Network Working Group                                           O. Friel
Internet-Draft                                                 R. Barnes
Intended status: Informational                                     Cisco
Expires: January 14, May 22, 2021                                     R. Shekh-Yusef
                                                                   Auth0
                                                           M. Richardson
                                                Sandelman Software Works
                                                           July 13,
                                                       November 18, 2020

                           ACME Integrations
                    draft-ietf-acme-integrations-01
                    draft-ietf-acme-integrations-02

Abstract

   This document outlines multiple advanced use cases and integrations
   that ACME facilitates without any modifications or enhancements
   required to the base ACME specification.  The use cases include ACME
   integration with EST, BRSKI and TEAP.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 14, May 22, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  ACME Integration with EST . . . . . . . . . . . . . . . . . .   3
   4.  ACME Integration with BRSKI . . . . . . . . . . . . . . . . .   6
   5.  ACME Integration with BRSKI Default Cloud Registrar . . . . .   8
   6.  ACME Integration with TEAP  . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  Informative References  . . . . . . . . . . . . . . . .
     8.1.  Denial of Service against ACME infrastructure . . .  14
   Appendix A.  Comments . . .  15
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15  17

1.  Introduction

   ACME [RFC8555] defines a protocol that a certificate authority (CA)
   and an applicant can use to automate the process of domain name
   ownership validation and X.509 (PKIX) certificate issuance.  The
   protocol is rich and flexible and enables multiple use cases that are
   not immediately obvious from reading the specification.  This
   document explicitly outlines multiple advanced ACME use cases
   including:

   o  ACME integration with EST [RFC7030]

   o  ACME integration with BRSKI
      [I-D.ietf-anima-bootstrapping-keyinfra]

   o  ACME integration with BRSKI Default Cloud Registrar
      [I-D.friel-anima-brski-cloud]

   o  ACME integration with TEAP [RFC7170] and TEAP Update and
      Extensions for Bootstrapping [I-D.lear-eap-teap-brski]

   The integrations with EST, BRSKI (which is based upon EST), and TEAP
   enable automated certificate enrolment for devices.

   ACME for subdomains [I-D.friel-acme-subdomains] outlines how ACME can
   be used by a client to obtain a certificate for a subdomain
   identifier from a certificate authority where the client has
   fulfilled a challenge against a parent domain, but does not need to
   fulfil a challenge against the explicit subdomain.  This is a useful
   optimization when ACME is used to issue certificates for large
   numbers of devices as it reduces the domain ownership proof traffic
   (DNS or HTTP) and ACME traffic overhead, but is not a necessary
   requirement.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are used in this document:

   o  BRSKI: Bootstrapping Remote Secure Key Infrastructures
      [I-D.ietf-anima-bootstrapping-keyinfra]

   o  CA: Certificate Authority

   o  CMC: Certificate Management over CMS

   o  CSR: Certificate Signing Request

   o  EST: Enrollment over Secure Transport [RFC7030]

   o  FQDN: Fully Qualified Domain Name

   o  RA: PKI Registration Authority

   o  TEAP: Tunneled Extensible Authentication Protocol [RFC7170]

3.  ACME Integration with EST

   EST [RFC7030] defines a mechanism for clients to enroll with a PKI
   Registration Authority by sending CMC messages over HTTP.  EST
   section 1 states:

   "Architecturally, the EST service is located between a Certification
   Authority (CA) and a client.  It performs several functions
   traditionally allocated to the Registration Authority (RA) role in a
   PKI."

   EST section 1.1 states that:

   "For certificate issuing services, the EST CA is reached through the
   EST server; the CA could be logically "behind" the EST server or
   embedded within it."
   When the CA is logically "behind" the EST RA, EST does not specify
   how the RA communicates with the CA.  EST section 1 states:

   "The nature of communication between an EST server and a CA is not
   described in this document."

