



Automated Certificate Management Environment                 G. De Marco
Internet-Draft                                               independent
Intended status: Standards Track                               B. Pitman
Expires: 14 December 2025                                   12 June 2025


    Automatic Certificate Management Environment (ACME) with OpenID
                             Federation 1.0
                draft-demarco-acme-openid-federation-00

Abstract

   The Automatic Certificate Management Environment (ACME) protocol
   allows server operators to obtain TLS certificates for their
   websites, based on a demonstration of control over the website's
   domain via a fully-automated challenge/response protocol.

   OpenID Federation 1.0 defines how to build a trust infrastructure
   using a trusted third-party model.  It uses a trust evaluation
   mechanism to attest the possession of public keys, protocol specific
   metadata and various administrative and technical information related
   to a specific entity.

   This document defines how X.509 certificates associated with a given
   OpenID Federation Entity can be issued by an X.509 Certification
   Authority through the ACME protocol to the organizations which are
   part of a federation built on top of OpenID Federation 1.0.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-demarco-acme-openid-
   federation/.

   Discussion of this document takes place on the Automated Certificate
   Management Environment Working Group mailing list
   (mailto:acme@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/acme/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/acme/.

   Source for this draft and an issue tracker can be found at
   https://github.com/peppelinux/draft-demarco-acme-openid-federation.







De Marco & Pitman       Expires 14 December 2025                [Page 1]

Internet-Draft           ACME OpenID Federation                June 2025


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 14 December 2025.

Copyright Notice

   Copyright (c) 2025 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 (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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Target Audience and Use Cases . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Conventions and Definitions . . . . . . . . . . . . . . . . .   5
   5.  Certificates issued using OpenID Federation . . . . . . . . .   5
   6.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   5
     6.2.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.4.  Entity Configuration Metadata . . . . . . . . . . . . . .   9
       6.4.1.  Issuer Metadata . . . . . . . . . . . . . . . . . . .   9
       6.4.2.  Requestor Metadata  . . . . . . . . . . . . . . . . .  10
     6.5.  OpenID Federation Identifier  . . . . . . . . . . . . . .  12
     6.6.  newOrder Request  . . . . . . . . . . . . . . . . . . . .  12
     6.7.  OpenID Federation Challenge Type  . . . . . . . . . . . .  12



De Marco & Pitman       Expires 14 December 2025                [Page 2]

Internet-Draft           ACME OpenID Federation                June 2025


       6.7.1.  CSR and Certificate Fields  . . . . . . . . . . . . .  15
   7.  Publication of the Certificates within the Federation . . . .  16
   8.  Certificate Lifecycle and Revocation  . . . . . . . . . . . .  16
   9.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  ACME Registry Group  . . . . . . . . . . . . . . . . . .  17
       11.1.1.  ACME Identifier Types  . . . . . . . . . . . . . . .  17
       11.1.2.  ACME Validation Methods  . . . . . . . . . . . . . .  17
       11.1.3.  ACME Error Types . . . . . . . . . . . . . . . . . .  18
     11.2.  Assign X.509 PKIX Other Name . . . . . . . . . . . . . .  18
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  18
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   This document describes extensions to the ACME protocol that
   integrate with OpenID Federation 1.0, allowing an ACME server to
   issue X.509 Certificates associated with a given OpenID Federation
   Entity.  X.509 Certificates can be provided to one or more
   organizations, without having pre-established any direct relationship
   or any stipulation of a contract.

   In a multilateral federation, composed by thousands of entities
   belonging to different organizations, all the participants adhere to
   the same regulation or trust framework.  OpenID Federation 1.0 allows
   each participant to recognize other participants using a trust
   evaluation mechanism, with RESTful services and cryptographic
   materials.

   Considering that a requestor is an entity requesting the issuance of
   an X.509 Certificate to a server and the issuer is the ACME server
   that validates the entitlements of the requestor before issuing the
   X.509 Certificate, this specification defines how ACME and OpenID
   Federation 1.0 can be integrated to allow efficient issuance of X.509
   Certificates to a requestor via the introduction of a new ACME
   challenge type.  The new challenge type extends the ACME protocol in
   the following ways:

   *  It associates a cryptographic key with an OpenID Entity, rather
      than a domain, since the authentication and authorization of the
      requestor is asserted with OpenID Federation 1.0.

