



Web Authorization Protocol                            A. Schwenkschuster
Internet-Draft                                             P. Kasselmann
Intended status: Standards Track                                   SPIRL
Expires: 2 January 2026                                      1 July 2025


                   OAuth SPIFFE Client Authentication
           draft-schwenkschuster-oauth-spiffe-client-auth-00

Abstract

   This specification profiles the Assertion Framework for OAuth 2.0
   Client Authentication and Authorization Grants [RFC7521] and JWT
   Profile for OAuth 2.0 Client Authentication and Authorization Grants
   [RFC7523] to enable the use of SPIFFE Verifiable Identity Documents
   (SVIDs) as client credentials in OAuth 2.0.  It defines how OAuth
   clients with SPIFFE credentials can authenticate to OAuth
   authorization servers using their JWT-SVIDs or X.509-SVIDs without
   the need for client secrets.  This approach enhances security by
   enabling seamless integration between SPIFFE-enabled workloads and
   OAuth authorization servers while eliminating the need to distribute
   and manage shared secrets such as static client secrets.

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-schwenkschuster-oauth-spiffe-
   client-auth/.

   Discussion of this document takes place on the Web Authorization
   Protocol Working Group mailing list (mailto:oauth@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/oauth/.
   Subscribe at https://www.ietf.org/mailman/listinfo/oauth/.

   Source for this draft and an issue tracker can be found at
   https://github.com/arndt-s/oauth-spiffe-client-authentication.

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/.



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   Internet-Drafts are draft documents valid for a maximum of six months
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   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  OAuth Client Authentication Using SPIFFE  . . . . . . . . . .   4
     3.1.  Client Authentication with JWT-SVIDs  . . . . . . . . . .   4
       3.1.1.  JWT-SVID example  . . . . . . . . . . . . . . . . . .   5
     3.2.  Client Authentication using X509-SVID . . . . . . . . . .   6
       3.2.1.  X509-SVID Example . . . . . . . . . . . . . . . . . .   7
   4.  SPIFFE Trust Establishment and Client Registration  . . . . .   9
   5.  SPIFFE Key Distribution and Validation  . . . . . . . . . . .   9
     5.1.  SPIFFE Bundle Endpoint  . . . . . . . . . . . . . . . . .   9
       5.1.1.  Example . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Alternative methods to avoid  . . . . . . . . . . . . . .  12
       5.2.1.  SPIFFE Workload API . . . . . . . . . . . . . . . . .  13
       5.2.2.  Manual configuration  . . . . . . . . . . . . . . . .  13
       5.2.3.  Using the system trust store  . . . . . . . . . . . .  13
       5.2.4.  Using the JWT-SVID iss claim  . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15






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1.  Introduction

   Traditional OAuth client authentication typically relies on client
   secrets or private key JWT authentication, both require an out of
   band distribution of secret material to the OAuth client.  In modern
   cloud-native architectures where identity is managed by SPIFFE
   (Secure Production Identity Framework for Everyone), there is a need
   to provision additional secret material for OAuth clients when
   attested identifiers and credentials such as SVIDs are already
   available.

   This specification profiles the Assertion Framework for OAuth 2.0
   Client Authentication and Authorization Grants [RFC7521] to allow
   SPIFFE-enabled workloads to use their SPIFFE Verifiable Identity
   Documents (SVIDs) — either X.509 certificates or JWT tokens — as
   client credentials for OAuth 2.0 client authentication.  JWT tokens
   make use of the profiled version of [RFC7523] - the JWT Profile for
   OAuth 2.0 Client Authentication and Authorization Grants [RFC7523].

   This profile focuses specifically on client authentication rather
   than authorization grants.  This focus is deliberate for several
   reasons:

   1.  In modern service-oriented architectures, services often need to
       authenticate as themselves to OAuth authorization servers.

   2.  Using SPIFFE as client authentication towards OAuth 2.0
       authorization servers is a bridge between SPIFFE, which covers
       workload identity and OAuth, which covers human identity.

