA00clA",
       "url": "https://example.com/acme/new-app"
     })
     "payload": base64url({
       "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
       "notBefore": "2016-01-01T00:00:00Z",
       "notAfter": "2016-01-08T00:00:00Z"
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   The CSR encodes the server client's requests with appropriate information regard to validate the
   challenge, then content of
   the certificate to be issued.  The CSR MUST indicate the requested
   identifiers, either in the commonName portion of the requested
   subject name, or in an extensionRequest attribute [RFC2985]
   requesting a subjectAltName extension.

   The server MUST return an HTTP error.  On receiving
   such an error, error if it cannot fulfil the client SHOULD undo any actions that have been
   taken to fulfill request as
   specified, and MUST NOT issue a certificate with contents other than
   those requested.  If the challenge, e.g., removing files that have been
   provisioned server requires the request to be modified
   in a web server.

   Presumably, certain way, it should indicate the client's responses provide required changes using an
   appropriate error code and description.

   If the server with enough
   information to validate one or more challenges.  The server is said willing to "finalize" the authorization when it has completed all issue the
   validations requested certificate, it
   responds with a 201 (Created) response.  The body of this response is going to complete,
   an application object reflecting the client's request and assigns any
   requirements the authorization client must fulfill before the certificate will be
   issued.

   HTTP/1.1 201 Created
   Location: https://example.com/acme/app/asdf

   {
     "status": "pending",
     "expires": "2015-03-01T14:09:00Z",

     "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "requirements": [
       {
         "type": "authorization",
         "status": "valid",
         "url": "https://example.com/acme/authz/1234"
       },
       {
         "type": "out-of-band",
         "status": "pending",
         "url": "https://example.com/acme/payment/1234"
       }
     ]
   }

   The application object returned by the server represents a promise
   that if the client fulfills the server's requirements before the
   "expires" time, then the server will issue the requested certificate.
   In the application object, any object in the "requirements" array
   whose status of "valid" or "invalid", corresponding to whether it considers is "pending" represents an action that the client must
   perform before the account authorized for server will issue the identifier. certificate.  If the final state is
   "valid", the server MUST add an "expires" field client
   fails to complete the authorization.
   When finalizing an authorization, required actions before the "expires" time,
   then the server SHOULD change the status of the application to
   "invalid" and MAY remove delete the
   "combinations" field (if present) or remove any challenges still
   pending. application resource.

   The server SHOULD NOT remove challenges with status
   "invalid".

   Usually, the validation process will take some time, so issue the client
   will need to poll requested certificate and update the authorization
   application resource to see when it is
   finalized.  For challenges where with a URL for the client can tell when certificate as soon as the server
   client has validated the challenge (e.g., by seeing an HTTP or DNS request
   from fulfilled the server), server's requirements.  If the client SHOULD NOT begin polling until it has
   seen the validation request from
   already satisfied the server.

   To check on server's requirements at the status time of an authorization, the client sends a GET this
   request to the (e.g., by obtaining authorization URI, and the server responds with for all of the
   current authorization object.  In responding to poll requests while identifiers
   in the validation is still certificate in progress, previous transactions), then the server MUST return a 202
   (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
   Host: example.com

   HTTP/1.1 200 OK

   {
     "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",
         "token": "IlirfxKKXAsHtmzK29Pj8A",
         "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
       }
     ]
   }

6.4.2.  Deleting an Authorization

   If a client wishes to relinquish its authorization to
   proactively issue
   certificates the requested certificate and provide a URL for an identifier, then it may request that
   in the server
   delete "certificate" field of the authorization. application.  The server MUST,
   however, still list the satisfied requirements in the "requirements"
   array, with the state "valid".

   Once the client makes this request by sending believes it has fulfilled the server's requirements,
   it should send a
   POST GET request to the authorization URI containing a payload in application resource to obtain
   its current state.  The status of the
   same format as in Section 6.3.2. application will indicate what
   action the client should take:

   o  "invalid": The only difference is certificate will not be issued.  Consider this
      application process abandoned.

   o  "pending": The server does not believe that the
   value of client has
      fulfilled the "resource" field is "authz".

   POST /acme/authz/asdf HTTP/1.1
   Host: example.com

   {
     "resource": "authz",
     "delete": true,
   }
   /* Signed as JWS */ requirements.  Check the "requirements" array for
      requirements that are still pending.

   o  "processing": The server MUST perform agrees that the same validity checks as in Section 6.3.2 requirements have been
      fulfilled, and reject is in the request if they fail.  If process of generating the certificate.
      Retry after the time given in the "Retry-After" header field of
      the response, if any.

   o  "valid": The server deletes has issued the
   account then it MUST send a response with a 200 (OK) status code certificate and
   an empty body.

6.5.  Certificate Issuance

   The holder of an account key pair authorized for one or more
   identifiers may use ACME provisioned its
      URL to request that a certificate be issued for
   any subset the "certificate" field of those identifiers.  The the application.  Download the
      certificate.

6.3.1.  Downloading the Certificate

   To download the issued certificate, the client makes this request by
   sending simply sends a POST GET
   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]. certificate URL.