   This section outlines how ACME could be used for communication
   between the EST RA and the CA.  The example call flow leverages
   [I-D.friel-acme-subdomains] and shows the RA proving ownership of a
   parent domain, with individual client certificates being subdomains
   under that parent domain.  This is an optimization that reduces DNS
   and ACME traffic overhead.  The RA could of course prove ownership of
   every explicit client certificate identifier.

   The call flow illustrates the client calling the EST /csrattrs API
   before calling the EST /simpleenroll API.  This enables the EST
   server to indicate to the client what attributes it expects the
   client to include in the CSR request sent in the /simpleenroll API.
   For example, EST servers could use this mechanism to tell the client
   what fields to include in the CSR Subject and Subject Alternative
   Name fields.

   The call flow illustrates the EST RA returning a 202 Retry-After
   response to the client's simpleenroll request.  This is an optional
   step and may be necessary if the interactions between the RA and the
   ACME server take some time to complete.  The exact details of when
   the RA returns a 202 Retry-After are implementation specific.

   +--------+             +--------+             +------+     +-----+
   | Pledge |             | EST RA |             | ACME |     | DNS |
   +--------+             +--------+             +------+     +-----+
       |                      |                      |           |
                  STEP 1: Pre-Authorization of parent domain
       |                      |                      |           |
       |                      | POST /newAuthz       |           |
       |                      |  "domain.com"  "example.com"       |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 201 authorizations   |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | Publish DNS TXT      |           |
       |                      | "domain.com" "example.com"        |           |
       |                      |--------------------------------->|
       |                      |                      |           |
       |                      | POST /challenge      |           |
       |                      |--------------------->|           |
       |                      |                      | Verify    |
       |                      |                      |---------->|
       |                      | 200 status=valid     |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | Delete DNS TXT       |           |
       |                      | "domain.com" "example.com"        |           |
       |                      |--------------------------------->|
       |                      |                      |           |
                  STEP 2: Pledge enrolls against RA
       |                      |                      |           |
       | GET /csrattrs        |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | SEQUENCE {AttrOrOID} |                      |           |
       | SAN OID:             |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 202 Retry-After      |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
                  STEP 3: RA places ACME order
       |                      |                      |           |
       |                      | POST /newOrder       |           |
       |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 201 status=ready     |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /finalize       |           |
       |                      | PKCS#10 CSR          |           |
       |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK status=valid  |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /certificate    |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK               |           |
       |                      | PEM                  |           |
       |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
       |                      |<---------------------|           |
       |                      |                      |           |
                  STEP 4: Pledge retries enroll
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | PKCS#7               |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |<---------------------|                      |           |

4.  ACME Integration with BRSKI

   BRSKI [I-D.ietf-anima-bootstrapping-keyinfra] is based upon EST
   [RFC7030] and defines how to autonomically bootstrap PKI trust
   anchors into devices via means of signed vouchers.  EST certificate
   enrollment may then optionally take place after trust has been
   established.  BRKSI voucher exchange and trust establishment are
   based on EST extensions and the certificate enrollment part of BRSKI
   is fully based on EST.  Similar to EST, BRSKI does not define how the
   EST RA communicates with the CA.  Therefore, the mechanisms outlined
   in the previous section for using ACME as the communications protocol
   between the EST RA and the CA are equally applicable to BRSKI.

   The following call flow shows how ACME may be integrated into a full
   BRSKI voucher plus EST enrollment workflow.  For brevity, it assumes
   that the EST RA has previously proven ownership of a parent domain
   and that pledge certificate identifiers are a subdomain of that
   parent domain.  The domain ownership exchanges between the RA, ACME
   and DNS are not shown.  Similarly, not all BRSKI interactions are
   shown and only the key protocol flows involving voucher exchange and
   EST enrollment are shown.

   Similar to the EST section above, the client calls EST /csrattrs API
   before calling the EST /simpleenroll API.  This enables the server to
   indicate what fields the pledge should include in the CSR that the
   client sends in the /simpleenroll API.

   The call flow illustrates the RA returning a 202 Retry-After response
   to the initial EST /simpleenroll API.  This may be appropriate if
   processing of the /simpleenroll request and ACME interactions takes
   some timme to complete.