   3.  Using SPIFFE as authorization grants for authorization requests
       where the workload itself is the resource owner is covered by
       other specifications, such as [Headless_JWT].

   The SPIFFE profile for client authentication enables seamless
   integration between SPIFFE-based and OAuth-based systems, allowing
   applications to leverage both ecosystems without requiring additional
   credential management.  It also enables a more secure authentication
   method by leveraging cryptographically verifiable credentials rather
   than shared secrets.

2.  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.



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2.1.  Terminology

   This specification uses the terms defined in OAuth 2.0 [RFC6749], the
   Assertion Framework for OAuth 2.0 [RFC7521], the JWT profile of it
   [RFC7523], and the SPIFFE specifications.  In particular, the
   following terms are particularly relevant:

   *Trust Domain*: As defined in SPIFFE; A trust domain represents a
   single trust root.  All SVIDs issued within a trust domain are
   verifiable via the trust domain's keys.

   *SPIFFE ID*: A unified resource identifier that uniquely and
   specifically identifies a workload using the spiffe scheme.  See
   [SPIFFE_ID] for details.

   *SVID*: A SPIFFE Verifiable Identity Document.  This document
   specifies the use of two types of SVIDs:

   *  *X.509-SVID*: An X.509 certificate that contains a SPIFFE ID in
      the URI SAN extension.  See [SPIFFE_X509] for details.

   *  *JWT-SVID*: A JSON Web Token (JWT) that contains a SPIFFE ID in
      the sub claim.  See [SPIFFE_JWT] for details.

   *SPIFFE Bundle*: A collection of public keys and associated metadata
   that allow validation of SVIDs issued by a trust domain.

   *SPIFFE Bundle Endpoint*: A URL that serves a SPIFFE bundle for a
   trust domain.

3.  OAuth Client Authentication Using SPIFFE

   This section describes how SPIFFE identity documents can be used for
   OAuth 2.0 client authentication, following the patterns established
   in [RFC7521] and, in case of JWT-SVID [RFC7523].

   OAuth 2.0 client authentication is used to authenticate the client to
   the authorization server when making requests to the token endpoint.
   When using SPIFFE for client authentication, the client presents its
   SVID (either JWT-SVID or X.509-SVID) to prove its identity.

3.1.  Client Authentication with JWT-SVIDs

   JWT-SVID based authentication naturally follows the JWT Profile for
   OAuth 2.0 Client Authentication [RFC7523], with specific adaptations
   for SPIFFE JWT-SVIDs.  [RFC7521] remains valid.





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   To identify the assertion content as a JWT-SVID this specification
   establishes the following client assertion type as an OAuth URI
   according to [RFC6755]:

   urn:ietf:params:oauth:client-assertion-type:jwt-spiffe

   Based on [RFC7523] the following request parameters MUST be present
   to perform client authentication in the context of this
   specification:

   *  client_assertion_type: MUST be set to
      urn:ietf:params:oauth:client-assertion-type:jwt-spiffe.

   *  client_assertion: MUST be a single SPIFFE JWT-SVID.

   To validate JWT-SVID client authentication requests the authorization
   server MUST:

   1.  Verify that the JWT is well-formed and contains all required
       claims (SPIFFE ID in sub, aud, and exp).

   2.  Verify that the JWT has not expired (check the exp claim).

   3.  Verify that the aud claim equals the endpoint of the request
       without fragment and query parameters.

   4.  Verify the JWT signature using the signing keys of the trust
       domains according to Section 5.

   5.  Verify that the SPIFFE ID in the sub claim matches a registered
       client identifier or is associated with a registered client
       identifier.

3.1.1.  JWT-SVID example

   The following examples illustrates an authorization_code request to
   the token endpoint of an OAuth 2.0 authorization server leveraging a
   SPIFFE JWT-SVID to authenticate the client.