   The CSR encodes the
   parameters default format of the requested certificate; authority to issue certificate is
   demonstrated by the JWS signature DER (application/pkix-cert).
   The client may request other formats by including an account key, from which the
   server can look up related authorizations.  Some attributes which
   cannot be reflected in a CSR are placed directly Accept header in the certificate
   its request.

   csr (required, string):  A CSR encoding  For example, the parameters for client may use the media type
   application/x-pem-file to request the certificate being requested.  The CSR is sent in the Base64url-
      encoded version of the DER PEM format.  (Note: This field uses

   The server provides metadata about the
      same modified Base64 encoding rules used elsewhere certificate in HTTP headers.
   In particular, the server MUST send one or more link relation header
   fields [RFC5988] with relation "up", each indicating a single
   certificate resource for the issuer of this
      document, so it is different from PEM.)

   notBefore (optional, string): certificate.  The requested value of server
   MAY also include the notBefore "up" links from these resources to enable the
   client to build a full certificate chain.

   The server MUST also provide a link relation header field in with
   relation "author" to indicate the certificate, in application under which this
   certificate was issued.

   If the date format defined CA participates in [RFC3339]

   notAfter (optional, string):  The requested value of Certificate Transparency (CT) [RFC6962],
   then they may want to provide the notAfter
      field 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, in
   certificate or as an extension to OCSP responses for the date format defined in [RFC3339]

   POST /acme/new-cert 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
   Host: example.com
   Accept: application/pkix-cert

   {
     "resource": "new-cert",
     "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z"
   }
   /* Signed as JWS */

   The CSR encodes

   HTTP/1.1 200 OK
   Content-Type: application/pkix-cert
   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/app/asdf>;rel="author"
   Link: <https://example.com/acme/sct/asdf>;rel="ct-sct"
   Link: <https://example.com/acme/some-directory>;rel="directory"

   [DER-encoded certificate]

   A certificate resource represents a single, immutable certificate.
   If the client's requests with regard client wishes to obtain a renewed certificate, the content of
   the certificate client
   initiates a new application process to be issued.  The CSR MUST indicate request one.

   Because certificate resources are immutable once issuance is
   complete, the requested
   identifiers, either in server MAY enable the commonName portion caching of the requested
   subject name, or in an extensionRequest attribute [RFC2985]
   requesting resource by adding
   Expires and Cache-Control headers specifying a subjectAltName extension.

   The values provided point in time in the CSR are only a request, and are not
   guaranteed.
   distant future.  These headers have no relation to the certificate's
   period of validity.

6.4.  Identifier Authorization

   The server SHOULD return identifier authorization process establishes the authorization of
   an error if it cannot fulfil account to manage certificates for a given identifier.  This
   process must assure the request as specified, but MAY issue server of two things: First, that the client
   controls the private key of the account key pair, and second, that
   the client holds the identifier in question.  This process may be
   repeated to associate multiple identifiers to a certificate key pair (e.g., to
   request certificates with contents
   other than those requested, according multiple identifiers), or to its local policy associate
   multiple accounts with an identifier (e.g.,
   removing identifiers for which the client is not authorized).

   It to allow multiple
   entities to manage certificates).  The server may declare that an
   authorization is up to only valid for a specific application by setting the server's local policy
   "scope" field of the authorization to decide which names are
   acceptable in a certificate, given the authorizations URI for that application.

   Authorization resources are created by the server
   associates with the client's in response to
   certificate applications submitted by an account key.  A server MAY consider a key holder; their
   URLs are provided to the client authorized for a wildcard domain if it in "authorization" requirement
   objects.  The authorization object is authorized for implicitly tied to the
   underlying domain name (without account
   key used to sign the "*" label).  Servers SHOULD NOT
   extend authorization across identifier types.  For example, if new-application request.

   When a client is authorized for "example.com", then receives an application from the server should not
   allow the client to issue a certificate with an iPAddress
   subjectAltName, even if
   "authorization" requirement, it contains an IP address downloads the authorization resource
   by sending a GET request to which
   example.com resolves.

   If the CA decides indicated URL.

   GET /acme/authz/1234 HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "status": "pending",

     "identifier": {
       "type": "dns",
       "value": "example.org"
     },

     "challenges": [
       {
         "type": "http-01",
         "uri": "https://example.com/authz/asdf/0",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
       },
       {
         "type": "dns-01",
         "uri": "https://example.com/authz/asdf/1",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       }
     ],

     "combinations": [[0], [1]]
   }

6.4.1.  Responding to issue a certificate, then Challenges

   To prove control of the server creates a
   new certificate resource identifier and returns a URI for it in receive authorization, the Location
   header field of a 201 (Created) response.

   HTTP/1.1 201 Created
   Location: https://example.com/acme/cert/asdf

   If
   client needs to respond with information to complete the certificate is available at challenges.
   To do this, the time of client updates the response, it is
   provided authorization object received from
   the server by filling in any required information in the body of elements of
   the "challenges" dictionary.  (This is also the stage where the response.  If
   client should perform any actions required by the CA has not yet issued challenge.)

   The client sends these updates back to the certificate, server in the body form of this a
   JSON object with the response will be empty.  The client
   should then send fields required by the challenge type,
   carried in a GET POST request to the certificate challenge URI (not authorization
   URI).  This allows the client to poll send information only for the
   certificate.  As long as the certificate challenges
   it is unavailable, responding to.

   For example, if the server
   MUST provide a 202 (Accepted) response and include a Retry-After
   header client were to respond to indicate when the server believes the certificate will be
   issued (as "http-01" challenge
   in the example above).

   GET /acme/cert/asdf above authorization, it would send the following request:

   POST /acme/authz/asdf/0 HTTP/1.1
   Host: example.com
   Accept: application/pkix-cert

   HTTP/1.1 202 Accepted
   Retry-After: 120
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
       "url": "https://example.com/acme/authz/asdf/0"
     })
     "payload": base64url({
       "type": "http-01",
       "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
     }),
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
   }

   The default format server updates the authorization document by updating its
   representation of the certificate is DER (application/pkix-cert).
   The client may request other formats challenge with the response fields provided by including an Accept header
   the client.  The server MUST ignore any fields in
   its request. the response object
   that are not specified as response fields for this type of challenge.
   The server provides metadata about a 200 (OK) response with the updated challenge
   object as its body.