   +--------+             +--------+             +------+     +------+
   | Pledge |             | EST RA |             | ACME |     | MASA |
   +--------+             +--------+             +------+     +------+
       |                      |                      |           |
                  NOTE: Pre-Authorization of "domain.com" "example.com" is complete
       |                      |                      |           |
                  STEP 1: Pledge requests Voucher
       |                      |                      |           |
       | POST /requestvoucher |                      |           |
       |--------------------->|                      |           |
       |                      | POST /requestvoucher |           |
       |                      |--------------------------------->|
       |                      |                      |           |
       |                      | 200 OK Voucher       |           |
       |                      |<---------------------------------|
       | 200 OK Voucher       |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
                  STEP 2: Pledge enrolls against RA
       |                      |                      |           |
       | GET /csrattrs        |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | SEQUENCE {AttrOrOID} |                      |           |
       | SAN OID:             |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 202 Retry-After      |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
                  STEP 3: RA places ACME order
       |                      |                      |           |
       |                      | POST /newOrder       |           |
       |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 201 status=ready     |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /finalize       |           |
       |                      | PKCS#10 CSR          |           |
       |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK status=valid  |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /certificate    |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK               |           |
       |                      | PEM                  |           |
       |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
       |                      |<---------------------|           |
       |                      |                      |           |
                  STEP 4: Pledge retries enroll
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | PKCS#7               |                      |           |
       | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
       |<---------------------|                      |           |

5.  ACME Integration with BRSKI Default Cloud Registrar

   BRSKI Cloud Registrar [I-D.friel-anima-brski-cloud] specifies the
   behaviour of a BRSKI Cloud Registrar, and how a pledge can interact
   with a BRSKI Cloud Registrar when bootstrapping.  Similar to the
   local domain registrar BRSKI flow, ACME can be easily integrated with
   a cloud registrar bootstrap flow.

   BRSKI cloud registrar is flexible and allows for multiple different
   local domain discovery and redirect scenarios.  In the example
   illustrated here, the extension to [RFC8366] Vouchers which is
   defined in [[TODO ID-TBD]] [I-D.friel-anima-brski-cloud], and allows the
   specification of a bootstrap DNS domain EST domain, is leveraged.  This
   extension allows the cloud registrar to specify the local domain RA
   that the pledge should connect to for the purposes of EST enrollment.

   Similar to the sectiosn above, the client calls EST /csrattrs API
   before calling the EST /simpleenroll API.

  +--------+             +--------+            +------+     +----------+
  | Pledge |             | EST RA |            | ACME |     | Cloud RA |
  +--------+             +--------+            +------+     |  / MASA  |
      |                                                     +----------+
      |                                                         |
           NOTE: Pre-Authorization of "domain.com" "example.com" is complete
      |                                                         |
           STEP 1: Pledge requests Voucher from Cloud Registrar
      |                                                         |
      | POST /requestvoucher                                    |
      |-------------------------------------------------------->|
      |                                                         |
      | 200 OK Voucher (EST RA domain) (includes 'est-domain')                  |
      |<--------------------------------------------------------|
      |                      |                      |           |
           STEP 2: Pledge enrolls against local domain RA
      |                      |                      |           |
      | GET /csrattrs        |                      |           |
      |--------------------->|                      |           |
      |                      |                      |           |
      | 200 OK               |                      |           |
      | SEQUENCE {AttrOrOID} |                      |           |
      | SAN OID:             |                      |           |
      | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
      |<---------------------|                      |           |
      |                      |                      |           |
      | POST /simpleenroll   |                      |           |
      | PCSK#10 CSR          |                      |           |
      | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
      |--------------------->|                      |           |
      |                      |                      |           |
      | 202 Retry-After      |                      |           |
      |<---------------------|                      |           |
      |                      |                      |           |
           STEP 3: RA places ACME order
      |                      |                      |           |
      |                      | POST /newOrder       |           |
      |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
      |                      |--------------------->|           |
      |                      |                      |           |
      |                      | 201 status=ready     |           |
      |                      |<---------------------|           |
      |                      |                      |           |
      |                      | POST /finalize       |           |
      |                      | PKCS#10 CSR          |           |
      |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
      |                      |--------------------->|           |
      |                      |                      |           |
      |                      | 200 OK status=valid  |           |
      |                      |<---------------------|           |
      |                      |                      |           |
      |                      | POST /certificate    |           |
      |                      |--------------------->|           |
      |                      |                      |           |
      |                      | 200 OK               |           |
      |                      | PEM                  |           |
      |                      | "pledgeid.domain.com"| "pledge.example.com" |           |
      |                      |<---------------------|           |
      |                      |                      |           |
           STEP 4: Pledge retries enroll
      |                      |                      |           |
      | POST /simpleenroll   |                      |           |
      | PCSK#10 CSR          |                      |           |
      | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
      |--------------------->|                      |           |
      |                      |                      |           |
      | 200 OK               |                      |           |
      | PKCS#7               |                      |           |
      | "pledgeid.domain.com"| "pledge.example.com" |                      |           |
      |<---------------------|                      |           |