   *  It defines how to use and validate a basic OpenID Federation
      component, called Entity Configuration, that is a signed JWT
      published in a well-known resource (/.well-known/openid-
      federation).



De Marco & Pitman       Expires 14 December 2025                [Page 3]

Internet-Draft           ACME OpenID Federation                June 2025


   *  It defines how the OpenID Federation Subordinate Statements can be
      used for the publication of the X.509 Certificates, by a Superior
      Entity, that were previously issued with ACME.

   *  It extends the ACME newOrder resource, as defined in Section 7.4
      of [RFC8555], defining a new payload identifier type called
      openid-federation.

2.  Target Audience and Use Cases

   The audience of the document are the multilateral federations that
   require automatic issuance of X.509 Certificates using an
   infrastructure of trust based on OpenID Federation 1.0.

   This specification can be implemented by:

   *  Federation Entities that join to a federation staging area using
      HTTP only transport to attest themselves as trustworthy, and then
      retrieve X.509 Certificates for their official HTTPS Federation
      Entity ID.

   *  Federation Entities that want to ask for and obtain X.509
      Certificate for use in other protocols.

3.  Terminology

   The terms "Federation Entity", "Trust Anchor", "Entity
   Configuration", "Subordinate Statement", "Superior Entity",
   "Immediate Superior Entity", "Federation Entity Keys", "Trust Mark"
   and "Trust Chain" used in this document are defined in Section 1.2 of
   [OPENID-FED].  The term "CSR" used in this document is defined in
   [RFC2986].  The term Certification Authority used in this document is
   defined in [RFC5280].  The terms "ACME Client" and "ACME Server" are
   defined in [RFC8555].

   The specification also defines the following terms:

   Requestor:  A Federation Entity which wants to request X.509
      certificates.  It operates a web server for hosting its Entity
      Configuration.  It also operates an ACME client, extended
      according to this document.

   Certificate Issuer:  A Federation Entity which issues X.509
      certificates.  It operates a web server for hosting its Entity
      Configuration.  It also operates an ACME server, extended
      according to this document.





De Marco & Pitman       Expires 14 December 2025                [Page 4]

Internet-Draft           ACME OpenID Federation                June 2025


4.  Conventions and Definitions

   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.

5.  Certificates issued using OpenID Federation

   The Certificate Issuer establishes the authorization of a Federation
   Entity to obtain X.509 Certificates for the identifier configured in
   the Requestor's Entity Configuration.

   The Certificate Issuer establishes if a Federation Entity is eligible
   to obtain X.509 Certificates for the identifier configured in the
   Requestor's Entity Configuration.

   The cryptographic keys published within the Requestor's Entity
   Configuration are used to satisfy the Certificate Issuer's challenge.

   The protocol assumes the following discovery preconditions are met.
   The Issuer has the guarantee that:

   1.  The Requestor controls the private key related to the public part
       published in its Entity Configuration.

   2.  The Requestor controls its identifier, having published the
       Entity Configuration.

6.  Protocol Flow

   This section presents the protocol flow.  The protocol flow is
   subdivided in the following phases:

   *  *Discovery*, the Requestor obtains the available CAs within a
      federation, inspecting the ACME issuer entity types.

   *  *Order request*, the Requestor requests a X.509 Certificate to a
      Certificate Issuer using the ACME protocol.

6.1.  Preconditions

   The protocol requires the following preconditions are met.

   1.  The Requestor and the Issuer MUST publish their Entity
       Configuration as defined in Section 9 of [OPENID-FED].




De Marco & Pitman       Expires 14 December 2025                [Page 5]

Internet-Draft           ACME OpenID Federation                June 2025


   2.  The Requestor and the Issuer MUST be able to establish a Trust
       Chain to each other, as defined in Section 4 of [OPENID-FED],
       from their respective Trust Anchors.

   3.  The Issuer MUST implement an ACME server, extended according to
       this document.

   4.  The Requestor MUST publish the entity type acme_requestor in its
       Entity Configuration, according to Section 6.4.2.

   5.  The Issuer MUST publish the entity type acme_issuer in its Entity
       Configuration, according to Section 6.4.1.

6.2.  Discovery

   The Requestor's ACME client may either be configured to use a
   particular ACME server, or to automatically discover a Certificate
   Issuer through the federation.  Requestors that use discovery MAY
   select any entity with an entity type of acme_issuer, or they may
   additionally require that such entities have a valid Trust Mark with
   a particular Trust Mark Identifier.

