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ACME Integrations for Device Certificate Enrollment
draft-ietf-acme-integrations-16

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Authors Owen Friel , Richard Barnes , Rifaat Shekh-Yusef , Michael Richardson
Last updated 2023-07-13 (Latest revision 2023-06-13)
Replaces draft-friel-acme-integrations
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ACME integration with with EST, BRSKI and TEAP use cases submitted to IESG
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draft-ietf-acme-integrations-16
Network Working Group                                           O. Friel
Internet-Draft                                                 R. Barnes
Intended status: Standards Track                                   Cisco
Expires: 15 December 2023                                 R. Shekh-Yusef
                                                           Ernst & Young
                                                           M. Richardson
                                                Sandelman Software Works
                                                            13 June 2023

          ACME Integrations for Device Certificate Enrollment
                    draft-ietf-acme-integrations-16

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 15 December 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Pre-requisites for Integration  . . . . . . . . . . . . . . .   4
   4.  ACME Integration with EST . . . . . . . . . . . . . . . . . .   4
   5.  ACME Integration with BRSKI . . . . . . . . . . . . . . . . .   8
   6.  ACME Integration with BRSKI Default Cloud Registrar . . . . .  10
   7.  ACME Integration with TEAP  . . . . . . . . . . . . . . . . .  12
   8.  ACME Integration Considerations . . . . . . . . . . . . . . .  15
     8.1.  Service Operators . . . . . . . . . . . . . . . . . . . .  15
     8.2.  CSR Attributes  . . . . . . . . . . . . . . . . . . . . .  16
     8.3.  Certificate Chains and Trust Anchors  . . . . . . . . . .  16
       8.3.1.  EST /cacerts  . . . . . . . . . . . . . . . . . . . .  16
       8.3.2.  TEAP PKCS#7 TLV . . . . . . . . . . . . . . . . . . .  17
     8.4.  id-kp-cmcRA . . . . . . . . . . . . . . . . . . . . . . .  17
     8.5.  Error Handling  . . . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
     10.1.  Denial of Service against ACME infrastructure  . . . . .  20
     10.2.  TLS Channel Bindings . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   ACME [RFC8555] defines a protocol that a certification authority (CA)
   and an applicant can use to automate the process of domain name
   ownership validation and X.509 (PKIX) [RFC5280] 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:

   *  ACME integration with EST [RFC7030]

   *  ACME integration with BRSKI [RFC8995]

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

   *  ACME integration with TEAP [RFC7170]

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

   Optionally, ACME for subdomains [I-D.ietf-acme-subdomains] offers a
   useful optimization when ACME is used to issue certificates for large
   numbers of devices in the same domain; it reduces the domain
   ownership proof traffic as well as the ACME traffic overhead.  This
   is accomplished by completing a challenge against the parent domain
   instead of a challenge against each explicit subdomain.  Use of ACME
   for subdomains is not a 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 defined in DNS Terminology [RFC8499],
   Section 2 and used in this document.  Please refer to [RFC8499],
   Section 2 for a definition of these terms.

   *  Label

   *  Domain Name

   *  Subdomain

   *  Fully-Qualified Domain Name (FQDN)

   The following terms are used in this document:

   *  BRSKI: Bootstrapping Remote Secure Key Infrastructures [RFC8995]

   *  Pledge: from [RFC8366], the prospective device attempting to find
      and securely join a domain.  When shipped, it only trusts
      authorized representatives of the manufacturer.

   *  Certification Authority (CA): An organization that is responsible
      for the creation, issuance, revocation, and management of
      Certificates.  The term applies equally to both Root CAs and
      Subordinate CAs

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   *  CMS: Cryptographic Message Syntax [RFC5652]

   *  CMC: Certificate Management over CMS [RFC5272]

   *  CSR: Certificate Signing Request [RFC2986]

   *  EST: Enrollment over Secure Transport [RFC7030]

   *  MASA: Manufacturer Authorized Signing Authority as defined in
      [RFC8995]

   *  PKCS: Public-Key Cryptography Standards [RFC8017]

   *  PKCS#7: PKCS Cryptographic Message Syntax [RFC2315]

   *  PKCS#10: PKCS Certification Request Syntax [RFC2986]

   *  RA: PKI Registration Authority [RFC2986]

   *  TEAP: Tunneled Extensible Authentication Protocol [RFC7170]

   *  TLV: Type-Length-Value format defined in TEAP [RFC7170]

3.  Pre-requisites for Integration

   In order for the EST server or TEAP server that is part of the BRSKI
   Registrar to use ACME to create new certificates it needs to have the
   ability to satisfy the dns-01 challenges that the ACME will issue.

