



Workload Identity in Multi System Environments        A. Schwenkschuster
Internet-Draft                                                     SPIRL
Intended status: Standards Track                       23 September 2025
Expires: 27 March 2026


               WIMSE Workload-to-Workload Authentication
                 draft-schwenkschuster-s2s-protocol-00

Abstract

   The WIMSE architecture defines authentication and authorization for
   software workloads in a variety of runtime environments, from the
   most basic ones up to complex multi-service, multi-cloud, multi-
   tenant deployments.  This document defines the simplest, atomic unit
   of this architecture: the protocol between two workloads that need to
   verify each other's identity in order to communicate securely.  The
   scope of this protocol is a single HTTP request-and-response pair.
   To address the needs of different setups, we propose two protocols,
   one at the application level and one that makes use of trusted TLS
   transport.  These two protocols are compatible, in the sense that a
   single call chain can have some calls use one protocol and some use
   the other.  Workload A can call Workload B with mutual TLS
   authentication, while the next call from Workload B to Workload C
   would be authenticated at the application level.

About This Document

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

   The latest revision of this draft can be found at https://ietf-wg-
   wimse.github.io/draft-ietf-wimse-s2s-protocol/draft-ietf-wimse-s2s-
   protocol.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-schwenkschuster-s2s-protocol/.

   Discussion of this document takes place on the Workload Identity in
   Multi System Environments Working Group mailing list
   (mailto:wimse@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/wimse/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/wimse/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-wimse/draft-ietf-wimse-s2s-protocol.

Status of This Memo

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



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Extending This Protocol to Other Use Cases  . . . . . . .   4
     1.2.  Deployment Architecture and Message Flow  . . . . . . . .   4
     1.3.  Workload Identifiers and Authentication Granularity . . .   6
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   7
   3.  Application Level Workload-to-Workload Authentication . . . .   7
     3.1.  The Workload Identity Token . . . . . . . . . . . . . . .   7
       3.1.1.  The WIT HTTP Header . . . . . . . . . . . . . . . . .  11
       3.1.2.  Including Additional Claims . . . . . . . . . . . . .  11
       3.1.3.  A note on iss claim and key distribution  . . . . . .  12
     3.2.  Profile Selection Guidance  . . . . . . . . . . . . . . .  12
     3.3.  Additional Profiles . . . . . . . . . . . . . . . . . . .  13
     3.4.  Error Conditions  . . . . . . . . . . . . . . . . . . . .  13
     3.5.  Coexistence with JWT Bearer Tokens  . . . . . . . . . . .  14
   4.  Using Mutual TLS for Workload-to-Workload Authentication  . .  14
     4.1.  The Workload Identity Certificate . . . . . . . . . . . .  14
     4.2.  Workload Identity Certificate Validation  . . . . . . . .  15
       4.2.1.  Server Name Validation  . . . . . . . . . . . . . . .  15
     4.3.  Client Authorization Using the Workload Identity  . . . .  16
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16



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     5.1.  Workload Identity . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Workload Identity Token and Proof of Possession . . . . .  16
     5.3.  Workload Identity Key Management  . . . . . . . . . . . .  17
     5.4.  Middle Boxes  . . . . . . . . . . . . . . . . . . . . . .  18
     5.5.  Privacy Considerations  . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Media Type Registration . . . . . . . . . . . . . . . . .  18
       6.1.1.  application/wit+jwt . . . . . . . . . . . . . . . . .  19
     6.2.  Hypertext Transfer Protocol (HTTP) Field Name
           Registration  . . . . . . . . . . . . . . . . . . . . . .  20
       6.2.1.  Workload-Identity-Token . . . . . . . . . . . . . . .  20
     6.3.  URI Scheme Registration . . . . . . . . . . . . . . . . .  20
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  23
     A.1.  draft-schwenkschuster-s2s-protocol-00 . . . . . . . . . .  23
     A.2.  draft-ietf-wimse-s2s-protocol-06  . . . . . . . . . . . .  23
     A.3.  draft-ietf-wimse-s2s-protocol-05  . . . . . . . . . . . .  23
     A.4.  draft-ietf-wimse-s2s-protocol-04  . . . . . . . . . . . .  24
     A.5.  draft-ietf-wimse-s2s-protocol-03  . . . . . . . . . . . .  24
     A.6.  draft-ietf-wimse-s2s-protocol-02  . . . . . . . . . . . .  24
     A.7.  draft-ietf-wimse-s2s-protocol-01  . . . . . . . . . . . .  24
     A.8.  draft-ietf-wimse-s2s-protocol-00  . . . . . . . . . . . .  25
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This document defines authentication and authorization in the context
   of interaction between two workloads.  This is the core component of
   the WIMSE architecture [I-D.ietf-wimse-arch].  For simplicity, this
   document focuses on HTTP-based services, and the workload-to-workload
   call consists of a single HTTP request and its response.  We define
   the credentials that both workloads should possess and how they are
   used to protect the HTTP exchange.

   There are multiple deployment styles in use today, and they result in
   different security properties.  We propose to address them
   differently.