   If the client's response is invalid for some reason, or does not
   provide the server with appropriate information to validate the
   challenge, then the server MUST return an HTTP error.  On receiving
   such an error, the client SHOULD undo any actions that have been
   taken to fulfill the challenge, e.g., removing files that have been
   provisioned to a web server.

   Presumably, the certificate in HTTP headers.
   In particular, client's responses provide the server MUST include a Link relation header field
   [RFC5988] with relation "up" enough
   information to provide a certificate under which
   this certificate was issued, and validate one with relation "author" to
   indicate the registration under which this certificate was issued. or more challenges.  The server MAY include an Expires header as a hint is said
   to "finalize" the client
   about authorization when to renew it has completed all the certificate.  (Of course,
   validations it is going to complete, and assigns the real expiration authorization a
   status of "valid" or "invalid", corresponding to whether it considers
   the certificate is controlled by the notAfter time in account authorized for the
   certificate itself.) identifier.  If the CA participates in Certificate Transparency (CT) [RFC6962],
   then they may want to provide final state is
   "valid", 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 server MUST add an extension in "expires" field to the
   certificate or as authorization.
   When finalizing an extension to OCSP responses for authorization, the certificate. server MAY remove the
   "combinations" field (if present) or remove any challenges still
   pending.  The server can also provide SHOULD NOT remove challenges with status
   "invalid".

   Usually, the validation process will take some time, so the client with direct access
   will need to an SCT
   for a certificate using a Link relation header field with relation
   "ct-sct".

   GET /acme/cert/asdf HTTP/1.1
   Host: example.com
   Accept: application/pkix-cert

   HTTP/1.1 200 OK
   Content-Type: application/pkix-cert
   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/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
   Content-Location: https://example.com/acme/cert-seq/12345

   [DER-encoded certificate]

   A certificate poll the authorization resource always represents to see when it is
   finalized.  For challenges where the most recent certificate
   issued for client can tell when the name/key binding expressed in server
   has validated the CSR.  If challenge (e.g., by seeing an HTTP or DNS request
   from the CA
   allows a certificate to be renewed, then server), the client SHOULD NOT begin polling until it publishes renewed
   versions of has
   seen the certificate through validation request from the same certificate URI.

   Clients retrieve renewed versions server.

   To check on the status of an authorization, the certificate using client sends a GET
   query
   request to the certificate authorization URI, which and the server should then return in
   a 200 (OK) response.  The server SHOULD provide a stable URI for each
   specific certificate in the Content-Location header field, as shown
   above.  Requests to stable certificate URIs MUST always result in the
   same certificate.

   To avoid unnecessary renewals, responds with the CA may choose not to issue a
   renewed certificate until it receives such a request (if it even
   allows renewal at all).
   current authorization object.  In such cases, if the CA requires some time responding to generate poll requests while
   the new certificate, validation is still in progress, the CA server MUST return a 202
   (Accepted) response, with and MAY include a Retry-After header field that indicates when to
   suggest a polling interval to the new
   certificate will be available.  The CA MAY include client.

   GET /acme/authz/asdf HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK

   {
     "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",
         "token": "IlirfxKKXAsHtmzK29Pj8A",
         "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
       }
     ]
   }

6.4.2.  Deactivating an Authorization

   If a client wishes to relinquish its authorization to issue
   certificates for an identifier, then it may request that the current (non-
   renewed) certificate as server
   deactivate each authorization associated with that identifier by
   sending a POST request with the body of static object {"status":
   "deactivated"}.

   POST /acme/authz/asdf HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "xWCM9lGbIyCgue8di6ueWQ",
       "url": "https://example.com/acme/authz/asdf"
     })
     "payload": base64url({
       "status": "deactivated"
     }),
     "signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4"
   }

   The server MUST verify that the response.

   Likewise, in order to prevent unnecessary renewal due to queries request is signed by
   parties other than the account key holder, certificate URIs should be
   structured as capability URLs [W3C.WD-capability-urls-20140218].

   From
   corresponding to the client's perspective, there is no difference between a
   certificate URI that allows renewal and one account that does not. owns the authorization.  If the
   client wishes to obtain
   server accepts the deactivation, it should reply with a renewed certificate, 200 (OK)
   status code and a GET request to the certificate URI does not yield one, then current contents of the client may initiate
   a new-certificate transaction to request one.

6.6. registration object.

   The server MUST NOT treat deactivated authorization objects as
   sufficient for issuing certificates.

6.5.  Certificate Revocation

   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
   is a JWS object whose JSON payload contains the certificate to be
   revoked:

   certificate (required, string):  The certificate to be revoked, in
      the base64url-encoded version of the DER format.  (Note: This
      field uses the same modified Base64 encoding rules used elsewhere
      in this document, so it is different from PEM.)

   reason (optional, int):  One of the revocation reasonCodes defined in
      RFC 5280 [RFC5280] Section 5.3.1 to be used when generating OCSP
      responses and CRLs.  If this field is not set the server SHOULD
      use the unspecified (0) reasonCode value when generating OCSP
      responses and CRLs.  The server MAY disallow a subset of
      reasonCodes from being used by the user.