   The EST Registration Authority (RA) is configured with the DNS domain
   for which it will issue certificates.  In the examples below, it is
   "example.com"

   The EST RA is configured with a credential that allows it to update
   the contents of the DNS domain.  This could be in the form of an
   [RFC3007] credential such as a TSIG key or a SIG(0) key.  It could
   also be some other proprietary credential that allows the EST RA to
   update the database on the DNS provider directly.  As a third option,
   the EST RA could maintain a zone itself, configured as a stealth
   primary, with a DNS NS zone cut pointing at the EST RA's DNS server.

4.  ACME Integration with EST

   EST [RFC7030] defines a mechanism for clients to enroll with a PKI
   Registration Authority by sending Certificate Management over CMS
   (CMC) [RFC5272] messages over HTTP.  EST [RFC7030] Section 1 states:

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   "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 [RFC7030] 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 [RFC7030] 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.ietf-acme-subdomains] and shows the RA proving ownership of a
   parent domain using the 'dns-01' challenge type, with individual
   client certificates being subdomains under that parent domain.  ACME
   [RFC8555], Section 8.4 defines how the ACME client, which in this
   example is the EST RA, and ACME server interact with the DNS system.
   Please refer to ACME [RFC8555] for details on all relevant DNS
   operations.

   Use of [I-D.ietf-acme-subdomains] is an optional 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 server to
   indicate what fields the client should include in the CSR that the
   client sends in the /simpleenroll API.  CSR Attributes handling are
   discussed in Section 8.2.

   If the CSR includes an identifier that the EST RA does not control,
   the RA MUST respond with a 4xx HTTP [RFC9110] error code.  Refer to
   section Section 8.5 for further details on error handling.

   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.

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   This example illustrates, and all subsequent examples in this
   document illustrate, the use of the ACME 'dns-01' challenge type.
   This does not preclude the use of any other ACME challenges, however,
   examples illustrating the use of other challenge types are not
   documented here.

   +--------+             +--------+            +--------+    +-----+
   | Client |             | EST RA |            |  ACME  |    | DNS |
   +--------+             +--------+            | Server |    +-----+
       |                      |                 +--------+       |
       |                      |                      |           |
                  STEP 1: Pre-Authorization of parent domain
       |                      |                      |           |
       |                      | POST /newAuthz       |           |
       |                      |  "example.com"       |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 201 authorizations   |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | Publish DNS TXT      |           |
       |                      | "example.com"        |           |
       |                      |--------------------------------->|
       |                      |                      |           |
       |                      | POST /challenge      |           |
       |                      |--------------------->|           |
       |                      |                      | Verify    |
       |                      |                      |---------->|
       |                      | 200 status=valid     |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | Delete DNS TXT       |           |
       |                      | "example.com"        |           |
       |                      |--------------------------------->|
       |                      |                      |           |
                  STEP 2: Client enrolls against RA
       |                      |                      |           |
       | GET /csrattrs        |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | SEQUENCE {AttrOrOID} |                      |           |
       | SAN OID:             |                      |           |
       | "client.example.com" |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |

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       | "client.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 202 Retry-After      |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
                  STEP 3: RA places ACME order
       |                      |                      |           |
       |                      | POST /newOrder       |           |
       |                      | "client.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 201 status=ready     |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /finalize       |           |
       |                      | PKCS#10 CSR          |           |
       |                      | "client.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK status=valid  |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /certificate    |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK               |           |
       |                      | PKCS#7               |           |
       |                      | "client.example.com" |           |
       |                      |<---------------------|           |
       |                      |                      |           |
                  STEP 4: Client retries enroll
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "client.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | PKCS#7               |                      |           |
       | "client.example.com" |                      |           |
       |<---------------------|                      |           |

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5.  ACME Integration with BRSKI

   BRSKI [RFC8995] is based upon EST [RFC7030] and defines how to
   autonomically bootstrap PKI trust anchors into devices via means of
   signed vouchers.  The signed vouchers are issued by the Manufacturer
   Authorized Signing Authority (MASA) service as described in BRSKI.

