WIMSE Service to Service Authentication

1.1. Deployment Architecture and Message Flow

Regardless of the transport between the workloads, we assume the following logical architecture:¶

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:¶

• Workload A obtains a credential from the Identity Server. This happens periodically, e.g. once every 24 hours.¶

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

• Workload B now authenticates Workload A and decides whether to authorize the call. In certain architectures, Workload B may need to consult with an external server to decide whether to accept the call.¶

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

4.1. 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. A WIT MUST contain the following:¶

• 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 wimse-id+jwt media type.¶

• in the JWT claims:¶

  • iss: The issuer of the token, which is the Identity Server, represented by a URI.¶

  • sub: The subject of the token, which is the identity of the workload, represented by a URI.¶

  • 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.¶

  • cnf: A confirmation claim containing the public key of the workload using the jwk member 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 Section 4.2 or Section 4.3. As such, it MUST NOT be used as a bearer token and is not intended for use in the Authorization header.¶

An example WIT might look like this (all examples, of course, are non-normative and with line breaks and extra space for readability):¶

eyJ0eXAiOiJ3aW1zZS1pZCtqd3QiLCJhbGciOiJFUzI1NiIsImtpZCI6Ikp1bmUgNSJ9.
eyJpc3MiOiJ3aW1zZTovL2V4YW1wbGUuY29tL3RydXN0ZWQtY2VudHJhbC1hdXRob3Jpd
HkiLCJleHAiOjE3MTc2MTI0NzAsInN1YiI6IndpbXNlOi8vZXhhbXBsZS5jb20vc3BlY2
lmaWMtd29ya2xvYWQiLCJqdGkiOiJ4LV8xQ1RMMmNjYTNDU0U0Y3diX18iLCJjbmYiOns
iandrIjp7Imt0eSI6Ik9LUCIsImNydiI6IkVkMjU1MTkiLCJ4IjoiX2FtUkMzWXJZYkho
SDFSdFlyTDhjU21URE1oWXRPVVRHNzhjR1RSNWV6ayJ9fX0.rOSUMR8I5WhM5C704l3iV
dY0zFqxhugJ8Jo2xo39G7FqUTbwTzAGdpz2lHp6eL1M486XmRgl3uyjj6R_iuzNOA

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

{
 "typ": "wimse-id+jwt",
 "alg": "ES256",
 "kid": "June 5"
}

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

{
 "iss": "wimse://example.com/trusted-central-authority",
 "exp": 1717612470,
 "sub": "wimse://example.com/specific-workload",
 "jti": "x-_1CTL2cca3CSE4cwb__",
 "cnf": {
  "jwk": {
   "kty": "OKP",
   "crv": "Ed25519",
   "x": "_amRC3YrYbHhH1RtYrL8cSmTDMhYtOUTG78cGTR5ezk"
  }
 }
}

The claims indicate that the example WIT:¶

• was issued by an Identity Server known as wimse://example.com/trusted-central-authority.¶

• is valid until May 15, 2024 3:28:45 PM GMT-06:00 (represented as NumericDate Section 2 of [RFC7519] value 1717612470).¶

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

• has a unique identifier of x-_1CTL2cca3CSE4cwb__.¶

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

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

{
 "kty":"OKP",
 "crv":"Ed25519",
 "x":"_amRC3YrYbHhH1RtYrL8cSmTDMhYtOUTG78cGTR5ezk",
 "d":"G4lGAYFtFq5rwyjlgSIRznIoCF7MtKDHByyUUZCqLiA"
}

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",
 "x":"kXqnA2Op7hgd4zRMbw0iFcc_hDxUxhojxOFVGjE2gks",
 "y":"n__VndPMR021-59UAs0b9qDTFT-EZtT6xSNs_xFskLo",
 "crv":"P-256"
}

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

Workload-Identity-Token: eyJ0eXAiOiJ3aW1zZS1pZCtqd3QiLCJhbGciOiJFUzI1
 NiIsImtpZCI6Ikp1bmUgNSJ9.eyJpc3MiOiJ3aW1zZTovL2V4YW1wbGUuY29tL3RydXN
 0ZWQtY2VudHJhbC1hdXRob3JpdHkiLCJleHAiOjE3MTc2MTI0NzAsInN1YiI6IndpbXN
 lOi8vZXhhbXBsZS5jb20vc3BlY2lmaWMtd29ya2xvYWQiLCJqdGkiOiJ4LV8xQ1RMMmN
 jYTNDU0U0Y3diX18iLCJjbmYiOnsiandrIjp7Imt0eSI6Ik9LUCIsImNydiI6IkVkMjU
 1MTkiLCJ4IjoiX2FtUkMzWXJZYkhoSDFSdFlyTDhjU21URE1oWXRPVVRHNzhjR1RSNWV
 6ayJ9fX0.rOSUMR8I5WhM5C704l3iVdY0zFqxhugJ8Jo2xo39G7FqUTbwTzAGdpz2lHp
 6eL1M486XmRgl3uyjj6R_iuzNOA