   POST /acme/revoke-cert HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "resource": "revoke-cert",
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/revoke-cert"
     })
     "payload": base64url({
       "certificate": "MIIEDTCCAvegAwIBAgIRAP8..." "MIIEDTCCAvegAwIBAgIRAP8...",
       "reason": 1
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }
   /* Signed as JWS */

   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
   the certificate.  Before revoking a certificate, the server MUST
   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 in the certificate.

   o  an account key that is authorized to act for all of the
      identifier(s) in the certificate.

   If the revocation succeeds, the server responds with status code 200
   (OK).  If the revocation fails, the server returns an error.

   HTTP/1.1 200 OK
   Content-Length: 0

   --- or ---

   HTTP/1.1 403 Forbidden
   Content-Type: application/problem+json
   Content-Language: en

   {
     "type": "urn:ietf:params:acme:error:unauthorized"
     "detail": "No authorization provided for name example.net"
     "instance": "http://example.com/doc/unauthorized"
   }

7.  Identifier Validation Challenges

   There are few types of identifiers in the world for which there is a
   standardized mechanism to prove possession of a given identifier.  In
   all practical cases, CAs rely on a variety of means to test whether
   an entity applying for a certificate with a given identifier actually
   controls that identifier.

   Challenges provide the server with assurance that an account key
   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
   successfully complete the challenge the entity must both:

   o  Hold the private key of the account key pair used to respond to
      the challenge

   o  Control the identifier in question

   Section 10 9 documents how the challenges defined in this document meet
   these requirements.  New challenges will need to document how they
   do.

   ACME uses an extensible challenge/response framework for identifier
   validation.  The server presents a set of challenge challenges in the
   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.

   This section describes an initial set of challenge types.  Each
   challenge must describe:

   1.  Content of challenge objects

   2.  Content of response objects

   3.  How the server uses the challenge and response to verify control
       of an identifier

   Challenge objects all contain the following basic fields:

   type (required, string):  The type of challenge encoded in the
      object.

   uri (required, string):  The URI to which a response can be posted.

   status (required, string):  The status of this authorization.
      Possible values are: "pending", "valid", and "invalid".  If this
      field is missing, then the default value is "pending".

   validated (optional, string):  The time at which this challenge was
      completed by the server, encoded in the format specified in RFC
      3339 [RFC3339].  This field is REQUIRED if the "status" field is
      "valid".

   error (optional, dictionary of string):  The error that occurred
      while the server was validating the challenge, if any.  This field
      is structured as a problem document
      [I-D.ietf-appsawg-http-problem]. [RFC7807].

   All additional fields are specified by the challenge type.  If the
   server sets a challenge's "status" to "invalid", it SHOULD also
   include the "error" field to help the client diagnose why they failed
   the challenge.

   Different challenges allow the server to obtain proof of different
   aspects of control over an identifier.  In some challenges, like HTTP
   and TLS SNI, the client directly proves its ability to do certain
   things related to the identifier.  The choice of which challenges to
   offer to a client under which circumstances is a matter of server
   policy.

   The identifier validation challenges described in this section all
   relate to validation of domain names.  If ACME is extended in the
   future to support other types of identifier, there will need to be
   new challenge types, and they will need to specify which types of
   identifier they apply to.

   [[ Editor's Note: In pre-RFC versions of this specification,
   challenges are labeled by type, and with the version of the draft in
   which they were introduced.  For example, if an HTTP challenge were
   introduced in version -03 and a breaking change made in version -05,
   then there would be a challenge labeled "http-03" and one labeled
   "http-05" - but not one labeled "http-04", since challenge in version
   -04 was compatible with one in version -04. ]]

   [[ Editor's Note: Operators SHOULD NOT issue "combinations" arrays in
   authorization objects that require the client to perform multiple
   challenges over the same type, e.g., ["http-03", "http-05"].
   Challenges within a type are testing the same capability of the
   domain owner, and it may not be possible to satisfy both at once. ]]

7.1.  Key Authorizations

   Several of the challenges in this document makes use of a key
   authorization string.  A key authorization is a string that expresses
   a domain holder's authorization for a specified key to satisfy a
   specified challenge, by concatenating the token for the challenge
   with a key fingerprint, separated by a "." character:

   key-authz = token || '.' || base64url(JWK\_Thumbprint(accountKey))

   The "JWK_Thumbprint" step indicates the computation specified in
   [RFC7638], using the SHA-256 digest.  As specified in the individual
   challenges below, the token for a challenge is a JSON string
   comprised entirely of characters in the URL-safe Base64 alphabet.
   The "||" operator indicates concatenation of strings.

   In computations involving key authorizations, such as the digest
   computations required for the DNS and TLS SNI challenges, the key
   authorization string MUST be represented in UTF-8 form (or,
   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

   With HTTP validation, the client in an ACME transaction proves its
   control over a domain name by proving that it can provision resources
   on an HTTP server that responds for that domain name.  The ACME
   server challenges the client to provision a file at a specific path,
   with a specific string as its content.

   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
   records.
   records, at its discretion.  Because many web servers webservers allocate a
   default HTTPS virtual host to a particular low-privilege tenant user
   in a subtle and non-
   intuitive non-intuitive manner, the challenge must be completed
   over HTTP, not HTTPS.

   type (required, string):  The string "http-01"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the URL-safe Base64 alphabet and
      MUST NOT contain any padding characters ("=").

   {
     "type": "http-01",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA", "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }
   A client responds to this challenge by constructing a key
   authorization from the "token" value provided in the challenge and
   the client's account key.  The client then provisions the key
   authorization as a resource on the HTTP server for the domain in
   question.