   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 the certificate
   identifier.  This ownership proof could have been by fulfilling an
   authorization challenge against the explicit identifier
   "pledge.example.com", or by fulfilling an authorization challenge
   against the parent domain "example.com" leveraging
   [I-D.ietf-acme-subdomains].

   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.  Refer to section Section 8.2
   for more details.

   If the CSR includes an identifier that the EST RA does not control,
   the RA MUST respond with a 4xx HTTP [RFC9110] error code.  Refer to
   section Section 8.5 for further details on error handling.

   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 time to complete.

   This example illustrates the use of the ACME 'dns-01' challenge type.

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   +--------+             +--------+            +--------+     +------+
   | Pledge |             | EST RA |            |  ACME  |     | MASA |
   +--------+             +--------+            | Server |     +------+
       |                      |                 +--------+       |
       |                      |                      |           |
            NOTE: Pre-Authorization of "pledge.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               |                      |           |
       | SAN:                 |                      |           |
       | "pledge.example.com" |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 202 Retry-After      |                      |           |
       |<---------------------|                      |           |
       |                      |                      |           |
            STEP 3: RA places ACME order
       |                      |                      |           |
       |                      | POST /newOrder       |           |
       |                      | "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 201 status=ready     |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /finalize       |           |
       |                      | PKCS#10 CSR          |           |

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       |                      | "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK status=valid  |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /certificate    |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK               |           |
       |                      | PKCS#7               |           |
       |                      | "pledge.example.com" |           |
       |                      |<---------------------|           |
       |                      |                      |           |
            STEP 4: Pledge retries enroll
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | PKCS#7               |                      |           |
       | "pledge.example.com" |                      |           |
       |<---------------------|                      |           |

6.  ACME Integration with BRSKI Default Cloud Registrar

   BRSKI Cloud Registrar [I-D.ietf-anima-brski-cloud] specifies the
   behavior 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.  The est-domain leaf
   defined in [I-D.ietf-anima-brski-cloud] allows the specification of a
   bootstrap EST domain.  In this example, the est-domain extension
   allows the cloud registrar to specify the local domain RA that the
   pledge should connect to for the purposes of EST enrollment.

   For brevity, it assumes that the EST RA has previously proven
   ownership of the certificate identifier.  This ownership proof could
   have been by fulfilling an authorization challenge against the
   explicit identifier "pledge.example.com", or by fulfilling an
   authorization challenge against the parent domain "example.com"
   leveraging [I-D.ietf-acme-subdomains].  The domain ownership
   exchanges between the RA, ACME and DNS are not shown.

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

   This example illustrates the use of the ACME 'dns-01' challenge type.

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

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       |                      | PKCS#10 CSR          |           |
       |                      | "pledge.example.com" |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK status=valid  |           |
       |                      |<---------------------|           |
       |                      |                      |           |
       |                      | POST /certificate    |           |
       |                      |--------------------->|           |
       |                      |                      |           |
       |                      | 200 OK               |           |
       |                      | PKCS#7               |           |
       |                      | "pledge.example.com" |           |
       |                      |<---------------------|           |
       |                      |                      |           |
            STEP 4: Pledge retries enroll
       |                      |                      |           |
       | POST /simpleenroll   |                      |           |
       | PCSK#10 CSR          |                      |           |
       | "pledge.example.com" |                      |           |
       |--------------------->|                      |           |
       |                      |                      |           |
       | 200 OK               |                      |           |
       | PKCS#7               |                      |           |
       | "pledge.example.com" |                      |           |
       |<---------------------|                      |           |

7.  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 [RFC7170] does not 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.ietf-acme-subdomains] and shows the TEAP server proving
   ownership of a parent domain, with individual client certificates
   being subdomains under that parent domain.

   For brevity, it assumes that the TEAP server has previously proven
   ownership of the certificate identifier.  This ownership proof could
   have been by fulfilling an authorization challenge against the
   explicit identifier "client.example.com", or by fulfilling an
   authorization challenge against the parent domain "example.com"
   leveraging [I-D.ietf-acme-subdomains].  The domain ownership
   exchanges between the TEAP server, ACME and DNS are not shown.