POST /token HTTP/1.1
Host: as.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&
code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&
client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3A
client-assertion-type%3Ajwt-spiffe&
client_assertion=eyJhbGciOiJFUzI1NiIsImtpZCI6IjR2QzhhZ3ljSHU2cm5rRUVKWUFINlZ1Q2U0Sm9Ta1BWIiwidHlwIjoiSldUIn0.eyJhdWQiOlsiaHR0cHM6Ly9hcy5leGFtcGxlLmNvbS90b2tlbiJdLCJleHAiOjE3NDcxMjQ1NDMsImlhdCI6MTc0NzEyNDI0Mywic3ViIjoic3BpZmZlOi8vZXhhbXBsZS5vcmcvbXktb2F1dGgtY2xpZW50In0.Xlv5lW4cbxDsQk4l0paewG4nXOR7MxF_FMn_c27DX45Bxr2HUZf9a6Untfq5S47xpwbw495HBL6_1Lc6TMJxmw



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   For clarify, the SPIFFE-JWT header and body decoded:

   {
     "alg": "ES256",
     "kid": "4vC8agycHu6rnkEEJYAH6VuCe4JoSkPV",
     "typ": "JWT"
   }.
   {
     "aud": [
       "https://as.example.com/token"
     ],
     "exp": 1747124543,
     "iat": 1747124243,
     "sub": "spiffe://example.org/my-oauth-client"
   }

3.2.  Client Authentication using X509-SVID

   X.509-SVID based authentication uses mutual TLS as defined in OAuth
   2.0 Mutual-TLS Client Authentication [RFC8705], with specific
   adaptations for SPIFFE X.509-SVIDs.

   To authenticate using an X.509-SVID, the client establishes a mutual
   TLS connection with the authorization server using its X.509-SVID as
   the client certificate.  The authorization server validates the
   client certificate as an X.509-SVID and extracts the SPIFFE ID from
   the URI SAN.  The server certificate MUST be validated by the client
   using its system trust store, and NOT the SPIFFE trust bundle.

   The request MUST include the client_id parameter containing the
   SPIFFE-ID of the client.  It MUST match the URI SAN of the presented
   X509-SVID client credential.

   The server validates the client certificates according the following
   rules

   1.  Perform standard X.509 path validation against the trust anchors
       according to Section 5.

   2.  Verify that the certificate contains exactly one URI SAN with a
       valid SPIFFE ID.

   3.  Verify that the certificate is a leaf certificate (Basic
       Constraints extension has CA=FALSE).

   4.  Verify that the certificate has the digitalSignature key usage
       bit set.




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   5.  Verify that the SPIFFE ID in the URI SAN matches a registered
       client identifier or is associated with a registered client
       identifier.

3.2.1.  X509-SVID Example

   The following request uses a refresh token to obtain a new access
   token.  The client is spiffe://example.org/my-oauth-client and is
   authenticted by performing this request over a mutual TLS connection.

   POST /token HTTP/1.1
   Host: as.example.com
   Content-Type: application/x-www-form-urlencoded

   grant_type=refresh_token&
   refresh_token=tGzv3JOkF0XG5Qx2TlKWIA&
   client_id=spiffe://example.org/my-oauth-client

   For clarity, the presented X509-SVID client certificate to the server
   decoded via openssl x509 -text is:































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Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            dd:48:ec:d4:a4:c6:b2:ea:8e:9b:54:35:e8:30:65:7b
        Signature Algorithm: ecdsa-with-SHA256
        Issuer: C=US, O=SPIFFE, serialNumber=6968729192859147614695638370388029008
        Validity
            Not Before: May 16 11:26:11 2025 GMT
            Not After : May 16 12:26:21 2025 GMT
        Subject: C=US, O=SPIRE
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:c2:0b:b6:8e:47:9a:20:ab:33:f1:a9:a5:77:97:
                    fa:a0:95:7d:2c:9f:e9:94:3d:e9:ed:e6:35:52:7f:
                    ff:82:34:74:20:97:5a:1b:4e:87:5f:32:3e:9d:da:
                    60:6a:05:8b:86:9d:0b:59:5f:67:be:93:3b:26:de:
                    ea:1e:18:98:96
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment, Key Agreement
            X509v3 Extended Key Usage:
                TLS Web Server Authentication, TLS Web Client Authentication
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Subject Key Identifier:
                D8:7A:2F:8B:E3:CF:08:83:EA:DD:5E:0A:59:33:6E:4C:E0:CC:6B:AD
            X509v3 Authority Key Identifier:
                C2:41:49:B0:ED:E0:94:7B:FA:7D:C2:F1:02:24:20:B9:1E:3D:56:FA
            X509v3 Subject Alternative Name:
                URI:spiffe://example.org/my-oauth-client
    Signature Algorithm: ecdsa-with-SHA256
    Signature Value:
        30:44:02:20:48:c3:5f:68:b2:c5:5d:96:c4:96:32:37:1f:af:
        b8:1c:1c:45:ad:41:26:dd:e2:92:b5:73:62:83:34:c6:16:2a:
        02:20:0f:48:02:8e:6b:1d:09:01:80:d8:85:2b:ca:25:c6:2c:
        9e:f2:27:c2:3c:e4:03:58:a8:47:21:f6:3c:5e:7a:c8










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4.  SPIFFE Trust Establishment and Client Registration

   This specification requires previously established trust between the
   OAuth 2.0 Authorization Server and the SPIFFE Trust Domain.  This
   needs to happen out of band and is not in scope of this
   specification.  However, the mechanisms of key distribution is in
   scope and described in Section 5.

   Similar to the trust establishment, corresponding OAuth clients need
   to be established prior of using SPIFFE as client authentication.
   This is also out of scope, implementors may for example choose to
   levarage OAuth 2.0 dynamic client registration according to [RFC7591]
   or configure them out of band.

5.  SPIFFE Key Distribution and Validation

   This section describes how an authorization server verifies the
   signature of an X509 or JWT-SVID.  It recommends two SPIFFE-native
   approaches.

   Trust bundles in general MUST be keyed by the trust domain identifier
   to prevent mix up between trust domain and their corresponding
   bundles.  The 2 approaches can be used in conjunction, for instance:

Trust domain "example.org": Workload API at unix:///var/secrets/spiffe/agent.sock
Trust domain "production": SPIFFE Bundle Endpoint at https://example.com/auth/spiffe/bundle.json

5.1.  SPIFFE Bundle Endpoint

   The SPIFFE Bundle Endpoint exposes the signing keys for X509-SVIDs
   and JWT-SVIDs over HTTP via a JSON Web Key Set according to
   [RFC7517].

   Server authentication on this endpoint is available in two flavors.
   For the sake of interoperability, in the context of this
   specification the WebPKI flavor MUST be used.  This effectively means
   that the server certificate of the bundle endpoint is trusted by the
   authorization server accessing it.  See Sec 5.2.1 of
   [SPIFFE_FEDERATION] for details.

   The authorization server SHOULD periodically poll the bundle endpoint
   to retrieve updated trust bundles, following the refresh hint and
   period provided in the bundle.  See [SPIFFE_FEDERATION] for details.

   The SPIFFE bundle endpoint cannot be derived from the JWT-SVID and
   X509-SVID and MUST be configured manually out of band.  Bundle
   endpoints MUST be keyed by the trust domain identifier.




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5.1.1.  Example

   The following examples showcase how the Authorization Server can
   perform key discovery for the trust domain example.org.  Important to
   note is the difference between example.org trust domain and
   example.com location for the SPIFFE Bundle Endpoint.  This highlights
   the importance of explicit configuration and undermines the fact that
   the SPIFFE Bundle Endpoint cannot be derived or discovered from the
   X509-SVID without explicit configuration.