   The path at which the resource is provisioned is comprised of the
   fixed prefix ".well-known/acme-challenge/", followed by the "token"
   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

   The client's response to this challenge indicates its agreement to
   this challenge by sending the server the key authorization covering
   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
      challenge.  This value MUST match the token from the challenge and
      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.

   /* BEGIN JWS-signed content */
   {
     "keyAuthorization": "evaGxfADs...62jcerQ"
   }
   /* Signed as JWS END JWS-signed content */

   On receiving a response, the server MUST verify that the key
   authorization in the response matches the "token" value in the
   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
   in which the client sent the challenge.

   Given a challenge/response pair, the server verifies the client's
   control of the domain by verifying that the resource was provisioned
   as expected.

   1.  Form a URI by populating the URI template [RFC6570]
       "http://{domain}/.well-known/acme-challenge/{token}", where:

       *  the domain field is set to the domain name being verified; and

       *  the token field is set to the token in the challenge.

   2.  Verify that the resulting URI is well-formed.

   3.  If the client has supplied an address to use, verify that the
       address is included in the A or AAAA records to which the domain
       name resolves.  If the address is not included in the result, the
       validation fails.

   4.  Dereference the URI using an HTTP GET request.  If an address was
       supplied by the client, use that address to establish the HTTP
       connection.

   5.

   4.  Verify that the body of the response is well-formed key
       authorization.  The server SHOULD ignore whitespace characters at
       the end of the body.

   6.

   5.  Verify that key authorization provided by the server matches the
       token for this challenge and the client's account key.

   If all of the above verifications succeed, then the validation is
   successful.  If the request fails, or the body does not pass these
   checks, then it has failed.

7.3.  TLS with Server Name Indication (TLS SNI)

   The TLS with Server Name Indication (TLS SNI) validation method
   proves control over a domain name by requiring the client to
   configure a TLS server referenced by an A/AAAA record under the
   domain name to respond to specific connection attempts utilizing the
   Server Name Indication extension [RFC6066].  The server verifies the
   client's challenge by accessing the reconfigured server and verifying
   a particular challenge certificate is presented.

   type (required, string):  The string "tls-sni-02"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the URL-safe Base64 alphabet and
      MUST NOT contain any padding characters ("=").

   {
     "type": "tls-sni-02",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   A client responds to this challenge by constructing a self-signed
   certificate which the client MUST provision at the domain name
   concerned in order to pass the challenge.

   The certificate may be constructed arbitrarily, except that each
   certificate MUST have exactly two subjectAlternativeNames, SAN A and
   SAN B.  Both MUST be dNSNames.

   SAN A MUST be constructed as follows: compute the SHA-256 digest of
   the UTF-8-encoded challenge token and encode it in lowercase
   hexadecimal form.  The dNSName is "x.y.token.acme.invalid", where x
   is the first half of the hexadecimal representation and y is the
   second half.

   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.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and
      the client's account key.

   /* BEGIN JWS-signed content */
   {
     "keyAuthorization": "evaGxfADs...62jcerQ", "evaGxfADs...62jcerQ"
   }
   /* Signed as JWS END JWS-signed content */

   On receiving a response, the server MUST verify that the key
   authorization in the response matches the "token" value in the
   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
   in which the client sent the challenge.

   Given a challenge/response pair, the ACME server verifies the
   client's control of the domain by verifying that the TLS server was
   configured appropriately, using these steps:

   1.  Compute SAN A and SAN B in the same way as the client.

   2.  Open a TLS connection to the domain name being validated on the
       requested port, presenting SAN A in the SNI field.  In the
       ClientHello initiating the TLS handshake, the server MUST include
       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.

   3.  Verify that the certificate contains a subjectAltName extension
       containing dNSName entries of SAN A and SAN B and no other
       entries.  The comparison MUST be insensitive to case and ordering
       of names.

   It is RECOMMENDED that the ACME server validation TLS connections
   from multiple vantage points to reduce the risk of DNS hijacking
   attacks.

   If all of the above verifications succeed, then the validation is
   successful.  Otherwise, the validation fails.

7.4.  DNS

   When the identifier being validated is a domain name, the client can
   prove control of that domain by provisioning a resource record under
   it.  The DNS challenge requires the client to provision a TXT record
   containing a designated value under a specific validation domain
   name.

   type (required, string):  The string "dns-01"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the URL-safe Base64 alphabet and
      MUST NOT contain any padding characters ("=").

   {
     "type": "dns-01",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   A client responds to this challenge by constructing a key
   authorization from the "token" value provided in the challenge and
   the client's account key.  The client then computes the SHA-256
   digest of the key authorization.

   The record provisioned to the DNS is the base64url encoding of this
   digest.  The client constructs the validation domain name by
   prepending the label "_acme-challenge" to the domain name being
   validated, then provisions a TXT record with the digest value under
   that name.  For example, if the domain name being validated is
   "example.com", then the client would provision the following DNS
   record:

   _acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM"
   The response to the DNS challenge provides the computed key
   authorization to acknowledge that the client is ready to fulfill this
   challenge.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and
      the client's account key.

   /* BEGIN JWS-signed content */
   {
     "keyAuthorization": "evaGxfADs...62jcerQ", "evaGxfADs...62jcerQ"
   }
   /* Signed as JWS END JWS-signed content */

   On receiving a response, the server MUST verify that the key
   authorization in the response matches the "token" value in the
   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
   in which the client sent the challenge.