6.3.  Overview

   1.  The Requestor checks if its superior Federation Entity supports
       the ACME protocol for OpenID Federation 1.0.  If not, the
       Requestor starts the discovery process to find Issuers within the
       federation.

   2.  The Requestor requests and obtains a new nonce from the
       Certificate Issuer, by sending a HTTP HEAD request to the
       Issuer's newNonce resource, as described in Section 7.2 of
       [RFC8555].

   3.  The Requestor creates an ACME Account with the Issuer, as
       described in Section 7.3 of [RFC8555].

   4.  The Requestor begins the X.509 Certificate issuance process by
       sending a HTTP POST request to the Certificate Issuer's newOrder
       resource, as described in Section 6.6, and follows the remainder
       of the ACME protocol as specified in [RFC8555], using the new
       challenge defined in Section 6.7.

   5.  The Certificate Issuer evaluates the trust to the Requestor by
       checking if it is part of its federation.  If not the CSR request
       MUST be rejected, with error type
       urn:ietf:params:acme:error:openIDFederationEntity, and an error
       code of invalid_trust_chain (Section 8.9 of [OPENID-FED]).



De Marco & Pitman       Expires 14 December 2025                [Page 6]

Internet-Draft           ACME OpenID Federation                June 2025


   There are two ways the Certificate Issuer is able to check if a
   Requestor is part of the federation, these are listed below:

   *  The Requestor adds the Trust Chain JWT header parameter related to
      itself, as described in Section 4.3 of [OPENID-FED].  This option
      is RECOMMENDED since it reduces the effort of the Certificate
      Issuer in evaluating the trust to the Requestor.

   *  The Requestor doesn't add the Trust Chain in the request.  The
      Certificate Issuer MUST start Federation Entity Discovery as
      described in Section 9 of [OPENID-FED] to obtain the Trust Chain
      related to the Requestor.

   The following diagram illustrates a successful interaction between
   Issuer and Requestor to retrieve an X.509 Certificate.  The diagram
   assumes the Requestor has already discovered the Issuer, and the
   Requestor has already created an ACME account with the Issuer.

,-----------------.
|Requestor's      |  ,-----------.
|OpenID Federation|  |Requestor's|               ,------------------------.  ,-----------------------.
| Web Server      |  |ACME Client|               |X.509 Certificate Issuer|  |Federation Trust Anchor|
`--------+--------'  `-----+-----'               `-----------+------------'  `-----------+-----------'
        |                  |           POST /acme/new-order  |                           |
        |                  |--------------------------------->                           |
        |                  |                                 |                           |
        |                  |Authorization at                 |                           |
        |                  |/acme/authz/[authz-id]           |                           |
        |                  |Finalize at                      |                           |
        |                  | /acme/order/[order-id]/finalize |                           |
        |                  |<- - - - - - - - - - - - - - - - -                           |
        |                  |                                 |                           |
        |                  | POST /acme/authz/[authz-id]     |                           |
        |                  |--------------------------------->                           |
        |                  |                                 |                           |
        |                  |  openid-federation-01 Challenge |                           |
        |                  |  at /acme/chall/[chall-id]      |                           |
        |                  |<- - - - - - - - - - - - - - - - -                           |
        |                  |                                 |                           |
        |                  ----.                             |                           |
        |                  |   | Sign challenge token        |                           |
        |                  |   | with private key            |                           |
        |                  <---'                             |                           |
        |                  |                                 |                           |
        |                  | POST /acme/chall/[chall-id]     |                           |
        |                  | with signed                     |                           |
        |                  | token and entity ID             |                           |
        |                  | set to Requestor's ID           |                           |



De Marco & Pitman       Expires 14 December 2025                [Page 7]

