OAuth Proof of Possession Tokens with HTTP Message Signatures

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Table of Contents

1. Introduction

The OAuth 2.0 framework provides methods for clients to get delegated access tokens from an authorization server for accessing protected resources. The access tokens at the center of OAuth 2.0 can be bound to a variety of different mechanisms, including bearer tokens, mutual TLS, or other presentation mechanisms.¶

Bearer tokens are simple to implement but also have the significant security downside of allowing anyone who sees the access token to use that token. This extension defines a token type that binds the token to a presentation key known to the client. The client uses HTTP Message Signatures to sign requests using its key, thereby proving its right to present the associated access token.¶

2. Token Response

When the client makes an access token request, the AS associates the generated access token with the client's registered key from the client's jwks or jwks_uri field. All presentations of this token at any RS MUST contain an HTTP message signature as described in Section 3.¶

A bound access token MUST have a token_type value of httpsig. The response MUST contain a keyid value which indicates the key the client MUST use when presenting the access token Section 3. The value of this keyid field MUST uniquely identify a key from the client's registered key set by its kid value.¶

{
    "access_token": "2340897.34j123-134uh2345n",
    "token_type": "httpsig",
    "keyid": "test-key-rsa-pss"
}

[[ Editor's note: while this document deals only with using a pre-registered key, it would be possible to have different key binding mechanisms, such as the client presenting an ephemeral key during the token request or the AS generating and assigning a key alongside the token. The WG needs to decide if this is in scope of this document or not. The presentation mechanisms would be the same. ]]¶

3. Presenting an HTTP Message Signature Bound Access Token

The algorithm and key used for the HTTP Message Signature are derived from the client's registered information. The key is taken from the client's registered jwks or jwks_uri field, identified by the keyid field of the token response Section 2. The signature algorithm is determined by the alg field of the identified key, following the method for JSON Web Algorithm selection described in [I-D.ietf-httpbis-message-signatures].¶

The client MUST include the access token value in an Authorization header using scheme HTTPSig. Note that the scheme value HTTPSig is not case sensitive.¶

Authorization: HTTPSig 2340897.34j123-134uh2345n

The client MUST include an HTTP Message Signature that covers, at minimum:¶

• The request target of the RS being called¶
• The Host header of the RS being called¶
• The Authorization header containing the access token value.¶

The signature parameters MUST include a created signature parameter. The RS SHOULD use this field to ensure freshness of the signed request, appropriate to the API being protected.¶

The client MUST NOT include an alg signature parameter, since the algorithm is determined by the client's registered key. The client MUST include the keyid signature parameter set to the value returned in the token response Section 2.¶

In this example, the client has a key with the kid value of test-key-rsa-pss which uses the JWA alg value of PS512. The signature input string is:¶

"@request-target": get /foo
"host": example.org
"authorization": HTTPSig 2340897.34j123-134uh2345n
"@signature-params": ("@request-target" "host" "authorization")\
  ;created=1618884475;keyid="test-key-rsa-pss"

This results in the following signed HTTP message, including the access token.¶

GET /foo HTTP/1.1
Host: example.com
Date: Tue, 20 Apr 2021 02:07:55 GMT
Authorization: HTTPSig 2340897.34j123-134uh2345n
Signature-Input: sig1=("@request-target" "host" "authorization")\
  ;created=1618884475;keyid="test-key-rsa-pss"
Signature: sig1=:o+Fy/a6IIWhHwnMFhsHqfXEpheWGBMOU3pheT50zA8rL5F8Nur\
  xBKAPylMGBWYCKH5Bd+TB0Co6vqANlXyOCM9Zr5c/UmR5WGex5/OgJJmfN7gOVOH5\
  pB2Zxa233xsohfwo9liBlctukN5//E3F04rKjIkoeTFJiS+hMcOzn29esgFSEl4Jy\
  oO5Q8snMIsC56ZAPYwU7rJis1Wvl6Y9/9tpW6gIn/SHwArhPQSAb0zZy6mCiw654n\
  CaKw5NYJ9S0DZlnV4T7nJtdZsHOkddF6kH4WVka3ev0xONI5kYkEdR1Gw0VAE9thi\
  p+3/aFoUVTJ/1J6JfehZpXqehwv3KNoQ==:

An RS receiving such a signed message and a bound access token MUST verify the HTTP Message Signature as described in [I-D.ietf-httpbis-message-signatures]. The RS MUST verify that all required portions of the HTTP request are covered by the signature by examining the contents of the signature parameters.¶

[[ Editor's note: we should define confirmation methods for access tokens here, including JWT values and introspection response values to allow the RS to verify the signature w/o the client's registration information. ]]¶

4. Acknowledgements

5. IANA Considerations

[[ TBD: register the token type and new parameters into their appropriate registries, as well as the JWT and introspection parameters. ]]¶

6. Security Considerations

[[ TBD: There are a lot of security considerations to add. ]]¶

All requests have to be over TLS or equivalent as per [BCP195].¶

7. Privacy Considerations

[[ TBD: There are a lot of privacy considerations to add. ]]¶

Appendix A. Document History

• -00¶

  • Initial individual draft.¶

Author's Address