   To validate a DNS challenge, the server performs the following steps:

   1.  Compute the SHA-256 digest of the key authorization

   2.  Query for TXT records under the validation domain name

   3.  Verify that the contents of one of the TXT records matches the
       digest value

   If all of the above verifications succeed, then the validation is
   successful.  If no DNS record is found, or DNS record and response
   payload do not pass these checks, then the validation fails.

7.5.  Out-of-Band

   There may be cases where a server cannot perform automated validation
   of an identifier, for example if validation requires some manual
   steps.  In such cases, the server may provide an "out of band" (OOB)
   challenge to request that the client perform some action outside of
   ACME in order to validate possession of the identifier.

   The OOB challenge requests that the client have a human user visit a
   web page to receive instructions on how to validate possession of the
   identifier, by providing a URL for that web page.

   type (required, string):  The string "oob-01"
   url (required, string):  The URL to be visited.  The scheme of this
      URL MUST be "http" or "https"

   {
     "type": "oob-01",
     "url": "https://example.com/validate/evaGxfADs6pSRb2LAv9IZ"
   }

   A client responds to this challenge by presenting the indicated URL
   for a human user to navigate to.  If the user choses to complete this
   challege (by vising the website and completing its instructions), the
   client indicates this by sending a simple acknowledgement response to
   the server.

   type (required, string):  The string "oob-01"

   /* BEGIN JWS-signed content */
   {
     "type": "oob-01"
   }
   /* END JWS-signed content */

   On receiving a response, the server MUST verify that the value of the
   "type" field is as required.  Otherwise, the steps the server takes
   to validate identifier possession are determined by the server's
   local policy.

8.  IANA Considerations

   [[ 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? ]]

9.

8.1.  Well-Known URI for the HTTP Challenge

   The "Well-Known URIs" registry should be updated with the following
   additional value (using the template from [RFC5785]):

   URI suffix: acme-challenge

   Change controller: IETF

   Specification document(s): This document, Section Section 7.2

   Related information: N/A

9.1.

8.2.  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.

8.3.  "url" JWS Header Parameter

   The "JSON Web Signature and Encryption Header Parameters" registry
   should be updated with the following additional value:

   o  Header Parameter Name: "url"

   o  Header Parameter Description: URL

   o  Header Parameter Usage Location(s): JWE, JWS

   o  Change Controller: IESG

   o  Specification Document(s): Section 5.3.1 of RFC XXXX

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

8.4.  "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.

8.5.  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.

8.6.  New Registries

   This document requests that IANA create three the following new registries:

   1.  ACME Error Codes

   2.  ACME Identifier Resource Types

   3.  ACME Identifier Types

   4.  ACME Challenge Types

   All of these registries should be administered under a Specification
   Required policy [RFC5226].

9.4.1.

8.6.1.  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 5.6 above, with the Reference field set to point to this
   specification.

9.4.2.

8.6.2.  Resource Types

   This registry lists the types of resources that ACME servers may list
   in their directory objects.

   Template:

   o  Key: The value to be used as a dictionary key in the directory
      object

   o  Resource type: The type of resource labeled by the key

   o  Reference: Where the identifier type is defined

   Initial contents:

             +-------------+--------------------+-----------+
             | Key         | Resource type      | Reference |
             +-------------+--------------------+-----------+
             | new-reg     | New registration   | RFC XXXX  |
             |             |                    |           |
             | new-app     | New application    | RFC XXXX  |
             |             |                    |           |
             | revoke-cert | Revoke certificate | RFC XXXX  |
             |             |                    |           |
             | key-change  | Key change         | RFC XXXX  |
             +-------------+--------------------+-----------+

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

8.6.3.  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.

8.6.4.  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.

9.  Security Considerations

   ACME is a protocol for managing certificates that attest to
   identifier/key bindings.  Thus the foremost security goal of ACME is
   to ensure the integrity of this process, i.e., to ensure that the
   bindings attested by certificates are correct, and that only
   authorized entities can manage certificates.  ACME identifies clients
   by their account keys, so this overall goal breaks down into two more
   precise goals:

   1.  Only an entity that controls an identifier can get an account key
       authorized for that identifier

   2.  Once authorized, an account key's authorizations cannot be
       improperly transferred to another account key

   In this section, we discuss the threat model that underlies ACME and
   the ways that ACME achieves these security goals within that threat
   model.  We also discuss the denial-of-service risks that ACME servers
   face, and a few other miscellaneous considerations.

10.1.

9.1.  Threat model

   As a service on the Internet, ACME broadly exists within the Internet
   threat model [RFC3552].  In analyzing ACME, it is useful to think of
   an ACME server interacting with other Internet hosts along three
   "channels":

   o  An ACME channel, over which the ACME HTTPS requests are exchanged

   o  A validation channel, over which the ACME server performs
      additional requests to validate a client's control of an
      identifier

   o  A contact channel, over which the ACME server sends messages to
      the registered contacts for ACME clients

   +------------+
   |    ACME    |     ACME Channel
   |   Client   |--------------------+
   +------------+                    |
          ^                          V
          |   Contact Channel  +------------+
          +--------------------|    ACME    |
                               |   Server   |
                               +------------+
   +------------+                    |
   | Validation |<-------------------+
   |   Server   |  Validation Channel
   +------------+

   In practice, the risks to these channels are not entirely separate,
   but they are different in most cases.  Each of the three channels,
   for example, uses a different communications pattern: the ACME
   channel will comprise inbound HTTPS connections to the ACME server,
   the validation channel outbound HTTP or DNS requests, and the contact
   channel will use channels such as email and PSTN.

   Broadly speaking, ACME aims to be secure against active and passive
   attackers on any individual channel.  Some vulnerabilities arise
   (noted below), when an attacker can exploit both the ACME channel and
   one of the others.