Internet-Draft           ACME OpenID Federation                June 2025


        |                  |--------------------------------->                           |
        |                  |                                 |                           |
        |                  |      Challenge validation       |                           |
        |                  |      beginning                  |                           |
        |                  |<- - - - - - - - - - - - - - - - -                           |
        |                  |                                 |                           |
        |          GET /.well-known/openid-federation        |                           |
        |<----------------------------------------------------                           |
        |                  |                                 |                           |
        |           Requestor's Entity Configuration         |                           |
        | - - -  - - - - - - - - - - - - - - - - - - - - - - >                           |
        |                  |                                 |                           |
        |                  |           ______________________________________________________
        |                  |           ! OPT  /  If requestor didn't provide Trust Chain |  !
        |                  |           !_____/               |                           |  !
        |                  |           !                     |  Determine Trust Chain    |  !
        |                  |           !                     |  from Issuer's            |  !
        |                  |           !                     |  Trust Anchor to Requestor|  !
        |                  |           !                     |  (Federation Discovery)   |  !
        |                  |           !                     | <------------------------>|  !
        |                  |           !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!
        |                  |                                 |                           |
        |                  |                                 |----.                      |
        |                  |                                 |    | Evaluate Trust Chain |
        |                  |                                 |<---'                      |
        |                  |                                 |                           |
        |                  |                                 |----.                      |
        |                  |                                 |    | Check                |
        |                  |                                 |    | Entity Configuration |
        |                  |                                 |    | sub matches          |
        |                  |                                 |    | Entity identifier    |
        |                  |                                 |<---' in the order         |
        |                  |                                 |                           |
        |                  |                                 |                           |
        |                  |                                 |----.                      |
        |                  |                                 |    | Check challenge      |
        |                  |                                 |    | sig is signed        |
        |                  |                                 |    | with key in          |
        |                  |                                 |<---' Entity Configuration |
        |                  |                                 |                           |
        |  _________________________________________________________________________     |
        |  ! LOOP  /  Poll until authz status                |                      !    |
        |  !      /  is "valid" or "invalid"                 |                      !    |
        |  !_____/         |                                 |                      !    |
        |  !               |    POST-as-GET                  |                      !    |
        |  !               |    /acme/authz/[authz-id]       |                      !    |
        |  !               |--------------------------------->                      !    |
        |  !               |                                 |                      !    |



De Marco & Pitman       Expires 14 December 2025                [Page 8]

Internet-Draft           ACME OpenID Federation                June 2025


        |  !               |           Current authz status  |                      !    |
        |  !               |<---------------------------------                      !    |
        |  !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!    |
        |                  |                                 |                           |
        |                  |                                 |                           |
        |  ___________________________________________________________________________   |
        |  ! OPT  /  If the authz status is "valid"          |                        !  |
        |  !_____/         |                                 |                        !  |
        |  !               | POST                            |                        !  |
        |  !               | /acme/orders/[order-id]/finalize|                        !  |
        |  !               | with CSR                        |                        !  |
        |  !               |--------------------------------->                        !  |
        |  !               |                                 |                        !  |
        |  !               |                                 |----.                   !  |
        |  !               |                                 |    | Check CSR         !  |
        |  !               |                                 |    | validity          !  |
        |  !               |                                 |    | according to      !  |
        |  !               |                                 |    | protocol          !  |
        |  !               |                                 |<---' and CA policy     !  |
        |  !               |                                 |                        !  |
        |  !               |                                 |                        !  |
        |  !               |  Order object with certificate  |                        !  |
        |  !               |  at /acme/cert/[cert-id]        |                        !  |
        |  !               |<- - - - - - - - - - - - - - - - -                        !  |
        |  !               |                                 |                        !  |
        |  !               |       POST /acme/cert/[cert-id] |                        !  |
        |  !               |--------------------------------->                        !  |
        |  !               |                                 |                        !  |
        |  !               |  Newly issued X.509 Certificate |                        !  |
        |  !               |<- - - - - - - - - - - - - - - - -                        !  |
        |  !~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!  |
,--------+--------.  ,-----+-----.               ,-----------+------------.  ,-----------+-----------.
|Requestor's      |  |Requestor's|               |X.509 Certificate Issuer|  |Federation Trust Anchor|
|OpenID Federation|  |ACME Client|               `------------------------'  `-----------------------'
| Web Server      |  `-----------'
`-----------------'

6.4.  Entity Configuration Metadata

   This section describes the metadata a Requestor and Issuer MUST
   publish in their respective Entity Configurations.

6.4.1.  Issuer Metadata

   The Issuer MUST publish its Entity Configuration including the
   acme_issuer Entity Type metadata within it.  The acme_issuer metadata
   contains one parameter, directory_url, which is the URL of the ACME
   Directory, as defined in Section 7.1.1 of [RFC8555].