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   After establishing the outer TLS tunnel, the TEAP server instructs
   the client to enroll for a certificate by sending a PKCS#10 TLV in
   the body of a Request-Action TLV.  The client then replies with a
   PKCS#10 TLV that contains its CSR.  The TEAP server interacts with
   the ACME server for certificate issuance and returns the certificate
   in a PKCS#7 TLV as per TEAP [RFC7170].

   This example illustrates the use of the ACME 'dns-01' challenge type.

   +------+                +-------------+          +--------+   +-----+
   | Peer |                | TEAP-Server |          |  ACME  |   | DNS |
   +------+                +-------------+          | Server |   +-----+
       |                         |                  +--------|      |
       |                         |                      |           |
            NOTE: Pre-Authorization of "client.example.com" is complete
       |                         |                      |           |
            STEP 1: Establish 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,            |                      |           |

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       |   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=Success,     |                      |           |
       |     Action=Process-TLV, |                      |           |
       |     TLV=PKCS#10}        |                      |           |
       |<------------------------|                      |           |
       |                         |                      |           |
            STEP 2: Enroll for certificate
       |                         |                      |           |
       |                         |                      |           |
       |  EAP-Response/          |                      |           |
       |   Type=TEAP,            |                      |           |
       |   {PKCS#10 TLV:         |                      |           |
       |   "client.example.com"} |                      |           |
       |------------------------>|                      |           |
       |                         | POST /newOrder       |           |
       |                         | "client.example.com" |           |
       |                         |--------------------->|           |
       |                         |                      |           |
       |                         | 201 status=ready     |           |
       |                         |<---------------------|           |
       |                         |                      |           |
       |                         | POST /finalize       |           |
       |                         | PKCS#10 CSR          |           |
       |                         | "client.example.com" |           |
       |                         |--------------------->|           |

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       |                         |                      |           |
       |                         | 200 OK status=valid  |           |
       |                         |<---------------------|           |
       |                         |                      |           |
       |                         | POST /certificate    |           |
       |                         |--------------------->|           |
       |                         |                      |           |
       |                         | 200 OK               |           |
       |                         | PKCS#7               |           |
       |                         | "client.example.com" |           |
       |                         |<---------------------|           |
       |                         |                      |           |
       |  EAP-Request/           |                      |           |
       |   Type=TEAP,            |                      |           |
       |   {PKCS#7 TLV,          |                      |           |
       |    Result TLV=Success}  |                      |           |
       |<------------------------|                      |           |
       |                         |                      |           |
       |  EAP-Response/          |                      |           |
       |   Type=TEAP,            |                      |           |
       |   {Result TLV=Success}  |                      |           |
       |------------------------>|                      |           |
       |                         |                      |           |
       |  EAP-Success            |                      |           |
       |<------------------------|                      |           |

8.  ACME Integration Considerations

8.1.  Service Operators

   The goal of these integrations is enabling issuance of certificates
   with identifiers in a given domain by an ACME server to a client.
   The operator of the EST RA or TEAP server must be able to fulfil ACME
   challenges that prove domain ownership for issuance of certificates
   with identifiers in that domain.  The ACME server is not necessarily
   operated by the organization that controls the domain.

   If the client sends a certificate enrollment request for an
   identifier in a domain that the EST RA or TEAP server does not have
   operational control over, the server MUST reject the request with a
   suitable error immediately, and MUST NOT send a certificate
   enrollment request to the ACME server.  See Section 8.5 for more
   information on error handling.

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8.2.  CSR Attributes

   In all EST and BRSKI integrations, the client MUST send a CSR
   Attributes request to the EST server prior to sending a certificate
   enrollment request.  This enables the server to indicate to the
   client what attributes, and what attribute values, it expects the
   client to include in the subsequent CSR request.  For example, the
   server could instruct the peer what Subject Alternative Name entries
   to include in its CSR.

   EST [RFC7030] is not clear on how the CSR Attributes response should
   be structured, and in particular is not clear on how a server can
   instruct a client to include specific attribute values in its CSR.
   [I-D.ietf-lamps-rfc7030-csrattrs] clarifies how a server can use CSR
   Attributes response to specify specific values for attributes that
   the client should include in its CSR.