   On the ACME channel, in addition to network-layer attackers, we also
   need to account for application-layer man in the middle attacks, and
   for abusive use of the protocol itself.  Protection against
   application-layer MitM addresses potential attackers such as Content
   Distribution Networks (CDNs) and middleboxes with a TLS MitM
   function.  Preventing abusive use of ACME means ensuring that an
   attacker with access to the validation or contact channels can't
   obtain illegitimate authorization by acting as an ACME client
   (legitimately, in terms of the protocol).

10.2.

9.2.  Integrity of Authorizations

   ACME allows anyone to request challenges for an identifier by
   registering an account key and sending a new-authorization new-application request
   under that account key.  The integrity of the authorization process
   thus depends on the identifier validation challenges to ensure that
   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
   in question.

   Validation responses need to be bound to an account key pair in order
   to avoid situations where an ACME MitM can switch out a legitimate
   domain holder's account key for one of his choosing, e.g.:

   o  Legitimate domain holder registers account key pair A

   o  MitM registers account key pair B

   o  Legitimate domain holder sends a new-authorization new-application request signed
      under account key A

   o  MitM suppresses the legitimate request, but sends the same request
      signed under account key B

   o  ACME server issues challenges and MitM forwards them to the
      legitimate domain holder

   o  Legitimate domain holder provisions the validation response
   o  ACME server performs validation query and sees the response
      provisioned by the legitimate domain holder

   o  Because the challenges were issued in response to a message signed
      account key B, the ACME server grants authorization to account key
      B (the MitM) instead of account key A (the legitimate domain
      holder)

   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 only the
   account private key holder can successfully respond to the validation
   query:

   o  HTTP: The value provided in the validation request is signed by

   All of the challenges above have a binding between the account
   private key.

   o  TLS SNI: The key and the validation TLS request uses query made by the account key pair as server, via the server's key pair.

   o  DNS:
   authorization.  The MAC covers the account key, and the MAC key authorization is derived
      from an ECDH public key signed with by the account
   private key. key, reflects the corresponding public key, and is provided
   to the server in the validation response.

   The association of challenges to identifiers is typically done by
   requiring the client to perform some action that only someone who
   effectively controls the identifier can perform.  For the challenges
   in this document, the actions are:

   o  HTTP: Provision files under .well-known on a web server for the
      domain

   o  TLS SNI: Configure a TLS server for the domain

   o  DNS: Provision DNS resource records for the domain

   There are several ways that these assumptions can be violated, both
   by misconfiguration and by attack.  For example, on a web server that
   allows non-administrative users to write to .well-known, any user can
   claim to own the server's hostname by responding to an HTTP
   challenge, and likewise for TLS configuration and TLS SNI.

   The use of hosting providers is a particular risk for ACME
   validation.  If the owner of the domain has outsourced operation of
   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
   outside world is concerned, the zone or web site provided by the
   hosting provider is the real thing.

   More limited forms of delegation can also lead to an unintended party
   gaining the ability to successfully complete a validation
   transaction.  For example, suppose an ACME server follows HTTP
   redirects in HTTP validation and a web site operator provisions a
   catch-all redirect rule that redirects requests for unknown resources
   to a different domain.  Then the target of the redirect could use
   that to get a certificate through HTTP validation, since the
   validation path will not be known to the primary server.

   The DNS is a common point of vulnerability for all of these
   challenges.  An entity that can provision false DNS records for a
   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
   validation query to a server of the attacker's choosing.  There are a
   few different mitigations that ACME servers can apply:

   o  Always querying the DNS using a DNSSEC-validating resolver
      (enhancing security for zones that are DNSSEC-enabled)

   o  Querying the DNS from multiple vantage points to address local
      attackers

   o  Applying mitigations against DNS off-path attackers, e.g., adding
      entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP

   Given these considerations, the ACME validation process makes it
   impossible for any attacker on the ACME channel, or a passive
   attacker on the validation channel to hijack the authorization
   process to authorize a key of the attacker's choice.

   An attacker that can only see the ACME channel would need to convince
   the validation server to provide a response that would authorize the
   attacker's account key, but this is prevented by binding the
   validation response to the account key used to request challenges.  A
   passive attacker on the validation channel can observe the correct
   validation response and even replay it, but that response can only be
   used with the account key for which it was generated.

   An active attacker on the validation channel can subvert the ACME
   process, by performing normal ACME transactions and providing a
   validation response for his own account key.  The risks due to
   hosting providers noted above are a particular case.  For identifiers
   where the server already has some public key associated with the
   domain this attack can be prevented by requiring the client to prove
   control of the corresponding private key.

10.3.

9.3.  Denial-of-Service Considerations

   As a protocol run over HTTPS, standard considerations for TCP-based
   and HTTP-based DoS mitigation also apply to ACME.

   At the application layer, ACME requires the server to perform a few
   potentially expensive operations.  Identifier validation transactions
   require the ACME server to make outbound connections to potentially
   attacker-controlled servers, and certificate issuance can require
   interactions with cryptographic hardware.

   In addition, an attacker can also cause the ACME server to send
   validation requests send
   validation requests to a domain of its choosing by submitting
   authorization requests for the victim domain.

   All of these attacks can be mitigated by the application of
   appropriate rate limits.  Issues closer to the front end, like POST
   body validation, can be addressed using HTTP request limiting.  For
   validation and certificate requests, there are other identifiers on
   which rate limits can be keyed.  For example, the server might limit
   the rate at which any individual account key can issue certificates,
   or the rate at which validation can be requested within a given
   subtree of the DNS.