4.2. Option 1: DPoP-Inspired Authentication

This option, inspired by the OAuth DPoP specification [RFC9449], uses a DPoP-like mechanism to authenticate the calling workload in the context of the request. The WIMSE Identity Token (Section 4.1) is sent in the request as described in Section 4.1.1. An additional JWT, the Workload Proof Token (WPT), is signed by the private key corresponding to the public key in the WIT. The WPT is sent in the Workload-Proof-Token header field of the request. A WPT contains the following:¶

• in the JOSE header:¶

  • alg: An identifier for an appropriate JWS asymmetric digital signature algorithm corresponding to the confirmation key in the associated WIT.¶

  • typ: the WPT is explicitly typed, as recommended in Section 3.11 of [RFC8725], using the application/wimse-proof+jwt media type.¶

• in the JWT claims:¶

  • iss: The issuer of the token, which is the calling workload, represented by the same value as the sub claim of the associated WIT.¶

  • aud: The audience of the token contains the HTTP target URI (Section 7.1 of [RFC9110]) of the request to which the WPT is attached, without query or fragment parts.¶

  • exp: The expiration time of the WIT (as defined in Section 4.1.4 of [RFC7519]). WPT lifetimes MUST be short, e.g., on the order of minutes or seconds.¶

  • jti: A unique identifier for the token.¶

  • ath: Hash of the OAuth access token, if present in the request, which might convey end-user identity and authorization context of the request. The value, as per Section 4.1 of [RFC9449], is the base64url encoding of the SHA-256 hash of the ASCII encoding of the access token's value.¶

  • tth: Hash of the Txn-Token [I-D.ietf-oauth-transaction-tokens], if present in the request, which might convey end-user identity and authorization context of the request. The value MUST be the result of a base64url encoding (as defined in Section 2 of [RFC7515]) of the SHA-256 hash of the ASCII encoding of the associated token's value.¶

  • oth: Hash of any other token in the request that might convey end-user identity and authorization context of the request. The value MUST be the result of a base64url encoding (as defined in Section 2 of [RFC7515]) of the SHA-256 hash of the ASCII encoding of the associated token's value. (note: this is less than ideal but seems we need something like this for extensibility)¶

An example WPT might look like the following:¶

eyJ0eXAiOiJ3aW1zZS1wcm9vZitqd3QiLCJhbGciOiJFZERTQSJ9.eyJpc3MiOiJ3aW1z
ZTovL2V4YW1wbGUuY29tL3NwZWNpZmljLXdvcmtsb2FkIiwiYXVkIjoiaHR0cHM6Ly9zZ
XJ2aWNlLmV4YW1wbGUuY29tL3BhdGgiLCJleHAiOjE3MTc2MTI4MjAsImp0aSI6Il9fYn
djNEVTQzNhY2MyTFRDMS1feCIsImF0aCI6IkNMNHdqZnBSbU5mLWJkWUliWUxuVjlkNXJ
NQVJHd0tZRTEwd1V3ekMwakkifQ.Zq50mcIVTUykQhOBS7lyF93py3q5QOSPIbnI_oESv
j6zSTWi-p0QNNHpKeB4IAgmC8Mt3dBM_rufwCxiKHSmDA

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

{
 "typ": "wimse-proof+jwt",
 "alg": "EdDSA"
}

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

{
 "iss": "wimse://example.com/specific-workload",
 "aud": "https://service.example.com/path",
 "exp": 1717612820,
 "jti": "__bwc4ESC3acc2LTC1-_x",
 "ath": "CL4wjfpRmNf-bdYIbYLnV9d5rMARGwKYE10wUwzC0jI"
}

An example of an HTTP request with both the WIT and WPT from prior examples is shown below:¶