   *  Many use cases have various middleboxes inserted between pairs of
      workloads, resulting in a transport layer that is not end-to-end
      encrypted.  We propose to address these use cases by protecting
      the HTTP messages at the application level (Section 3).






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   *  The other commonly deployed architecture has a mutual-TLS
      connection between each pair of workloads.  This setup can be
      addressed by a simpler solution (Section 4).

   It is an explicit goal of this protocol that a workload deployment
   can include both architectures across a multi-chain call.  In other
   words, Workload A can call Workload B with mutual TLS protection,
   while the next call to Workload C is protected at the application
   level.

   For application-level protection we define two alternative approaches
   in separate companion documents: one inspired by DPoP [RFC9449]
   defined in [I-D.ietf-schwenkschuster-s2s-jwt-pop] and one which is a
   profile of HTTP Message Signatures [RFC9421] defined in
   [I-D.ietf-schwenkschuster-s2s-http-sig].  Alternative protocol-
   specific options are also possible.

1.1.  Extending This Protocol to Other Use Cases

   The protocol defined here is narrowly scoped, targeting only HTTP-
   based request/response services.  To secure workloads communicating
   over other transports, new protocol bindings will need to be defined.
   We note though that this protocol is designed to allow some level of
   reuse.  In particular, we expect that the Workload Identity Token
   (WIT) construct will be reusable in other settings.  The Workload
   Proof Token (WPT) may be adaptable with some changes to different
   environments.

1.2.  Deployment Architecture and Message Flow

   Regardless of the transport between the workloads, we assume the
   following logical architecture (numbers refer to the sequence of
   steps listed below):


















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   +------------+               +------------+
   |            |      (1)      |            |
   |            |<=============>|            |
   |            |               |            |
   | Workload A |      (3)      | Workload B |
   |            |==============>|            |
   |            |               |            |
   |            |      (5)      |   +--------+
   |            |<==============|   |  PEP   |
   +------------+               +---+--------+
         ^                        ^     ^
         |            (2)         |     |
     (2) | +----------------------+     | (4)
         | |                            |
         v v                            v
   +------------+               +------------+
   |            |               |            |
   |  Identity  |               |    PDP     |
   |   Server   |               | (optional) |
   |            |               |            |
   +------------+               +------------+

                      Figure 1: Sequence of Operations

   The Identity Server provisions credentials to each of the workloads.
   At least Workload A (and possibly both) must be provisioned with a
   credential before the call can proceed.  Details of communication
   with the Identity Server are out of scope of this document, however
   we do describe the credential received by the workload.

   PEP is a Policy Enforcement Point, the component that allows the call
   to go through or blocks it.  PDP is an optional Policy Decision
   Point, which may be deployed in architectures where policy management
   is centralized.  All details of policy management and message
   authorization are out of scope of this document.

   The high-level message flow is as follows:

   1.  A transport connection is set up.  In the case of mutual TLS,
       this includes authentication of both workloads to one another.
       In the case of application-level security, the TLS connection is
       typically one-way authenticated, and workload-level
       authentication does not yet take place.

   2.  Workload A (and similarly, Workload B) obtains a credential from
       the Identity Server.  This happens periodically, e.g. once every
       24 hours.




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   3.  Workload A makes an HTTP call into Workload B.  This is a regular
       HTTP request, with the additional protection mechanisms defined
       below.

   4.  In the case of application-level security, Workload B
       authenticates Workload A (when using mutual TLS, this happened in
       step 1).  In either case, Workload B decides whether to authorize
       the call.  In certain architectures, Workload B may need to
       consult with an external server when making this decision.

   5.  Workload B returns a response to Workload A, which may be an
       error response or a regular one.

1.3.  Workload Identifiers and Authentication Granularity

   The specific format of workload identifiers (see
   [I-D.ietf-wimse-arch]) is set by local policy for each deployment,
   and this choice has several implications.

   Prior to WIMSE, many use cases did not allow for fully granular
   authentication in containerized runtime platforms.  For instance,
   with mutual TLS, there's often no clear way to map the request's
   external access reference (e.g., Kubernetes Ingress path, service
   name, or host header) to the SubjectAltName value in the server
   certificate.  This means that the client could only verify if the
   server certificate is valid within a trust domain, not if it's tied
   to a specific workload.

   To enable mutual and granular authentication between workloads, two
   things must be in place:

   *  Each workload must know its own identifier.

   *  There needs to be an explicit mapping from the external handle
      used to access a workload (such as an Ingress path or service DNS
      name) to its workload identifier.

   Once these conditions are met, the methods described in this document
   can be used for the caller and callee to mutually authenticate.

   Implementations MUST allow for defining this mapping between the
   workload's access path and the workload identifier (e.g., through
   callback functions).  Deployments SHOULD use these features to
   establish a consistent set of identifiers within their environment.







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2.  Conventions and Definitions

   All terminology in this document follows [I-D.ietf-wimse-arch].

   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.

3.  Application Level Workload-to-Workload Authentication

   As noted in the Introduction, for many deployments communication
   between workloads cannot use end-to-end TLS.  For these deployment
   styles, application-level protections are required.