6.  ACME Integration with TEAP

   TEAP [RFC7170] defines a tunnel-based EAP method that enables secure
   communication between a peer and a server by using TLS to establish a
   mutually authenticated tunnel.  TEAP enables certificate provisioning
   within the tunnel.  TEAP Update and Extensions for Bootstrapping
   [I-D.lear-eap-teap-brski] defines extensions to TEAP that includes
   additional TLVs for certificate enrollment and BRSKI handling inside
   the TEAP tunnel.  Neither TEAP [RFC7170] or TEAP Update and
   Extensions for Bootstrapping [I-D.lear-eap-teap-brski] define how the
   TEAP server communicates with the CA.

   This section outlines how ACME could be used for communication
   between the TEAP server and the CA.  The example call flow leverages
   [I-D.friel-acme-subdomains] and shows the TEAP server proving
   ownership of a parent domain, with individual client certificates
   being subdomains under that parent domain.

   The example illustrates the TEAP server sending a Request-Action TLV
   including a CSR-Attributes TLV instructing the peer to send a CSR-
   Attributes TLV to the server.  This enables the server to indicate
   what fields the peer should include in the CSR that the peer sends in
   the PKCS#10 TLV.  For example, the TEAP server could instruct the
   peer what Subject or SAN entries to include in its CSR.

   Althought not explicitly illustrated in this call flow, the Peer and
   TEAP Server could exchange BRSKI TLVs, and a BRSKI integration and
   voucher exchange with a MASA server could take place over TEAP.

   Whether a BRSKI TLV exchange takes place or not does not impact the
   ACME specific message exchanges.