   Example configuration at the OAuth Authorization Server in the JSON
   format

   {
     "example.org": {
       "spiffe_bundle_endpoint": {
         "url": "https://example.com/bundle.json"
       }
     }
   }

      Note difference between example.org and example.com

   Example SPIFFE Bundle Endpoint request, response:



























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GET /bundle.json HTTP/1.1
Host: example.com

{
  "keys": [
    {
      "use": "x509-svid",
      "kty": "EC",
      "crv": "P-384",
      "x": "9XBzty8W_ex4Xr0RdzUBgie_okdaUTheSF0PQvVAaTsXaP1J7yv0Dhlaw45I7Cv9",
      "y": "HP21HOmMxIlZ0XeqsOl9sM5H57HBQWu0bINXfw4jdeHdB5vk1XyNyBQQxeUpSxhn",
      "x5c": [
        " "
      ]
    },
    {
      "use": "jwt-svid",
      "kty": "EC",
      "kid": "6d02Vc2oU62mXVH5nlggHGLmfIhrlnNW",
      "crv": "P-256",
      "x": "S2V42XlFjNp30CFmOidbWQT9IpZHqJ8JuuJgDBvkdZA",
      "y": "vN0y5TK36VRxZo_E3Gc7S5c0jIRIaHZ53f2UiJ1NFto"
    }
  ],
  "spiffe_sequence": 10,
  "spiffe_refresh_hint": 300
}

      The use parameter in the JSON Web Key indicates the credential
      format the key is indended for.  Multiple keys of the same use can
      be present.

   The X509-SVID signing certificate (.keys[0].x5c[0] from response
   above) in text form:

















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Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            5c:4b:d5:2d:f9:c1:6e:78:2c:32:a6:bb:6c:73:f0:b8:f4:be:13:09
        Signature Algorithm: ecdsa-with-SHA512
        Issuer: C=US, O=SPIFFE
        Validity
            Not Before: May 16 11:23:19 2025 GMT
            Not After : May 15 11:23:19 2030 GMT
        Subject: C=US, O=SPIFFE
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (384 bit)
                pub:
                    04:ef:3f:db:67:2b:e8:5c:a1:64:23:e7:f2:fd:f0:
                    3b:16:55:68:17:55:17:d4:bd:cd:6d:04:fd:cc:8f:
                    99:31:f7:8c:ac:b0:1e:31:60:18:45:32:8b:a1:17:
                    4b:2f:01:75:27:6c:3f:c3:a5:b9:da:56:fb:29:54:
                    63:cb:08:96:81:35:0e:96:04:03:40:fe:51:0d:26:
                    da:d5:99:6c:8f:c2:45:43:cb:2c:b4:8d:9b:68:78:
                    9f:c0:2d:68:36:b8:5e
                ASN1 OID: secp384r1
                NIST CURVE: P-384
        X509v3 extensions:
            X509v3 Subject Key Identifier:
                8D:79:D2:26:5E:4C:83:30:40:C7:E9:1D:E1:35:12:F6:60:CF:0B:DB
            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Key Usage: critical
                Certificate Sign, CRL Sign
            X509v3 Subject Alternative Name:
                URI:spiffe://example.org
    Signature Algorithm: ecdsa-with-SHA512
    Signature Value:
        30:64:02:30:0a:e9:fd:d4:cd:99:52:90:cb:14:86:93:4e:f8:
        02:52:d6:17:12:9f:2e:65:99:0e:38:b6:b9:a6:fe:43:0f:60:
        30:04:87:ec:24:20:80:a4:75:ee:3c:ad:9d:a2:72:0d:02:30:
        55:93:0e:14:8c:47:47:3b:74:7c:a7:2a:2a:96:1d:a4:85:46:
        4f:3f:95:a4:c2:ab:3c:2e:04:b3:1b:cf:02:0f:33:fc:dd:dc:
        d5:2f:44:c8:2a:dc:ce:3f:c5:c6:89:d0

      Arndt: Bundle doesn't match X509-SVID.  This needs to be fixed.

5.2.  Alternative methods to avoid

   The following key distribution mechanisms are alternatives and SHOULD
   be avoided for interopability reasons.