   Two alternative approaches for application-level protection are
   defined in companion documents, both using the Workload Identity
   Token (Section 3.1) defined in this document.  The first alternative
   [I-D.ietf-schwenkschuster-s2s-jwt-pop] is inspired by the OAuth DPoP
   specification and uses Workload Proof Tokens.  The second alternative
   [I-D.ietf-schwenkschuster-s2s-http-sig] is based on HTTP Message
   Signatures.  A comparison of the two alternatives is provided in
   Section 3.2.

3.1.  The Workload Identity Token

   The Workload Identity Token (WIT) is a JWS [RFC7515] signed JWT
   [RFC7519] that represents the identity of a workload.  It is issued
   by the Identity Server and binds a public key to the workload
   identity.  See Section 5.3 for security considerations.

   A WIT MUST contain the following claims, except where noted:

   *  in the JOSE header:

      -  alg: An identifier for a JWS asymmetric digital signature
         algorithm (registered algorithm identifiers are listed in the
         IANA JOSE Algorithms registry [IANA.JOSE.ALGS]).  The value
         none MUST NOT be used.

      -  typ: the WIT is explicitly typed, as recommended in
         Section 3.11 of [RFC8725], using the wit+jwt media type.

   *  in the JWT claims:






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      -  iss: The issuer of the token, which is the Identity Server,
         represented by a URI.  The iss claim is RECOMMENDED but
         optional, see Section 3.1.3 for more.

      -  sub: The subject of the token, which is the identity of the
         workload, represented by a URI.  See [I-D.ietf-wimse-arch] for
         details of the Workload Identifier.  And see Section 1.3 for
         security implications of these identifiers.

      -  exp: The expiration time of the token (as defined in
         Section 4.1.4 of [RFC7519]).  WITs should be refreshed
         regularly, e.g. on the order of hours.

      -  jti: A unique identifier for the token.  This claim is
         OPTIONAL.  The jti claim is frequently useful for auditing
         issuance of individual WITs or to revoke them, but some token
         generation environments do not support it.

      -  cnf: A confirmation claim referencing the public key of the
         workload.

         o  jwk: Within the cnf claim, a jwk key MUST be present that
            contains the public key of the workload as defined in
            Section 3.2 of [RFC7800].  The workload MUST prove
            possession of the corresponding private key when presenting
            the WIT to another party, which can be accomplished by using
            it in conjunction with one of the methods in
            [I-D.ietf-schwenkschuster-s2s-jwt-pop] or
            [I-D.ietf-schwenkschuster-s2s-http-sig].  As such, it MUST
            NOT be used as a bearer token and is not intended for use in
            the Authorization header.

            +  alg: Within the jwk object, an alg field MUST be present.
               Allowed values are listed in the IANA "JSON Web Signature
               and Encryption Algorithms" registry established by
               [RFC7518].  The presented proof (WPT or http-sig) MUST be
               produced with the algorithm specified in this field.  The
               value none MUST NOT be used.  Algorithms used in
               combination with symmetric keys MUST NOT be used.  Also
               encryption algorithms MUST NOT be used as this would
               require additional key distribution outside of the WIT.
               To promote interoperability, the ES256 signing algorithm
               MUST be supported by general purpose implementations of
               this document.

   As noted in [I-D.ietf-wimse-arch], a workload identifier is a URI
   with a trust domain component.  The runtime environment often
   determines which URI scheme is used, e.g. if SPIFFE is used to



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   authenticate workloads, it mandates "spiffe" URIs.  However for those
   deployments where this is not the case, this document (Section 6.3)
   defines the "wimse" URI scheme which can be used by any deployment
   that implements this protocol.

   An example WIT might look like this:

   =============== NOTE: '\' line wrapping per RFC 8792 ================

   eyJhbGciOiJFUzI1NiIsImtpZCI6Ikp1bmUgNSIsInR5cCI6IndpdCtqd3QifQ.eyJjb\
   mYiOnsiandrIjp7ImFsZyI6IkVkRFNBIiwiY3J2IjoiRWQyNTUxOSIsImt0eSI6Ik9LU\
   CIsIngiOiIxQ1hYdmZsTl9MVlZzSXNZWHNVdkIwM0ptbEdXZUNIcVFWdW91Q0Y5MmJnI\
   n19LCJleHAiOjE3NDU1MTI1MTAsImlhdCI6MTc0NTUwODkxMCwianRpIjoieC1fMUNUT\
   DJjY2EzQ1NFNGN3Yl9sIiwic3ViIjoid2ltc2U6Ly9leGFtcGxlLmNvbS9zcGVjaWZpY\
   y13b3JrbG9hZCJ9.6KraSQUxWdl9dxFQ3Fj6dPY0Vi88OkwFwZpAIOhLeq6AbXAnLLQg\
   Op8U9UDGcBuYF3KiNv6oKQD1ZWAzrMZOJw

             Figure 2: An example Workload Identity Token (WIT)

   The decoded JOSE header of the WIT from the example above is shown
   here:

   {
     "alg": "ES256",
     "kid": "June 5",
     "typ": "wit+jwt"
   }

                     Figure 3: Example WIT JOSE Header

   The decoded JWT claims of the WIT from the example above are shown
   here:

   {
     "cnf": {
       "jwk": {
         "alg": "EdDSA",
         "crv": "Ed25519",
         "kty": "OKP",
         "x": "1CXXvflN_LVVsIsYXsUvB03JmlGWeCHqQVuouCF92bg"
       }
     },
     "exp": 1745512510,
     "iat": 1745508910,
     "jti": "x-_1CTL2cca3CSE4cwb_l",
     "sub": "wimse://example.com/specific-workload"
   }




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                        Figure 4: Example WIT Claims

   The claims indicate that the example WIT:

   *  is valid until Thu Apr 24 2025 16:35:10 GMT (represented as
      NumericDate Section 2 of [RFC7519] value 1745512510).

   *  identifies the workload to which the token was issued as
      wimse://example.com/specific-workload.

   *  has a unique identifier of x-_1CTL2cca3CSE4cwb_l.

   *  binds the public key represented by the jwk confirmation method to
      the workload wimse://example.com/specific-workload.

   *  requires the proof to be produced with the EdDSA signature
      algorithm.

   For elucidative purposes only, the workload's key, including the
   private part, is shown below in JWK [RFC7517] format:

   {
    "kty": "OKP",
    "crv": "Ed25519",
    "x": "1CXXvflN_LVVsIsYXsUvB03JmlGWeCHqQVuouCF92bg",
    "d": "sdLX8yCYKqo_XvGBLn-ZWeKT7llYeeQpgeCaXVxb5kY"
   }

                      Figure 5: Example Workload's Key

   The afore-exampled WIT is signed with the private key of the Identity
   Server.  The public key(s) of the Identity Server need to be known to
   all workloads in order to verify the signature of the WIT.  The
   Identity Server's public key from this example is shown below in JWK
   [RFC7517] format:

   {
    "kty": "EC",
    "kid": "June 5",
    "crv": "P-256",
    "x": "kXqnA2Op7hgd4zRMbw0iFcc_hDxUxhojxOFVGjE2gks",
    "y": "n__VndPMR021-59UAs0b9qDTFT-EZtT6xSNs_xFskLo"
   }

                   Figure 6: Example Identity Server Key






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3.1.1.  The WIT HTTP Header

   A WIT is conveyed in an HTTP header field named Workload-Identity-
   Token.

   ABNF [RFC5234] for the value of Workload-Identity-Token header field
   is provided in Figure 7:

   ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
   DIGIT = %x30-39 ; 0-9
   base64url = 1*(ALPHA / DIGIT / "-" / "_")
   JWT =  base64url "." base64url "." base64url
   WIT =  JWT

            Figure 7: Workload-Identity-Token Header Field ABNF

   The following shows the WIT from Figure 2 in an example of a
   Workload-Identity-Token header field:

   =============== NOTE: '\' line wrapping per RFC 8792 ================

   Workload-Identity-Token: eyJhbGciOiJFUzI1NiIsImtpZCI6Ikp1bmUgNSIsInR\
   5cCI6IndpdCtqd3QifQ.eyJjbmYiOnsiandrIjp7ImFsZyI6IkVkRFNBIiwiY3J2Ijoi\
   RWQyNTUxOSIsImt0eSI6Ik9LUCIsIngiOiIxQ1hYdmZsTl9MVlZzSXNZWHNVdkIwM0pt\
   bEdXZUNIcVFWdW91Q0Y5MmJnIn19LCJleHAiOjE3NDU1MTI1MTAsImlhdCI6MTc0NTUw\
   ODkxMCwianRpIjoieC1fMUNUTDJjY2EzQ1NFNGN3Yl9sIiwic3ViIjoid2ltc2U6Ly9l\
   eGFtcGxlLmNvbS9zcGVjaWZpYy13b3JrbG9hZCJ9.6KraSQUxWdl9dxFQ3Fj6dPY0Vi8\
   8OkwFwZpAIOhLeq6AbXAnLLQgOp8U9UDGcBuYF3KiNv6oKQD1ZWAzrMZOJw

       Figure 8: An example Workload Identity Token HTTP Header Field

   Note that per [RFC9110], header field names are case insensitive;
   thus, Workload-Identity-Token, workload-identity-token, WORKLOAD-
   IDENTITY-TOKEN, etc., are all valid and equivalent header field
   names.  However, case is significant in the header field value.

3.1.2.  Including Additional Claims

   The WIT contains JSON structures and therefore can be trivially
   extended by adding more claims beyond those defined in the current
   specification.  This, however, could result in interoperability
   issues, which the following rules are addressing.

   *  To ensure interoperability in WIMSE environments, the use of
      Private claim names (Sec. 4.3 of [RFC7519]) is NOT RECOMMENDED.






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   *  In closed environments, deployers MAY freely add claims to the
      WIT.  Such claims SHOULD be collision-resistant, such as
      example.com/myclaim.

   *  A recipient that does not understand such claims MUST ignore them,
      as per Sec. 4 of [RFC7519].