De Marco & Pitman       Expires 14 December 2025                [Page 9]

Internet-Draft           ACME OpenID Federation                June 2025


   Requestors MUST use the ACME Directory provided in the Issuer's
   Entity Configuration for client configuration of ACME endpoints.

   The following is a non-normative example of an Entity Configuration
   including the acme_issuer metadata:

   {
     "iss": "https://issuer.example.com",
     "sub": "https://issuer.example.com",
     "iat": 1516239022,
     "exp": 1516298022,
     "jwks": {
       "keys": [
         {
           "kty": "RSA",
           "alg": "RS256",
           "use": "sig",
           "kid": "NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs",
           "n": "pnXBOusEANuug6ewezb9J_...",
           "e": "AQAB"
         }
       ]
     },
     "metadata": {
       "acme_issuer": {
         "directory_url": "https://issuer.example.com/acme/directory"
       }
     }
   }

6.4.2.  Requestor Metadata

   The Requestor MUST publish in its Entity Configuration an
   acme_requestor metadata containing a JWK set, according to
   Section 5.2.1 of [OPENID-FED].  The keys in the set are used to
   respond to ACME challenges.

   The following is a non-normative example of an Entity Configuration
   including the acme_requestor metadata and using the jwks metadata
   parameter.











De Marco & Pitman       Expires 14 December 2025               [Page 10]

Internet-Draft           ACME OpenID Federation                June 2025


{
  "iss": "https://requestor.example.com",
  "sub": "https://requestor.example.com",
  "iat": 1516239022,
  "exp": 1516298022,
  "jwks": {
    "keys": [
      {
        "kty": "RSA",
        "alg": "RS256",
        "use": "sig",
        "kid": "NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs",
        "n": "pnXBOusEANuug6ewezb9J_...",
        "e": "AQAB"
      }
    ]
  },
  "metadata": {
    "acme_requestor": {
      "jwks": {
        "keys": [
          {
            "kty": "RSA",
            "kid": "SUdtUndEWVY2cUFDeD...",
            "n": "y_Zc8rByfeRIC9fFZrD...",
            "e": "AQAB"
          },
          {
            "kty": "EC",
            "kid": "MFYycG1raTI4SkZvVDBIMF9CNGw3VEZYUmxQLVN2T21nSWlkd3",
            "crv": "P-256",
            "x": "qAOdPQROkHfZY1daGofOmSNQWpYK8c9G2m2Rbkpbd4c",
            "y": "G_7fF-T8n2vONKM15Mzj4KR_shvHBxKGjMosF6FdoPY"
          }
        ]
      }
    }
  }
}

   The Issuer MUST only use the Requestor's acme_requestor to validate
   an ACME challenge.  Therefore, after completing the challenge, the
   Requestor MAY remove the acme_requestor metadata from its Entity
   Configuration.







De Marco & Pitman       Expires 14 December 2025               [Page 11]

Internet-Draft           ACME OpenID Federation                June 2025


6.5.  OpenID Federation Identifier

   This document defines a new ACME identifier type for OpenID
   Federation entities, openid-federation, whose value is the sub
   parameter of the requestor's Entity Configuration, as defined in
   Section 1.2 of [OPENID-FED].

6.6.  newOrder Request

   The Requestor begins certificate issuance by sending a HTTP POST
   request to the Issuer's newOrder resource, as specified in
   Section 7.4 of [RFC8555].

   A non-normative example of an ACME newOrder request:

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

      {
        "protected": base64url({
          "alg": "ES256",
          "kid": "https://issuer.example.com/acme/acct/evOfKhNU60wg",
          "nonce": "5XJ1L3lEkMG7tR6pA00clA",
          "url": "https://issuer.example.com/acme/new-order"
        }),
        "payload": base64url({
          "identifiers": [
            {
              "type": "openid-federation",
              "value": "https://requestor.example.com"
            }
          ],
          "notBefore": "2016-01-01T00:04:00+04:00",
          "notAfter": "2016-01-08T00:04:00+04:00"
        }),
        "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
      }

6.7.  OpenID Federation Challenge Type

   The OpenID Federation challenge type allows a Requestor to prove
   control of a Federation Entity using the trust evaluation mechanism
   provided by [OPENID-FED].  The Requestor demonstrates control of a
   cryptographic public key published in its OpenID Federation Entity
   Configuration.