POST /path HTTP/1.1
Host: service.example.com
Content-Type: application/json
Authorization: Bearer 16_mAd0GiwaZokU26_0902100
Workload-Identity-Token: eyJ0eXAiOiJ3aW1zZS1pZCtqd3QiLCJhbGciOiJFUzI1
 NiIsImtpZCI6Ikp1bmUgNSJ9.eyJpc3MiOiJ3aW1zZTovL2V4YW1wbGUuY29tL3RydXN
 0ZWQtY2VudHJhbC1hdXRob3JpdHkiLCJleHAiOjE3MTc2MTI0NzAsInN1YiI6IndpbXN
 lOi8vZXhhbXBsZS5jb20vc3BlY2lmaWMtd29ya2xvYWQiLCJqdGkiOiJ4LV8xQ1RMMmN
 jYTNDU0U0Y3diX18iLCJjbmYiOnsiandrIjp7Imt0eSI6Ik9LUCIsImNydiI6IkVkMjU
 1MTkiLCJ4IjoiX2FtUkMzWXJZYkhoSDFSdFlyTDhjU21URE1oWXRPVVRHNzhjR1RSNWV
 6ayJ9fX0.rOSUMR8I5WhM5C704l3iVdY0zFqxhugJ8Jo2xo39G7FqUTbwTzAGdpz2lHp
 6eL1M486XmRgl3uyjj6R_iuzNOA
Workload-Proof-Token: eyJ0eXAiOiJ3aW1zZS1wcm9vZitqd3QiLCJhbGciOiJFZER
 TQSJ9.eyJpc3MiOiJ3aW1zZTovL2V4YW1wbGUuY29tL3NwZWNpZmljLXdvcmtsb2FkIi
 wiYXVkIjoiaHR0cHM6Ly9zZXJ2aWNlLmV4YW1wbGUuY29tL3BhdGgiLCJleHAiOjE3MT
 c2MTI4MjAsImp0aSI6Il9fYndjNEVTQzNhY2MyTFRDMS1feCIsImF0aCI6IkNMNHdqZn
 BSbU5mLWJkWUliWUxuVjlkNXJNQVJHd0tZRTEwd1V3ekMwakkifQ.Zq50mcIVTUykQhO
 BS7lyF93py3q5QOSPIbnI_oESvj6zSTWi-p0QNNHpKeB4IAgmC8Mt3dBM_rufwCxiKHS
 mDA

{"do stuff":"please"}

To validate the WPT in the request, the recipient MUST ensure the following:¶

• There is exactly one Workload-Proof-Token header field in the request.¶

• The Workload-Proof-Token header field value is a single and well-formed JWT.¶

• The WPT signature is valid using the public key from the confirmation claim of the WIT.¶

• The typ JOSE header parameter of the WPT conveys a media type of wimse-proof+jwt.¶

• The iss claim of the WPT matches the sub claim of the WIT. (note: not sure iss in the WPT is useful or necessary)¶

• The aud claim of the WPT matches the target URI, or an acceptable alias or normalization thereof, of the HTTP request in which the WPT was received, ignoring any query and fragment parts.¶

• The exp claim is present and conveys a time that has not passed. WPTs with an expiration time unreasonably far in the future SHOULD be rejected.¶

• Optionally, check that the value of the jti claim has not been used before in the time window in which the respective WPT would be considered valid.¶

• If presented in conjunction with an OAuth access token, the value of the ath claim matches the hash of that token's value.¶

• If presented in conjunction with a Txn-Token, the value of the tth claim matches the hash of that token's value.¶

• If presented in conjunction with a token conveying end-user identity or authorization context, the value of the oth claim matches the hash of that token's value.¶

4.3. Option 2: Authentication Based on HTTP Message Signatures

This option uses the WIMSE Identity Token (Section 4.1) to sign the request and optionally, the response. This specification defines a profile of the Message Signatures specification [RFC9421].¶

The request is signed as per [RFC9421]. The following derived components MUST be signed:¶

• @method¶

• @request-target¶

In addition, the following request headers MUST be signed when they exist:¶

• Content-Type¶

• Content-Digest¶

• Authorization¶

• Txn-Token [I-D.ietf-oauth-transaction-tokens]¶

• Workload-Identity-Token¶

If the response is signed, the following components MUST be signed:¶

• @status¶

• @method;req¶

• @request-target;req¶

• Content-Type if it exists¶

• Content-Digest if it exists¶

• Workload-Identity-Token¶

For both requests and responses, the following signature parameters MUST be included:¶

• created¶

• expires - expiration MUST be short, e.g. on the order of minutes. The WIMSE architecture will provide separate mechanisms in support of long-lived compute processes.¶

• nonce¶

• tag - the value for implementations of this specification is wimse-service-to-service¶

Since the signing key is sent along with the message, the keyid parameter SHOULD NOT be used.¶

It is RECOMMENDED to include only one signature with the HTTP message. If multiple ones are included, then the signature label included in both the Signature-Input and Signature headers SHOULD be wimse.¶

A sender MUST ensure that each nonce it generates is unique, at least among messages sent to the same recipient. To detect message replays, a recipient MAY reject a message (request or response) if a nonce is repeated.¶

To promote interoperability, the ecdsa-p256-sha256 signing algorithm MUST be implemented by general purpose implementations of this spec.¶

OPEN ISSUE: do we use the `Accept-Signature` field to signal that the response must be signed?¶

Following is a non-normative example of a signed request and a signed response, where the caller is using the keys specified in Figure 4.¶