   *  Outside of closed environments, new claims MUST be registered with
      IANA [IANA.JWT.CLAIMS] before they can be used.

3.1.3.  A note on iss claim and key distribution

   It is RECOMMENDED that the WIT carries an iss claim.  This
   specification itself does not make use of a potential iss claim but
   also carries the trust domain in the workload identifier (see
   [I-D.ietf-wimse-arch] for a definition of the identifier and related
   rules).  Implementations MAY include the iss claim in the form of a
   https URL to facilitate key distribution via mechanisms like the
   jwks_uri from [RFC8414] but alternative key distribution methods may
   make use of the trust domain included in the workload identifier
   which is carried in the mandatory sub claim.

3.2.  Profile Selection Guidance

   The two workload protection options defined in the companion
   documents [I-D.ietf-schwenkschuster-s2s-jwt-pop] and
   [I-D.ietf-schwenkschuster-s2s-http-sig] have different strengths and
   weaknesses regarding implementation complexity, extensibility, and
   security.  Here is a summary of the main differences between the
   DPoP-inspired approach and the HTTP Message Signatures approach.

   *  The DPoP-inspired solution is less HTTP-specific, making it easier
      to adapt for other protocols beyond HTTP.  This flexibility is
      particularly valuable for asynchronous communication scenarios,
      such as event-driven systems.

   *  Message Signatures, on the other hand, benefit from an existing
      HTTP-specific RFC with some established implementations.  This
      existing groundwork means that this option could be simpler to
      deploy, to the extent such implementations are available and
      easily integrated.

   *  Given that the WIT (Workload Identity Token) is a type of JWT, the
      DPoP-inspired approach that also uses JWT is less complex and
      technology-intensive than Message Signatures.  In contrast,
      Message Signatures introduce an additional layer of technology,
      potentially increasing the complexity of the overall system.




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   *  Message Signatures offer superior integrity protection,
      particularly by mitigating message modification by middleboxes.
      See also Section 5.4.

   *  A key advantage of Message Signatures is that they support
      response signing.  This opens up the possibility for future
      decisions about whether to make response signing mandatory,
      allowing for flexibility in the specification and/or in specific
      deployment scenarios.

   *  In general, Message Signatures provide greater flexibility
      compared to the DPoP-inspired approach.  Future versions of this
      draft (and subsequent implementations) can decide whether specific
      aspects of message signing, such as coverage of particular fields,
      should be mandatory or optional.  Covering more fields will
      constrain the proof so it cannot be easily reused in another
      context, which is often a security improvement.  The DPoP inspired
      approach could be designed to include extensibility to sign other
      fields, but this would make it closer to trying to reinvent
      Message Signatures.

3.3.  Additional Profiles

   The WIMSE architecture is designed to be extensible, allowing for
   additional authentication profiles to be defined as separate
   companion documents.  While this document defines the foundational
   Workload Identity Token (WIT) and establishes the framework for
   application-level authentication, the current two profiles (JWT-based
   proof of possession and HTTP Message Signatures) represent initial
   approaches to address common deployment scenarios.

   When developing additional profiles, implementers SHOULD use the
   Workload Identity Token or Workload Identity Certificate as defined
   in this document as Workload Identity presentation.

3.4.  Error Conditions

   Errors may occur during the processing of application-level
   authentication mechanisms defined in the companion documents.  If
   signature verification fails for any reason, such as an invalid
   signature, an expired validity time window, or a malformed data
   structure, an error is returned.  Typically, this will be in response
   to an API call, so an HTTP status code such as 400 (Bad Request) is
   appropriate.  This response could include more details as per
   [RFC9457], such as an indicator that the wrong key material or
   algorithm was used.  The use of HTTP status code 401 is NOT
   RECOMMENDED for this purpose because it requires a WWW-Authenticate
   with acceptable http auth mechanisms in the error response and an



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   associated Authorization header in the subsequent request.  The use
   of these headers for the WIT or proof of possession mechanisms is not
   compatible with this specification.

3.5.  Coexistence with JWT Bearer Tokens

   The WIT and the proof of possession mechanisms defined in the
   companion documents use new HTTP headers.  They can therefore be
   presented along with existing headers used for JWT bearer tokens.
   This property allows for transition from mechanisms using identity
   tokens based on bearer JWTs to proof of possession based WITs.  A
   workload may implement a policy that accepts both bearer tokens and
   WITs during a transition period.  This policy may be configurable
   per-caller to allow the workload to reject bearer tokens from callers
   that support WITs.  Once a deployment fully supports WITs, then the
   use of bearer tokens for identity can be disabled through policy.
   Implementations should be careful when implementing such a transition
   strategy, since the decision which token to prefer is made when the
   caller's identity has still not been authenticated, and needs to be
   revalidated following the authentication step.

   The WIT can also coexist with tokens used to establish security
   context, such as transaction tokens
   [I-D.ietf-oauth-transaction-tokens].  In this case a workload's
   authorization policy may take into account both the sending
   workload's identity and the information in the context token.  For
   example, the identity in the WIT may be used to establish which API
   calls can be made and information in the context token may be used to
   determine which specific resources can be accessed.