   Servers MUST use this mechanism to tell the client what identifiers
   to include in CSR request.  ACME [RFC8555] allows the identifier to
   be included in either CSR Subject or Subject Alternative Name fields,
   however [I-D.ietf-uta-use-san] states that Subject Alternative Name
   field MUST be used.  This document aligns with [I-D.ietf-uta-use-san]
   and Subject Alternate Name field MUST be used.  The identifier MUST
   be a subdomain of a domain that the server has control over and can
   fulfill ACME challenges against.  The leftmost part of the identifier
   MAY be a field that the client presented to the server in an IEEE
   802.1AR [IDevID].

   Servers MAY use this field to instruct the client to include other
   attributes such as specific policy OIDs.  Refer to EST [RFC7030]
   Section 2.6 for further details.

8.3.  Certificate Chains and Trust Anchors

   ACME [RFC8555] Section 9.1 states that ACME servers may return a
   certificate chain to an ACME client where an end entity certificate
   is followed by certificates that certify it.  The trust anchor
   certificate SHOULD be omitted from the chain as it is assumed that
   the trust anchor is already known by the ACME client i.e. the EST or
   TEAP server.

8.3.1.  EST /cacerts

   EST [RFC7030] Section 4.2.3 states that the /simpleenroll response
   contains "only the certificate that was issued".  EST [RFC7030]
   Section 4.1.3 states that the /cacerts response "MUST include any
   additional certificates the client would need to build a chain from
   an EST CA-issued certificate to the current EST CA TA".

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   Therefore, the EST server MUST return only the ACME end entity
   certificate in the /simpleenroll response.  The EST server MUST
   return the remainder of the chain returned by the ACME server to the
   EST server in the /cacerts response to the client, appending the
   trust anchor root CA if necessary.

8.3.2.  TEAP PKCS#7 TLV

   TEAP [RFC7170] Section 4.2.16 allows for download of a PKCS#7
   [RFC2315] certificate chain in response to a TEAP PKCS#10 [RFC2986]
   TLV request.  TEAP also allows for download of multiple PKCS#7
   certificates in response to a TEAP Trusted-Server-Root TLV request.

   The TEAP server MUST return the full ACME client certificate chain in
   the PKCS#7 response to the PKCS#10 TLV request.  The TEAP server MUST
   return the ACME server trust anchor in a PKCS#7 response to a
   Trusted-Server-Root TLV request.  As outlined in Section 8.4, the
   TEAP server SHOULD also return the trust anchor that was used for
   issuing its own identity certificate, if different from the ACME
   server trust anchor.

8.4.  id-kp-cmcRA

   BRSKI [RFC8995] mandates that the id-kp-cmcRA extended key usage OID
   is set in the Registrar (or EST RA) end entity certificate that the
   Registrar uses when signing voucher request messages sent to the
   MASA.  Public ACME servers may not be willing to issue end entity
   certificates that have the id-kp-cmcRA extended key usage OID set.
   In these scenarios, the EST RA may be used by the pledge to get
   issued certificates by a public ACME server, but the EST RA itself
   will need an end entity certificate that has been issued by a
   different CA (e.g. an operator deployed private CA) and that has the
   id-kp-cmcRA OID set.

8.5.  Error Handling

   ACME [RFC8555] Section 6.7 defines multiple errors that may be
   returned by an ACME server to an ACME client.  TEAP [RFC7170]
   Section 4.2.6 defines multiple errors that may be returned by a TEAP
   server to a client in an Error TLV.  EST [RFC7030] Section 4.2.3
   defines how an EST server may return an error encoded in a CMC
   [RFC5272] response, or may return a human readable error in the
   response body.

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   If a client sends a certificate enrollment request to an EST RA for
   an identifier that the RA does not control, the RA MUST respond with
   a suitable 4xx HTTP [RFC9110] error code, and MUST NOT send an
   enrollment request to the ACME server.  The RA MAY include a
   CMCFailInfo [RFC5272] error code of badIdentity.

   If a client sends a certificate enrollment request to a TEAP server
   for an identifier that the TEAP server does not control, the TEAP
   server MUST respond with an Error TLV with error code 1024 Bad
   Identity In Certificate Signing Request, and MUST NOT send an
   enrollment request to the ACME server.

   If the EST RA or TEAP server sends an enrollment request to the ACME
   server and receives an error response from the ACME server, the
   following mapping from ACME errors to CMC [RFC5272] Section 6.1.4
   CMCFailInfo and TEAP [RFC7170] Section 4.2.6 error codes is
   RECOMMENDED.