  +------+                +-------------+           +------+     +-----+
  | Peer |                | TEAP-Server |           | ACME |     | DNS |
  +------+                +-------------+           +------+     +-----+
      |                         |                      |           |
                 STEP 1: Pre-Authorization of parent domain
      |                         |                      |           |
      |                         | POST /newAuthz       |           |
      |                         |  "domain.com"  "example.com"       |           |
      |                         |--------------------->|           |
      |                         |                      |           |
      |                         | 201 authorizations   |           |
      |                         |<---------------------|           |
      |                         |                      |           |
      |                         | Publish DNS TXT      |           |
      |                         | "domain.com" "example.com"        |           |
      |                         |--------------------------------->|
      |                         |                      |           |
      |                         | POST /challenge      |           |
      |                         |--------------------->|           |
      |                         |                      | Verify    |
      |                         |                      |---------->|
      |                         | 200 status=valid     |           |
      |                         |<---------------------|           |
      |                         |                      |           |
      |                         | Delete DNS TXT       |           |
      |                         | "domain.com" "example.com"        |           |
      |                         |--------------------------------->|
      |                         |                      |           |
      |                         |                      |           |
                 STEP 2: Establsh EAP Outer Tunnel
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=Identity         |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=Identity         |                      |           |
      |------------------------>|                      |           |
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=TEAP,            |                      |           |
      |   TEAP Start,           |                      |           |
      |   Authority-ID TLV      |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=TEAP,            |                      |           |
      |   TLS(ClientHello)      |                      |           |
      |------------------------>|                      |           |
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=TEAP,            |                      |           |
      |   TLS(ServerHello,      |                      |           |
      |   Certificate,          |                      |           |
      |   ServerKeyExchange,    |                      |           |
      |   CertificateRequest,   |                      |           |
      |   ServerHelloDone)      |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=TEAP,            |                      |           |
      |   TLS(Certificate,      |                      |           |
      |   ClientKeyExchange,    |                      |           |
      |   CertificateVerify,    |                      |           |
      |   ChangeCipherSpec,     |                      |           |
      |   Finished)             |                      |           |
      |------------------------>|                      |           |
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=TEAP,            |                      |           |
      |   TLS(ChangeCipherSpec, |                      |           |
      |   Finished),            |                      |           |
      |   {Crypto-Binding TLV,  |                      |           |
      |   Result TLV=Success}   |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {Crypto-Binding TLV,  |                      |           |
      |   Result TLV=Success}   |                      |           |
      |------------------------>|                      |           |
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {Request-Action TLV:  |                      |           |
      |     Status=Failure,     |                      |           |
      |     Action=Process-TLV, |                      |           |
      |     TLV=CSR-Attributes, |                      |           |
      |     TLV=PKCS#10}        |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
                 STEP 3: Enroll for certificate
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {CSR-Attributes TLV}  |                      |           |
      |------------------------>|                      |           |
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {CSR-Attributes TLV}  |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {PKCS#10 TLV:         |                      |           |
      |   "pledgeid.domain.com"}|   "pledge.example.com"} |                      |           |
      |------------------------>|                      |           |
      |                         | POST /newOrder       |           |
      |                         | "pledgeid.domain.com"| "pledge.example.com" |           |
      |                         |--------------------->|           |
      |                         |                      |           |
      |                         | 201 status=ready     |           |
      |                         |<---------------------|           |
      |                         |                      |           |
      |                         | POST /finalize       |           |
      |                         | PKCS#10 CSR          |           |
      |                         | "pledgeid.domain.com"| "pledge.example.com" |           |
      |                         |--------------------->|           |
      |                         |                      |           |
      |                         | 200 OK status=valid  |           |
      |                         |<---------------------|           |
      |                         |                      |           |
      |                         | POST /certificate    |           |
      |                         |--------------------->|           |
      |                         |                      |           |
      |                         | 200 OK               |           |
      |                         | PEM                  |           |
      |                         | "pledgeid.domain.com"| "pledge.example.com" |           |
      |                         |<---------------------|           |
      |                         |                      |           |
      |  EAP-Request/           |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {PKCS#7 TLV,          |                      |           |
      |    Result TLV=Success}  |                      |           |
      |<------------------------|                      |           |
      |                         |                      |           |
      |  EAP-Response/          |                      |           |
      |   Type=TEAP,            |                      |           |
      |   {Result TLV=Success}  |                      |           |
      |------------------------>|                      |           |
      |                         |                      |           |
      |  EAP-Success            |                      |           |
      |<------------------------|                      |           |

7.  IANA Considerations

   [todo]

   This document does not make any requests to IANA.

8.  Security Considerations

   [todo]

   This draft is informational and makes no changes to the referenced
   specifications.  All security considerations from these referenced
   documents are applicable here:

   o  EST [RFC7030]

   o  BRSKI [I-D.ietf-anima-bootstrapping-keyinfra]

   o  BRSKI Default Cloud Registrar [I-D.friel-anima-brski-cloud]

   o  TEAP [RFC7170] and TEAP Update and Extensions for Bootstrapping
      [I-D.lear-eap-teap-brski]

   Additionally, all Security Considerations in ACME in the following
   areas are equally applicable to ACME Integrations.