4.  Using Mutual TLS for Workload-to-Workload Authentication

   As noted in the introduction, for many deployments, transport-level
   protection of application traffic using TLS is ideal.

4.1.  The Workload Identity Certificate

   The Workload Identity Certificate is an X.509 certificate.  The
   workload identity MUST be encoded in a SubjectAltName extension of
   type URI.  There MUST be only one SubjectAltName extension of type
   URI in a Workload Identity Certificate.  If the workload will act as
   a TLS server for clients that do not understand workload identities
   it is RECOMMENDED that the Workload Identity Certificate contain a
   SubjectAltName of type DNSName with the appropriate DNS names for the
   server.  The certificate MAY contain SubjectAltName extensions of
   other types.





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4.2.  Workload Identity Certificate Validation

   Workload Identity Certificates may be used to authenticate both the
   server and client side of the connections.  When validating a
   Workload Identity Certificate, the relying party MUST use the trust
   anchors configured for the trust domain in the workload identity to
   validate the peer's certificate.  Other PKIX [RFC5280] path
   validation rules apply.  Workloads acting as TLS clients and servers
   MUST validate that the trust domain portion of the Workload Identity
   Certificate matches the expected trust domain for the other side of
   the connection.

   Servers wishing to use the Workload Identity Certificate for
   authorizing the client MUST require client certificate authentication
   in the TLS handshake.  Other methods of post handshake authentication
   are not specified by this document.

   Workload Identity Certificates used by TLS servers SHOULD have the
   id-kp-serverAuth extended key usage [RFC5280] field set and Workload
   Identity Certificates used by TLS clients SHOULD have the id-kp-
   clientAuth extended key usage field set.  A certificate that is used
   for both client and server connections may have both fields set.
   This specification does not make any other requirements beyond
   [RFC5280] on the contents of Workload Identity Certificates or on the
   certification authorities that issue workload certificates.

4.2.1.  Server Name Validation

   If the WIMSE client uses a hostname to connect to the server and the
   server certificate contain a DNS SAN the client MUST perform standard
   host name validation (Section 6.3 of [RFC9525]) unless it is
   configured with the additional information necessary to perform
   alternate validation of the peer's workload identity.  If the client
   did not perform standard host name validation then the WIMSE client
   SHOULD further use the workload identifier to validate the server.
   The host portion of the workload identifier is NOT treated as a host
   name as specified in section 6.4 of [RFC9525] but rather as a trust
   domain.  The server identity is encoded in the path portion of the
   workload identifier in a deployment specific way.  Validating the
   workload identity could be a simple match on the trust domain and
   path portions of the identifier or validation may be based on the
   specific details on how the identifier is constructed.  The path
   portion of the WIMSE identifier MUST always be considered in the
   scope of the trust domain.  In most cases it is preferable to
   validate the entire workload identifier, see Section 1.3 for
   additional implementation advice.





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4.3.  Client Authorization Using the Workload Identity

   The server application retrieves the workload identifier from the
   client certificate subjectAltName, which in turn is obtained from the
   TLS layer.  The identifier is used in authorization, accounting and
   auditing.  For example, the full workload identifier may be matched
   against ACLs to authorize actions requested by the peer and the
   identifier may be included in log messages to associate actions to
   the client workload for audit purposes.  A deployment may specify
   other authorization policies based on the specific details of how the
   workload identifier is constructed.  The path portion of the workload
   identifier MUST always be considered in the scope of the trust
   domain.  See Section 1.3 on additional security implications of
   workload identifiers.

5.  Security Considerations

5.1.  Workload Identity

   The Workload Identifier is scoped within an issuer and therefore any
   sub-components (path portion of Identifier) are only unique within a
   trust domain defined by the issuer.  Using a Workload Identifier
   without taking into account the trust domain could allow one domain
   to issue tokens to spoof identities in another domain.  Additionally,
   the trust domain must be tied to an authorized issuer cryptographic
   trust anchor through some mechanism such as a JWKS or X.509
   certificate chain.  The association of an issuer, trust domain and a
   cryptographic trust anchor MUST be communicated securely out of band.

5.2.  Workload Identity Token and Proof of Possession

   The Workload Identity Token (WIT) is bound to a secret cryptographic
   key and is always presented with a proof of possession as described
   in Section 3.1.  The WIT is a general purpose token that can be
   presented in multiple contexts.  The WIT and its PoP are only used in
   the application-level options, and both are not used in MTLS.  The
   WIT MUST NOT be used as a bearer token.  While this helps reduce the
   sensitivity of the token it is still possible that a token and its
   proof of possession may be captured and replayed within the PoP's
   lifetime.  The following are some mitigations for the capture and
   reuse of the proof of possession (PoP):

   *  Preventing Eavesdropping and Interception with TLS

   An attacker observing or intercepting the communication channel can
   view the token and its proof of possession and attempt to replay it
   to gain an advantage.  In order to prevent this the token and proof
   of possession MUST be sent over a secure, server authenticated TLS



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   connection unless a secure channel is provided by some other
   mechanisms.  Host name validation according to Section 4.2.1 MUST be
   performed by the client.