    +====================+=================+==========================+
    | ACME               | CMCFailInfo     | TEAP Error Code          |
    +====================+=================+==========================+
    | badCSR             | badRequest      | 1025 Bad CSR             |
    +--------------------+-----------------+--------------------------+
    | caa                | badRequest      | 1025 Bad CSR             |
    +--------------------+-----------------+--------------------------+
    | rejectedIdentifier | badIdentity     | 1024 Bad Identity In CSR |
    +--------------------+-----------------+--------------------------+
    | all other errors   | internalCAError | 1026 Internal CA Error   |
    +--------------------+-----------------+--------------------------+

                                  Table 1

9.  IANA Considerations

   This document does not make any requests to IANA.

10.  Security Considerations

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

   *  EST [RFC7030]

   *  BRSKI [RFC8995]

   *  BRSKI Default Cloud Registrar [I-D.ietf-anima-brski-cloud]

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   *  TEAP [RFC7170]

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

   It is expected that 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.  This can be done in a variety of ways,
   including use of [RFC3007] Dynamic updates (with [RFC2136]), secured
   with either SIG(0) [RFC2931], or TSIG keys.  Other proprietary APIs
   and interactions are also common, secured by some local credential.

   A concern is the disclosure of the credential used to update the DNS
   records.  If an attacker gains access to the credential, they can
   provision their own certificates into 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 enrollment automation.
   For example, if all IoT devices in an organization enroll 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.

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10.1.  Denial of Service against ACME infrastructure

   The intermediate 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 public ACME servers have per-day, per-IP and per-
   subjectAltName limits, it is prudent not to request identical
   certificates too often.  When the limits are hit, it is often a sign
   of operator or installer error: Multiple configuration resets
   occurring within a short period of time.

   Many private CA relationships use [RFC8555] as their enrollment
   protocol, and in those cases, there may be very different limits.
   But, rate limiting and caching still has some value in protecting
   external infrastructure.

   The cache should be indexed 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 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.

10.2.  TLS Channel Bindings

   EST [RFC7030], Section 3.5 and TEAP [RFC7170], Section 3.8.2 specify
   mechanisms to bind the PKCS#10 CSR request with the TLS tunnel used
   to transport the CSR request by using the tls-unique value from the
   TLS subsystem.  It is RECOMMENDED that implementations use these tls-
   unique channel binding mechanisms.

11.  References

11.1.  Normative References

   [I-D.ietf-acme-subdomains]
              Friel, O., Barnes, R., Hollebeek, T., and M. Richardson,
              "Automated Certificate Management Environment (ACME) for
              Subdomains", Work in Progress, Internet-Draft, draft-ietf-
              acme-subdomains-07, 1 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-acme-
              subdomains-07>.

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   [I-D.ietf-anima-brski-cloud]
              Friel, O., Shekh-Yusef, R., and M. Richardson, "BRSKI
              Cloud Registrar", Work in Progress, Internet-Draft, draft-
              ietf-anima-brski-cloud-06, 17 May 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-anima-
              brski-cloud-06>.

   [I-D.ietf-lamps-rfc7030-csrattrs]
              Richardson, M., Friel, O., von Oheimb, D., and D. Harkins,
              "Clarification of RFC7030 CSR Attributes definition", Work
              in Progress, Internet-Draft, draft-ietf-lamps-rfc7030-
              csrattrs-02, 8 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              rfc7030-csrattrs-02>.

   [I-D.ietf-uta-use-san]
              Salz, R., "Update to Verifying TLS Server Identities with
              X.509 Certificates", Work in Progress, Internet-Draft,
              draft-ietf-uta-use-san-00, 1 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-use-
              san-00>.

   [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>.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <https://www.rfc-editor.org/info/rfc5272>.

   [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>.

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   [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>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

11.2.  Informative References

   [IDevID]   IEEE, "IEEE Standard for Local and metropolitan area
              networks - Secure Device Identity", n.d.,
              <https://1.ieee802.org/security/802-1ar>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
              <https://www.rfc-editor.org/info/rfc2315>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

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

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

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   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

Authors' Addresses

   Owen Friel
   Cisco
   Email: ofriel@cisco.com

   Richard Barnes
   Cisco
   Email: rlb@ipv.sx

   Rifaat Shekh-Yusef
   Ernst & Young
   Email: rifaat.s.ietf@gmail.com

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

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