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5.2.1.  SPIFFE Workload API

   The SPIFFE Workload API allows workloads to retrieve a trust bundle.
   It requires the authorization server to be part of a SPIFFE trust
   domain and be considered a workload within it.  The SPIFFE Workload
   API is build in a way that the workload proactively retrieves trust
   bundles updates and does not need to poll them, which reduces the
   time to distribute them.  In addition to the trust bundle of the
   trust domain the workload resides in, the SPIFFE Workload API also
   allows to retrieve trust bundles from federated trust domains.

   This approach is NOT RECOMMENDED for OAuth SPIFFE Client
   Authentication for several reasons:

   *  OAuth Authorization Server needs to be a workload within a SPIFFE
      trust domain, which is a significant limitation for deployment
      scenarios.

   *  Federated trust domain bundles create ambiguity about how they are
      handled.  When distributed via the SPIFFE Workload API the trust
      relationship and points where they are established become
      ambiguous.

5.2.2.  Manual configuration

   In small, static environments the authorization server MAY be
   configured with the SPIFFE bundles manually.  This approach requires
   human interaction to set up, rotate and manage keying material and is
   thus generally NOT RECOMMENDED.

5.2.3.  Using the system trust store

   X509-SVIDs MUST NOT be validated using the system trust store.  The
   SPIFFE ID carried in the URI SAN is rarely a verifiable attribute in
   the broader X.509 ecosystem.  Using the system trust store as trust
   anchor would allow ANY certificate authority in it to issue a trusted
   X509-SVID for ANY SPIFFE ID.  In comparison: using SPIFFE-native
   validation methods restricts the signing of SPIFFE-IDs to the
   corresponding trust domain signing keys.

5.2.4.  Using the JWT-SVID iss claim

   JWT-SVIDs carrying iss claims could technically be validated by
   retrieving the signing keys via OpenID Connect Discovery or OAuth 2.0
   Authorization Server Metadata.  This approach only applies for JWT-
   SVIDs and only works when the iss claim is present, which is not
   guaranteed and not part of the JWT-SVID specification.




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   The narrow scope of applicability does not make it a viable
   alternative to the SPIFFE Bundle Endpoint.  In combination with
   interoperability concerns, this approach is NOT RECOMMENDED.

6.  Security Considerations

   TODO Security

7.  IANA Considerations

   This document has no IANA actions.

8.  Normative References

   [Headless_JWT]
              "Headless-JWT", n.d., <foo>.

   [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>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6749>.

   [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
              for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6755>.

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

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/rfc/rfc7521>.

   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <https://www.rfc-editor.org/rfc/rfc7523>.

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/rfc/rfc7591>.



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   [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>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8705>.

   [SPIFFE_BUNDLE]
              "SPIFFE Bundle", n.d.,
              <https://github.com/spiffe/spiffe/blob/main/standards/
              SPIFFE_Trust_Domain_and_Bundle.md#4-spiffe-bundle-format>.

   [SPIFFE_FEDERATION]
              "SPIFFE Federation", n.d.,
              <https://github.com/spiffe/spiffe/blob/main/standards/
              SPIFFE_Federation.md>.

   [SPIFFE_ID]
              "SPIFFE-ID", n.d.,
              <https://github.com/spiffe/spiffe/blob/main/standards/
              SPIFFE-ID.md>.

   [SPIFFE_JWT]
              "JWT-SVID", n.d.,
              <https://github.com/spiffe/spiffe/blob/main/standards/JWT-
              SVID.md>.

   [SPIFFE_X509]
              "X509-SVID", n.d.,
              <https://github.com/spiffe/spiffe/blob/main/standards/
              X509-SVID.md>.

Acknowledgments

   TODO acknowledge.

Authors' Addresses

   Arndt Schwenkschuster
   SPIRL
   Email: arndts.ietf@gmail.com


   Pieter Kasselmann
   SPIRL



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   Email: pieter@spirl.com


















































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