   *  Limiting Proof of Possession Lifespan

   The proof of possession MUST be time limited.  A PoP should only be
   valid over the time necessary for it to be successfully used for the
   purpose it is needed.  This will typically be on the order of
   minutes.  PoPs received outside their validity time MUST be rejected.

   *  Limiting Proof of Possession Scope

   In order to reduce the risk of theft and replay the PoP should have a
   limited scope.  For example, a PoP may be targeted for use with a
   specific workload and even a specific transaction to reduce the
   impact of a stolen PoP.  In some cases a workload may wish to reuse a
   PoP for a period of time or have it accepted by multiple target
   workloads.  A careful analysis is warranted to understand the impacts
   to the system if a PoP is disclosed allowing it to be presented by an
   attacker along with a captured WIT.

   *  Replay Protection

   A proof of possession includes the jti claim that MUST uniquely
   identify it, within the scope of a particular sender.  This claim
   SHOULD be used by the receiver to perform basic replay protection
   against tokens it has already seen.  Depending upon the design of the
   system it may be difficult to synchronize the replay cache across all
   token validators.  If an attacker can somehow influence the identity
   of the validator (e.g. which cluster member receives the message)
   then replay protection would not be effective.

   *  Binding to TLS Endpoint

   The POP MAY be bound to a transport layer sender such as the client
   identity of a TLS session or TLS channel binding parameters.  The
   mechanisms for binding are outside the scope of this specification.

5.3.  Workload Identity Key Management

   Both the Workload Identity Token and the Workload Identity
   Certificate carry a public key.  The corresponding private key:

   *  MUST be kept private

   *  MUST be individual for each Workload Identifier (see
      [I-D.ietf-wimse-arch])



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   *  MUST NOT be used once the credential is expired

   *  SHOULD be re-generated for each new Workload Identity Token or
      Certificate.

5.4.  Middle Boxes

   In some deployments the Workload Identity Token and proof of
   possession may pass through multiple systems.  The communication
   between the systems is over TLS, but the token and PoP are available
   in the clear at each intermediary.  While the intermediary cannot
   modify the token or the information within the PoP they can attempt
   to capture and replay the token or modify the data not protected by
   the PoP.

   Mitigations listed in Section 3 can be used to provide some
   protection from middle boxes.  However we note that the DPoP-inspired
   solution [I-D.ietf-schwenkschuster-s2s-jwt-pop] does not protect
   major portions of the request and response and therefore does not
   provide protection from an actively malicious middle box.
   Deployments should perform analysis on their situation to determine
   if it is appropriate to trust and allow traffic to pass through a
   middle box.

5.5.  Privacy Considerations

   WITs and the proofs of possession may contain private information
   such as user names or other identities.  Care should be taken to
   prevent the disclosure of this information.  The use of TLS helps
   protect the privacy of WITs and proofs of possession.

   WITs and certificates with workload identifiers are typically
   associated with a workload and not a specific user, however in some
   deployments the workload may be associated directly to a user.  While
   these are exceptional cases a deployment should evaluate if the
   disclosure of WITs or certificates can be used to track a user.

6.  IANA Considerations

6.1.  Media Type Registration

   IANA is requested to register the following entries to the "Media
   Types" registry [IANA.MEDIA.TYPES]:

   *  application/wit+jwt, per Section 6.1.1.






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6.1.1.  application/wit+jwt

   Type name: application

   Subtype name: wit+jwt

   Required parameters: N/A

   Optional parameters: N/A

   Encoding considerations: Encoding considerations are identical to
   those specified for the "application/jwt" media type.  See [RFC7519].

   Security considerations: See the Security Considerations section of
   RFC XXX.

   Interoperability considerations: N/A

   Published specification: RFC XXX, Section 3.1.

   Applications that use this media type: Identity servers that vend
   Workload Identity Tokens, and Workloads that use these tokens to
   authenticate to each other.

   Fragment identifier considerations: N/A

   Additional information:

   Deprecated alias names for this type: N/A

   Magic number(s): N/A

   File extension(s): None

   Macintosh file type code(s): N/A

   Person & email address to contact for further information:

   See the Authors' Addresses section of RFC XXX.

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: See the Authors' Addresses section of RFC XXX.

   Change controller: Internet Engineering Task Force (iesg@ietf.org).




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6.2.  Hypertext Transfer Protocol (HTTP) Field Name Registration

   IANA is requested to register the following entries to the "Hypertext
   Transfer Protocol (HTTP) Field Name Registry" [IANA.HTTP.FIELDS]:

   *  Workload-Identity-Token, per Section 6.2.1.

6.2.1.  Workload-Identity-Token

   *  Field Name: Workload-Identity-Token

   *  Status: permanent

   *  Structured Type: N/A

   *  Specification Document: RFC XXX, Section 3.1.1

   *  Comments: see reference above for an ABNF syntax of this field

6.3.  URI Scheme Registration

   IANA is requested to register the "wimse" scheme to the "URI Schemes"
   registry [IANA.URI.SCHEMES]:

   *  Scheme name: wimse

   *  Status: permanent

   *  Applications/protocols that use this scheme name: the WIMSE
      workload-to-workload authentication protocol.

   *  Contact: IETF Chair chair@ietf.org (mailto:chair@ietf.org)

   *  Change controller: IESG iesg@ietf.org (mailto:iesg@ietf.org)

   *  References: Section 3 of this document (RFC XXX).

7.  References

7.1.  Normative References

   [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>.






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   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

   [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>.

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

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

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7519>.

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,
              <https://www.rfc-editor.org/rfc/rfc7800>.

   [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>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/rfc/rfc8414>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8725>.






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   [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/rfc/rfc9110>.

   [RFC9421]  Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
              Message Signatures", RFC 9421, DOI 10.17487/RFC9421,
              February 2024, <https://www.rfc-editor.org/rfc/rfc9421>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9525>.

7.2.  Informative References

   [I-D.ietf-oauth-transaction-tokens]
              Tulshibagwale, A., Fletcher, G., and P. Kasselman,
              "Transaction Tokens", Work in Progress, Internet-Draft,
              draft-ietf-oauth-transaction-tokens-06, 28 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              transaction-tokens-06>.

   [I-D.ietf-schwenkschuster-s2s-http-sig]
              "*** BROKEN REFERENCE ***".

   [I-D.ietf-schwenkschuster-s2s-jwt-pop]
              "*** BROKEN REFERENCE ***".

   [I-D.ietf-wimse-arch]
              Salowey, J. A., Rosomakho, Y., and H. Tschofenig,
              "Workload Identity in a Multi System Environment (WIMSE)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-wimse-arch-05, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-wimse-
              arch-05>.

   [IANA.HTTP.FIELDS]
              IANA, "Hypertext Transfer Protocol (HTTP) Field Name
              Registry", <https://www.iana.org/assignments/http-fields>.

   [IANA.JOSE.ALGS]
              IANA, "JSON Web Signature and Encryption Algorithms",
              <https://www.iana.org/assignments/jose>.

   [IANA.JWT.CLAIMS]
              IANA, "JSON Web Token Claims",
              <https://www.iana.org/assignments/jwt>.




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   [IANA.MEDIA.TYPES]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [IANA.URI.SCHEMES]
              IANA, "Uniform Resource Identifier (URI) Schemes",
              <https://www.iana.org/assignments/uri-schemes>.

   [RFC9449]  Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
              Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
              September 2023, <https://www.rfc-editor.org/rfc/rfc9449>.

   [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>.

Appendix A.  Document History


   // RFC Editor: please remove before publication.

A.1.  draft-schwenkschuster-s2s-protocol-00

   *  Rework the WPT's oth claim

   *  update the [media]typ[e] values

   *  Remove WPT and HTTP-SIG POP presentation

A.2.  draft-ietf-wimse-s2s-protocol-06

   *  Explicit definition of the Workload Identity Certificate.

   *  Definition of the validation of workload identifiers as part of
      workload authentication.  Still work in progress.

A.3.  draft-ietf-wimse-s2s-protocol-05

   *  Removed the entire Workload Identity section which is now covered
      in the Architecture document.

   *  Content-Digest is mandatory with HTTP-Sig.

   *  Some wording on extending the protocol beyond HTTP.

   *  IANA considerations.




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A.4.  draft-ietf-wimse-s2s-protocol-04

   *  Require cnf.jwk.alg in WIT which restricts signature algorithm of
      WPT or HTTP-Sig.

   *  Replay protection as a SHOULD for both WPT and HTTP-Sig.

   *  Consolidate terminology with the Architecture draft.

A.5.  draft-ietf-wimse-s2s-protocol-03

   *  Consistently use "workload".

   *  Implement comments from the SPIFFE community.

   *  Make iss claim in WIT optional and add wording about its relation
      to key distribution.

   *  Remove iss claim from WPT.

   *  Make jti claim in WIT optional.

   *  Error handling for the application level methods.

A.6.  draft-ietf-wimse-s2s-protocol-02

   *  Coexistence with bearer tokens.

   *  Improve the architecture diagram.

   *  Some more ABNF.

   *  Clarified identifiers and URIs.

   *  Moved an author to acknowledgments.

A.7.  draft-ietf-wimse-s2s-protocol-01

   *  Addressed multiple comments from Pieter.

   *  Clarified WIMSE identity concepts, specifically "trust domain" and
      "workload identifier".

   *  Much more detail around mTLS, including some normative language.

   *  WIT (the identity token) is now included in the WPT proof of
      possession.




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   *  Added a section comparing the DPoP-inspired app-level security
      option to the Message Signature-based alternative.

A.8.  draft-ietf-wimse-s2s-protocol-00

   *  Initial WG draft, an exact copy of draft-sheffer-wimse-s2s-
      protocol-00

   *  Added this document history section

Acknowledgments

   The authors would like to thank Pieter Kasselman for his detailed
   comments.

   We thank Daniel Feldman for his contributions to earlier versions of
   this document.

Author's Address

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




























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