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

GET /gimme-ice-cream?flavor=vanilla HTTP/1.1
Host: example.com
Signature: wimse=:K4dfGnguF5f1L4DKBSp5XeFXosLGj8Y9fiUX06rL/wdOF+x3zT\
WmsvKWiY0B1oFZaOtm2FHru+YLjdkqa2WfCQ==:
Signature-Input: wimse=("@method" "@request-target" "workload-identi\
ty-token");created=1718291357;expires=1718291657;nonce="abcd1111";ta\
g="wimse-service-to-service"
Workload-Identity-Token: aGVhZGVyCg.VGhpcyBpcyBub3QgYSByZWFsIHRva2Vu\
Lgo.c2lnbmF0dXJlCg

Assuming that the workload being called has the following keypair:¶

{
 "kty":"OKP",
 "crv":"Ed25519",
 "x":"CfaY1XX-aHJpenRP8ATm3yGlbcKA_treqOfwKrilwyg",
 "d":"fycSKS-iHZ6TC1BNwN6cE0sOBP3-4KgR-eqxNpnyhws"
}

A signed response would be:¶

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

HTTP/1.1 404 Not Found
Connection: close
Content-Digest: sha-256=:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU\
=:
Content-Type: text/plain
Signature: wimse=:NMrMn3xhI6m9PI8mKVfpnH5qFGcEfuFxiCmsB5PJhGjUHT/5J4\
612EZwRw3V4kU4gGJmO+ER8RC4DM2HKVOYDQ==:
Signature-Input: wimse=("@status" "workload-identity-token" "content\
-type" "content-digest" "@method";req "@request-target";req);created\
=1718295368;expires=1718295670;nonce="abcd2222";tag="wimse-service-t\
o-service"
Workload-Identity-Token: aGVhZGVyCg.VGhpcyBhaW4ndCBvbmUsIHRvby4K.c2l\
nbmF0dXJlCg

No ice cream today.

5.1. Host Name Validation

TODO: need to define trust root used to validate the certificate is appropriate for the trust domain.¶

It is RECOMMENDED that the server certificate contain a DNS SAN that the client can use to perform standard host name validation (Section 6.3 of [RFC9525]). The client SHOULD also extract the WIMSE identity from the certificate if it is present and validate that the WIMSE trust domain matches the intended trust domain for the server. The client MAY then further use the WIMSE identity in applying authorization policy to the server. If the client does not use the DNS SAN then the client MUST match the WIMSE identity in the certificate against the WIMSE identity of the workload of the intended server according to a locally defined policy. The host portion of the WIMSE URI 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 WIMSE identity in a deployment specific way.¶

5.2. Authorization Using the WIMSE Identity

The client or server application may retrieve the WIMSE identity from the TLS layer for use in authorization, accounting and auditing. For example, the full URI may be matched against ACLs and other policy constructs to authorize actions requested by the peer.¶

6.1. WIMSE Identity

The WIMSE ID is scoped within an issuer and therefore any sub-components (path portion of ID) are only unique within a trust domain defined by the issuer. Using a WIMSE ID 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 root through some mechanism such as a JWKS or X.509 certificate chain. The association of an issuer, trust domain and a cryptographic trust root MUST be communicated securely out of band.¶

TODO: Should there be a DNS name to Trust domain mapping defined or some other discovery mechanism?¶

6.2. Workload ID Token and Proof of Possession

The Workload ID token (WIT) is bound to a secret cryptographic key and is always presented with a proof of possession as described in Section 4.1. 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 connection unless a secure channel is provided by some other mechanisms. Host name validation according to Section 5 MUST be performed. The WIT itself is not usable without a proof of possession.¶

• 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.¶

• Binding to a Timestamp or Nonce¶

A proof of possession should include information that can be used to uniquely identify it such as a unique timestamp or nonce. This can 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. In this case, if the PoP is not sufficiently scoped it may be usable with another workload. While a fresh nonce could be included in the PoP, a mechanism for distributing a fresh challenge nonce from the validator to provide single use properties of a PoP is outside the scope of this specification.¶

• 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.¶

6.3. Middle Boxes

In some deployments the WIMSE 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 the previous section can be used to provide some protection from middle boxes. Deployments should perform analysis on their situation to determine if it is appropriate to trust and allow traffic to pass through a middle box.¶

6.4. 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 WIMSE 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.¶

7.1. Media Type Registration

TODO: application/wimse-id+jwt or appropriately bikeshedded media type name (despite my ongoing unease with using media types for typing JWTs) in Media Types.¶

TODO: application/wimse-proof+jwt ...¶

7.2. Hypertext Transfer Protocol (HTTP) Field Name Registration

TODO: Workload-Identity-Token from Section 4.1.1¶

TODO: Workload-Proof-Token from Section 4.2¶