   The integration mechanisms proposed here will primarily use the
   DNS-01 challenge documented in [RFC8555] section 8.4.  The security
   considerations in RFC8555 says:

   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 HTTP validation
   query to a remote server of the attacker's choosing.

   It is expected that the TEAP-EAP server/EST Registrar will perform
   DNS dynamic updates to a DNS primary server using [RFC3007] Dynamic
   updates, secured with with either SIG(0), or TSIG keys.

   A major source of vulnerability is the disclosure of these DNS key
   records.  An attacker that has access to them, can provision their
   own certificates into the the name space of the entity.

   For many uses, this may allow the attacker to get access to some
   enterprise resource.  When used to provision, for instance, a (SIP)
   phone system this would permit an attacker to impersonate a
   legitimate phone.  Not only does this allow for redirection of phone
   calls, but possibly also toll fraud.

   Operators should consider restricting the integration server such
   that it can only update the DNS records for a specific zone or zones
   where ACME is required for client certificate enrolment automation.
   For example, if all IoT devices in an organisation enrol using EST
   against an EST RA, and all IoT devices will be issued certificates in
   a subdomain under iot.example.com, then the integration server could
   be issued a credential that only allows updating of DNS records in a
   zone that includes domains in the iot.example.com namespace, but does
   not allow updating of DNS records under any other example.com DNS
   namespace.

   When performing challenge fulfilment via writing files to HTTP
   webservers, write access should only be granted to a specific set of
   servers, and only to a specific set of directories for storage of
   challenge files.

8.1.  Denial of Service against ACME infrastructure

   The intermdiate node (the TEAP-EAP server, or the EST Registrar)
   should cache the resulting certificates such that if the
   communication with the pledge is lost, subsequent attempts to enroll
   will result in the cache certificate being returned.

   As many ACME servers have per-day, per-IP and per-subjectAltName
   limits, it is prudent not to request identical certificates too
   often.  This could be due to operator or installer error, with
   multiple configuration resets occuring within a short period of time.

   The cache should be keyed by the complete contents of the Certificate
   Signing Request, and should not persist beyond the notAfter date in
   the certificate.

   This means that if the private/public keypair changes on the pledge,
   then a new certificate will be issued.  If the the requested
   SubjectAltName changes, then a new certificate will be requested.

   In a case where a device is simply factory reset, and enrolls again,
   then the same certificate can be returned.

9.  Informative References

   [I-D.friel-acme-subdomains]
              Friel, O., Barnes, R., Hollebeek, T., and M. Richardson,
              "ACME for Subdomains", draft-friel-acme-subdomains-02 draft-friel-acme-subdomains-03
              (work in progress), March October 2020.

   [I-D.friel-anima-brski-cloud]
              Friel, O., Shekh-Yusef, R., and M. Richardson, "BRSKI
              Cloud Registrar", draft-friel-anima-brski-cloud-02 draft-friel-anima-brski-cloud-03 (work
              in progress), May September 2020.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-41
              keyinfra-45 (work in progress), April November 2020.

   [I-D.lear-eap-teap-brski]
              Lear, E., Friel, O., Cam-Winget, N., and D. Harkins, "TEAP
              Update and Extensions for Bootstrapping", draft-lear-eap-
              teap-brski-05 (work in progress), November 2019.

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

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [RFC7170]  Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
              "Tunnel Extensible Authentication Protocol (TEAP) Version
              1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
              <https://www.rfc-editor.org/info/rfc7170>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,
              <https://www.rfc-editor.org/info/rfc8366>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

Appendix A.  Comments

Authors' Addresses

   Owen Friel
   Cisco

   Email: ofriel@cisco.com

   Richard Barnes
   Cisco

   Email: rlb@ipv.sx

   Rifaat Shekh-Yusef
   Auth0

   Email: rifaat.s.ietf@gmail.com

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca