HTTP Unprompted Authentication

Existing HTTP authentication mechanisms are probeable in the sense that it is possible for an unauthenticated client to probe whether an origin serves resources that require authentication. It is possible for an origin to hide the fact that it requires authentication by not generating Unauthorized status codes, however that only works with non-cryptographic authentication schemes: cryptographic schemes (such as signatures or message authentication codes) require a fresh nonce to be signed, and there is no existing way for the origin to share such a nonce without exposing the fact that it serves resources that require authentication. This document proposes a new non-probeable cryptographic authentication scheme.¶

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Existing HTTP authentication mechanisms (see Section 11 of [HTTP]) are probeable in the sense that it is possible for an unauthenticated client to probe whether an origin serves resources that require authentication. It is possible for an origin to hide the fact that it requires authentication by not generating Unauthorized status codes, however that only works with non-cryptographic authentication schemes: cryptographic schemes (such as signatures or message authentication codes) require a fresh nonce to be signed, and there is no existing way for the origin to share such a nonce without exposing the fact that it serves resources that require authentication. This document proposes a new non-probeable cryptographic authentication scheme.¶

There are scenarios where servers may want to expose the fact that authentication is required for access to specific resources. This is left for future work.¶

This document only defines the Signature and HMAC authentication schemes for uses of HTTP with TLS [TLS]. This includes any use of HTTP over TLS as typically used for HTTP/2 [HTTP/2], or HTTP/3 [HTTP/3] where the transport protocol uses TLS as its authentication and key exchange mechanism [QUIC-TLS].¶

The user agent leverages a TLS keying material exporter [KEY-EXPORT] to generate a nonce which can be signed using the user's key. The keying material exporter uses a label that starts with the characters "EXPORTER-HTTP-Unprompted-Authentication-" (see Section 5 for the labels and contexts used by each scheme). The TLS keying material exporter is used to generate a 32-byte key which is then used as a nonce.¶

The "Unprompted-Authentication" header field allows a user agent to authenticate with an origin server. The authentication is scoped to the HTTP request associated with this header field. The value of the Unprompted-Authentication header field is a credentials object, as defined in Section 11.4 of [HTTP]. Credentials contain an authentication scheme followed by optional authentication parameters.¶

This specification defines the following authentication parameters, they can be used by the authentication schemes defined in Section 5.¶

This document defines the "Signature" and "HMAC" HTTP authentication schemes.¶

Since the Signature and HMAC HTTP Authentication Schemes leverage TLS keying material exporters, their output cannot be transparently forwarded by HTTP intermediaries. HTTP intermediaries that support this specification will validate the authentication received from the client themselves, then inform the upstream HTTP server of the presence of valid authentication using some other mechanism.¶

Unprompted Authentication allows a user-agent to authenticate to an origin server while guaranteeing freshness and without the need for the server to transmit a nonce to the user agent. This allows the server to accept authenticated clients without revealing that it supports or expects authentication for some resources. It also allows authentication without the user agent leaking the presence of authentication to observers due to clear-text TLS Client Hello extensions.¶

The authentication proofs described in this document are not bound to individual HTTP requests; if the same user sends an authentication proof on multiple requests they will all be identical. This allows for better compression when sending over the wire, but implies that client implementations that multiplex different security contexts over a single HTTP connection need to ensure that those contexts cannot read each other's header fields. Otherwise, one context would be able to replay the unprompted authentication header field of another. This constraint is met by modern Web browsers. If an attacker were to compromise the browser such that it could access another context's memory, the attacker might also be able to access the corresponding key, so binding authentication to requests would not provide much benefit in practice.¶

The authors would like to thank many members of the IETF community, as this document is the fruit of many hallway conversations. Ben Schwartz contributed ideas to this document.¶