De Marco & Pitman       Expires 14 December 2025               [Page 12]

Internet-Draft           ACME OpenID Federation                June 2025


   The openid-federation-01 ACME challenge object has the following
   format:

   type (required, string): The string "openid-federation-01"

   token (required, string): A random value that uniquely identifies the
   challenge.  This value MUST have at least 128 bits of entropy.  It
   MUST NOT contain any characters outside the base64url alphabet as
   described in Section 5 of [RFC4648].  Trailing '=' padding characters
   MUST be stripped.  See [RFC4086] for additional information on
   randomness requirements.

   trustAnchors (optional, array of string): An array of strings
   containing Entity Identifiers of the Issuer's Trust Anchors.  When
   solving the challenge, the Requestor can construct a Trust Chain from
   itself to one of these Trust Anchors.  It is RECOMMENDED that the
   Issuer includes this field to make it easier for the Requestor to
   construct a Trust Chain.

   A non-normative example of a challenge with trustAnchors specified:

      {
        "type": "openid-federation-01",
        "url": "https://issuer.example.com/acme/chall/prV_B7yEyA4",
        "status": "pending",
        "token": "LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0",
        "trustAnchors": [
          "https://trust-anchor-1.example.com",
          "https://trust-anchor-2.example.com"
        ]
      }

   The openid-federation-01 challenge MUST NOT be used to issue X.509
   Certificates for any identifiers except openid-federation
   identifiers.

   The openid-federation identifier MUST NOT be validated except by the
   openid-federation-01 challenge.

   The Requestor responds to the challenge with an object with the
   following format:

   sig (required, string): the compact JSON serialization (as described
   in Section 7.1 of [RFC7515]) of a JWS, signing the key authorization
   encoded in UTF-8.  The key authorization is computed from the token
   in the challenge and the Requestor's ACME account key, as defined in
   Section 8.1 of [RFC8555].  The signature must be made by one of the
   keys published in the Requestor's acme_requestor metadata in its



De Marco & Pitman       Expires 14 December 2025               [Page 13]

Internet-Draft           ACME OpenID Federation                June 2025


   Entity Configuration, as specified in Section 6.4.2.  The JWS MUST
   include a kid header parameter corresponding to the key used to sign
   the key authorization and a typ header parameter set to "signed-acme-
   challenge+jwt".

   trustChain (optional, array of string): an array of strings
   containing signed JWTs, representing a Trust Chain from the Requestor
   to one of the Issuer's Trust Anchors (see Section 4 of [OPENID-FED]).
   The Entity Configuration of the Trust Chain subject MUST contain
   acme_requestor metadata that is valid under the Trust Chain's
   resolved metadata policy (Section 6.1 of [OPENID-FED]) and contains
   the key used to compute sig.  It is RECOMMENDED that the Requestor
   includes this field.  If the Requestor cannot construct a Trust Chain
   to one of the Trust Anchors indicated by the Issuer, or if no Trust
   Anchors were indicated, it MAY use some other Trust Anchor that it
   believes the Issuer trusts.  If the Requestor cannot construct a
   Trust Chain to any Trust Anchor, it MAY omit the trustChain field
   from the challenge response.

   A non-normative example for an authorization with trustChain
   specified:

      POST /acme/chall/prV_B7yEyA4
      Host: issuer.example.com
      Content-Type: application/jose+json

      {
        "protected": base64url({
          "alg": "ES256",
          "kid": "https://issuer.example.com/acme/acct/evOfKhNU60wg",
          "nonce": "UQI1PoRi5OuXzxuX7V7wL0",
          "url": "https://issuer.example.com/acme/chall/prV_B7yEyA4"
        }),
        "payload": base64url({
         "sig": "wQAvHlPV1tVxRW0vZUa4BQ...",
         "trustChain": ["eyJhbGciOiJFU...", "eyJhbGci..."]
        }),
        "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
      }

   On receiving a challenge response, the Certificate Issuer verifies
   that the Requestor is trusted.  If the Requestor did not provide a
   trustChain, the Issuer MUST perform Federation Entity Discovery
   (Section 10 of [OPENID-FED]) to obtain a Trust Chain for the
   Requestor.