9.4.  Server-Side Request Forgery

   Server-Side Request Forgery (SSRF) attacks can arise when an attacker
   can cause a server to perform HTTP requests to an attacker-chosen
   URL.  In the ACME HTTP challenge validation process, the ACME server
   performs an HTTP GET request to a URL in which the attacker can
   choose the domain.  This request is made before the server has
   verified that the client controls the domain, so any client can cause
   a query to any domain.

   Some server implementations include information from the validation
   server's response (in order to facilitate debugging).  Such
   implementations enable an attacker to extract this information from
   any web server that is accessible to the ACME server, even if it is
   not accessible to a domain the ACME client.

   It might seem that the risk of its choosing SSRF through this channel is limited
   by submitting
   authorization requests for the victim domain.

   All of these attacks fact that the attacker can be mitigated by only control the application domain of
   appropriate rate limits.  Issues closer to the front end, like POST
   body validation, URL,
   not the path.  However, if the attacker first sets the domain to one
   they control, then they can be addressed using send the server an HTTP request limiting.  For
   validation and certificate requests, there are other identifiers on redirect (e.g., a
   302 response) which rate limits can be keyed.  For example, will cause the server might to query an arbitrary URI.

   In order to further limit the rate at which any individual account key can issue certificates,
   or the rate at which SSRF risk, ACME server operators should
   ensure that validation queries can only be requested sent to servers on the
   public Internet, and not, say, web services within the server
   operator's internal network.  Since the attacker could make requests
   to these public servers himself, he can't gain anything extra through
   an SSRF attack on ACME aside from a given
   subtree layer of the DNS.

10.4. anonymization.

9.5.  CA Policy Considerations

   The controls on issuance enabled by ACME are focused on validating
   that a certificate applicant controls the identifier he claims.
   Before issuing a certificate, however, there are many other checks
   that a CA might need to perform, for example:

   o  Has the client agreed to a subscriber agreement?

   o  Is the claimed identifier syntactically valid?

   o  For domain names:

      *  If the leftmost label is a '*', then have the appropriate
         checks been applied?

      *  Is the name on the Public Suffix List?

      *  Is the name a high-value name?

      *  Is the name a known phishing domain?

   o  Is the key in the CSR sufficiently strong?

   o  Is the CSR signed with an acceptable algorithm?

   CAs that use ACME to automate issuance will need to ensure that their
   servers perform all necessary checks before issuing.

11.

10.  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

10.1.  DNS over TCP

   As noted above, DNS forgery attacks against the ACME server can
   result in the server making incorrect decisions about domain control
   and thus mis-issuing certificates.  Servers SHOULD verify DNSSEC when
   it is advisable to available for a domain.  When DNSSEC is not available, servers
   SHOULD perform DNS queries via TCP over TCP, which provides better resistance
   to mitigate DNS some forgery attacks than DNS over UDP

   [[ TODO: Other operational considerations ]]

11.1. UDP.

10.2.  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 comes 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.

10.3.  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.

11.  Acknowledgements

   In addition to the editors listed on the front page, this document
   has benefited from contributions from a broad set of contributors,
   all the way back to its inception.

   o  Peter Eckersley, EFF

   o  Eric Rescorla, Mozilla

   o  Seth Schoen, EFF
   o  Alex Halderman, University of Michigan

   o  Martin Thomson, Mozilla

   o  Jakub Warmuz, University of Oxford

   This document draws on many concepts established by Eric Rescorla's
   "Automated Certificate Issuance Protocol" draft.  Martin Thomson
   provided helpful guidance in the use of HTTP.

13.

12.  References

13.1.

12.1.  Normative References

   [I-D.ietf-appsawg-http-problem]
              mnot, m. and E. Wilde, "Problem Details for HTTP APIs",
              draft-ietf-appsawg-http-problem-03 (work in progress),
              January 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2314]  Kaliski, B., "PKCS #10: Certification Request Syntax
              Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998,
              <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
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <http://www.rfc-editor.org/info/rfc2985>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <http://www.rfc-editor.org/info/rfc2986>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie,

   [RFC3986]  Berners-Lee, T., Fielding, R., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <http://www.rfc-editor.org/info/rfc3553>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 4291, 3986, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>. 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <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
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <http://www.rfc-editor.org/info/rfc7518>.

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <http://www.rfc-editor.org/info/rfc7638>.

13.2.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <http://www.rfc-editor.org/info/rfc7807>.

12.2.  Informative References

   [I-D.vixie-dnsext-dns0x20]
              Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to
              Improve Transaction Identity", draft-vixie-dnsext-
              dns0x20-00 (work in progress), March 2008.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <http://www.rfc-editor.org/info/rfc3553>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

   [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>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

   [W3C.CR-cors-20130129]
              Kesteren, A., "Cross-Origin Resource Sharing", World Wide
              Web Consortium CR CR-cors-20130129, January 2013,
              <http://www.w3.org/TR/2013/CR-cors-20130129>.

   [W3C.WD-capability-urls-20140218]
              Tennison, J., "Good Practices for Capability URLs", World
              Wide Web Consortium WD WD-capability-urls-20140218,
              February 2014,
              <http://www.w3.org/TR/2014/WD-capability-urls-20140218>.

Authors' Addresses

   Richard Barnes
   Mozilla

   Email: rlb@ipv.sx

   Jacob Hoffman-Andrews
   EFF

   Email: jsha@eff.org

   James Kasten
   University of Michigan

   Email: jdkasten@umich.edu