   Once it has obtained a Trust Chain, the Issuer verifies:




De Marco & Pitman       Expires 14 December 2025               [Page 14]

Internet-Draft           ACME OpenID Federation                June 2025


   *  That the Requestor's acme_requestor metadata is valid under the
      Trust Chain's resolved metadata policy (Section 6.1 of
      [OPENID-FED]).

   *  That the requested openid-federation identifier value matches the
      sub parameter of the Requestor's Entity Configuration.

   *  That the sig field of the payload is the compact JSON
      serialization of a JWS signing the key authorization, signed with
      a key from the Requestor's acme_requestor metadata (Section 6.4.2)
      whose kid matches the kid claim in the challenge response.

   If all of the above verifications succeed, then the validation is
   successful.  Otherwise, it has failed.  In either case, the
   Certificate Issuer responds according to Section 7.5.1 of [RFC8555].

   A non-normative example for the challenge object post-validation:

      {
        "type": "openid-federation-01",
        "url": "https://issuer.example.com/acme/chall/prV_B7yEyA4",
        "status": "valid",
        "validated": "2024-10-01T12:05:13.72Z",
        "token": "LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0"
      }

6.7.1.  CSR and Certificate Fields

   Depending on the Certificate Issuer's X.509 Certificate profile, the
   CSR and X.509 Certificate MAY associate the X.509 Certificate to the
   Federation Entity by including the Entity ID in the X.509
   Certificate.  To do so, the Issuer includes a Subject Alternative
   Name extension containing an otherName with a type-id of id-on-
   OpenIdFederationEntityId.  The value of the name is an Octet String
   containing the UTF-8 encoding of the Entity ID (i.e., the URI in the
   corresponding openid-federation identifier from the newOrder
   request).

      id-on-OpenIdFederationEntityId OBJECT IDENTIFIER ::= { id-on XXX }

      OpenIdFederationEntityId ::= UTF8String










De Marco & Pitman       Expires 14 December 2025               [Page 15]

Internet-Draft           ACME OpenID Federation                June 2025


7.  Publication of the Certificates within the Federation

   The X.509 Certificates issued by federation Immediate Superior
   Entities pertaining to one or more Federation Entity Keys in control
   of their Subordinates MAY publish this information by including the
   x5c member in each JWK contained within the matching Subordinate
   Statement.  The contents of the published x5c member, including
   whether it contains a full or partial Trust Chain, and if so, to what
   Trust Anchor, are policy decisions out of scope for this document.

8.  Certificate Lifecycle and Revocation

   The identity of the Requestor is verified through proof of possession
   of a private key corresponding to a public key attested within a
   Trust Chain.  It is up to the Certificate Issuer to decide the
   expiration time of the X.509 Certificate.  In some cases, and when
   required, it MAY be set to match the expiration of the Trust Chain.

   A Requestor SHOULD request the revocation of its X.509 Certificate
   when the related cryptographic material is revoked.  The Requestor
   SHOULD publish the revoked or expired cryptographic keys in the
   Federation Historical Key Registry.

   The X.509 Certificate revocation request is defined in Section 7.6 of
   [RFC8555].

9.  Errors

   This document defines one new error type URI to be used in problem
   documents [RFC9457], as described in Section 6.7 of [RFC8555].

   The error type urn:ietf:params:acme:error:openIDFederationEntity can
   be used to encapsulate any OAuth error code returned while resolving
   OpenID Federation Entities.  The title of this error type is "OpenID
   Federation Error".  The detail member of the problem document MAY
   include the description of the particular OAuth error code that
   caused the error.  The problem document for this error type SHOULD
   include an extension member named error_code.  The error_code member
   SHOULD be set to the OAuth error code, taken from the IANA "OAuth
   Extensions Error Registry" [IANA-OAUTH].

10.  Security Considerations

   The openid-federation-01 challenge defined in Section 6.7 defends
   against replay attacks by malicious ACME servers because the
   signature in challenge responses is over an ACME key authorization,
   which binds the ACME account key.




De Marco & Pitman       Expires 14 December 2025               [Page 16]

Internet-Draft           ACME OpenID Federation                June 2025


   The cryptographic keys in the acme_requestor metadata SHOULD NOT be
   reused for other purposes than signing responses to acme-
   federation-01 challenges.  For example, the same keys SHOULD NOT be
   reused in the issued X.509 Certificate.  If the keys are reused for
   other purposes, cross-protocol attacks MUST be considered.

   The cryptographic keys in the acme_requestor metadata SHOULD be
   rotated periodically.

11.  IANA Considerations

   IANA is kindly asked to make the following updates to registries:

11.1.  ACME Registry Group

   The following updates are all assignments in the "Automated
   Certificate Management Environment (ACME) Protocol" registry group
   [IANA-ACME].

11.1.1.  ACME Identifier Types

   IANA is asked to add to the "ACME Identifier Types" registry, defined
   in Section 9.7.7 of [RFC8555], the entry below, as specified here in
   Section 6.5:

                   +===================+===============+
                   | Label             | Reference     |
                   +===================+===============+
                   | openid-federation | this document |
                   +-------------------+---------------+

                                  Table 1

11.1.2.  ACME Validation Methods

   IANA is also asked to add to the "ACME Validation Methods" registry,
   defined in Section 9.7.8 of [RFC8555], the entry below, as specified
   here in Section 6.7:

       +======================+===================+===============+
       | Label                | Identifier Type   | Reference     |
       +======================+===================+===============+
       | openid-federation-01 | openid-federation | this document |
       +----------------------+-------------------+---------------+

                                 Table 2





De Marco & Pitman       Expires 14 December 2025               [Page 17]

Internet-Draft           ACME OpenID Federation                June 2025


11.1.3.  ACME Error Types

   IANA is also asked to add to the "ACME Error Types" registry, defined
   in Section 9.7.4 of [RFC8555], the entry below, as specified here in
   Section 9:

     +========================+=========================+===========+
     | Type                   | Description             | Reference |
     +========================+=========================+===========+
     | openIDFederationEntity | An error occurred while | this      |
     |                        | resolving an OpenID     | document  |
     |                        | Federation entity       |           |
     +------------------------+-------------------------+-----------+

                                 Table 3

11.2.  Assign X.509 PKIX Other Name

   IANA is asked to add to the "PKIX Other Name Forms" registry
   (1.3.6.1.5.5.7.8 (https://www.iana.org/assignments/smi-numbers/smi-
   numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.8)) the entry below, as
   specified here in Section 6.7.1

       +=========+================================+===============+
       | Decimal | Description                    | Reference     |
       +=========+================================+===============+
       | TBA     | id-on-OpenIdFederationEntityId | this document |
       +---------+--------------------------------+---------------+

                                 Table 4

12.  Normative References

   [IANA-ACME]
              IANA, "Automated Certificate Management Environment (ACME)
              Protocol", n.d., <https://www.iana.org/assignments/acme>.

   [IANA-OAUTH]
              IANA, "OAuth Parameters", n.d.,
              <https://www.iana.org/assignments/oauth-parameters>.

   [OPENID-FED]
              Hedberg, R., Jones, M. B., Solberg, A. Å., Bradley, J., De
              Marco, G., and V. Dzhuvinov, "OpenID Federation 1.0 -
              draft 41", 4 December 2024,
              <https://openid.net/specs/openid-federation-1_0-41.html>.





De Marco & Pitman       Expires 14 December 2025               [Page 18]

Internet-Draft           ACME OpenID Federation                June 2025


   [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/rfc/rfc2119>.

   [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/rfc/rfc2986>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/rfc/rfc4086>.

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

   [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/rfc/rfc5280>.

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

   [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/rfc/rfc8174>.

   [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/rfc/rfc8555>.

   [RFC9457]  Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
              for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023,
              <https://www.rfc-editor.org/rfc/rfc9457>.

Contributors

   David Cook
   ISRG
   Email: divergentdave@gmail.com




De Marco & Pitman       Expires 14 December 2025               [Page 19]

Internet-Draft           ACME OpenID Federation                June 2025


   Ameer Ghani
   ISRG
   Email: inahga@letsencrypt.org


   J.C. Jones
   ISRG
   Email: ietf@insufficient.coffee


   Tim Geoghegan
   ISRG
   Email: timgeog+ietf@gmail.com


Authors' Addresses

   Giuseppe De Marco
   independent
   Email: demarcog83@gmail.com


   Brandon Pitman
   Email: bran@bran.land



























De Marco & Pitman       Expires 14 December 2025               [Page 20]
