Internet-Draft | HTTP Transport Authentication | July 2022 |
Schinazi & Oliver | Expires 12 January 2023 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://DavidSchinazi.github.io/draft-schinazi-httpbis-transport-auth/draft-schinazi-httpbis-transport-auth.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-schinazi-httpbis-transport-auth/.¶
Discussion of this document takes place on the HTTP Working Group mailing list (mailto:ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.¶
Source for this draft and an issue tracker can be found at https://github.com/DavidSchinazi/draft-schinazi-httpbis-transport-auth.¶
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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.¶
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.¶
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.¶
This document uses the Augmented BNF defined in [ABNF] and updated by [ABNF2] along with the "#rule" extension defined in Section 5.6.1 of [HTTP]. The rules below are defined in [HTTP] and [OID].¶
OWS = <OWS, see {{Section 5.6.3 of HTTP}}> quoted-string = <quoted-string, see {{Section 5.6.4 of HTTP}}> token = <token, see {{Section 5.6.2 of HTTP}}> token68 = <token68, see {{Section 5.6.3 of HTTP}}> oid = <oid, see {{Section 2 of OID}}>¶
This document only defines Transport Authentication for uses of HTTP with TLS. This includes any use of HTTP over TLS as typically used for HTTP/2, or 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-id's key. The keying material exporter uses a label that starts with the characters "EXPORTER-HTTP-Transport-Authentication-" (see Section 4 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 "Transport-Authentication" header allows a user agent to authenticate with an origin server. The authentication is scoped to the HTTP request associated with this header.¶
Transport-Authentication = tpauth-scheme *( OWS ";" OWS param ) tpauth-scheme = token param = token "=" ( token / quoted-string )¶
The OPTIONAL "u" (user-id) directive specifies the user-id that the user agent wishes to authenticate. It is encoded using Base64 (Section 4 of [BASE64]).¶
u = token68¶
The OPTIONAL "p" (proof) directive specifies the proof that the user agent provides to attest to possessing the credential that matches its user-id. It is encoded using Base64 (Section 4 of [BASE64]).¶
p = token68¶
The OPTIONAL "a" (algorithm) directive specifies the algorithm used to compute the proof transmitted in the "p" directive.¶
a = oid¶
The Transport Authentication Framework allows defining Transport Authentication Schemes, which specify how to authenticate user-ids. This documents defined the "Signature" and "HMAC" schemes.¶
The "Signature" Transport Authentication Scheme uses asymmetric cyptography. User agents possess a user-id and a public/private key pair, and origin servers maintain a mapping of authorized user-ids to their associated public keys. When using this scheme, the "u", "p", and "a" directives are REQUIRED. The TLS keying material export label for this scheme is "EXPORTER-HTTP-Transport-Authentication-Signature" and the associated context is empty. The nonce is then signed using the selected asymmetric signature algorithm and transmitted as the proof directive.¶
For example, the user-id "john.doe" authenticating using Ed25519 [ED25519] could produce the following header (lines are folded to fit):¶
Transport-Authentication: Signature u="am9obi5kb2U="; a=1.3.101.112; p="SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ=="¶
The "HMAC" Transport Authentication Scheme uses symmetric cyptography. User agents possess a user-id and a secret key, and origin servers maintain a mapping of authorized user-ids to their associated secret key. When using this scheme, the "u", "p", and "a" directives are REQUIRED. The TLS keying material export label for this scheme is "EXPORTER-HTTP-Transport-Authentication-HMAC" and the associated context is empty. The nonce is then HMACed using the selected HMAC algorithm and transmitted as the proof directive.¶
For example, the user-id "john.doe" authenticating using HMAC-SHA-512 [SHA] could produce the following header (lines are folded to fit):¶
Transport-Authentication: HMAC u="am9obi5kb2U="; a=2.16.840.1.101.3.4.2.3; p="SW5zZXJ0IEhNQUMgb2Ygbm9uY2UgaGVyZSB3aGljaCB0YWtl cyA1MTIgYml0cyBmb3IgU0hBLTUxMiEhISEhIQ=="¶
Since Transport Authentication authenticates the underlying transport by leveraging TLS keying material exporters, it 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.¶
Transport 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.¶
This document will request IANA to register the following entry in the "HTTP Field Name" registry maintained at <https://www.iana.org/assignments/http-fields>:¶
This document, if approved, requests IANA to create a new "HTTP Transport Authentication Schemes" Registry. This new registry contains strings and is covered by the First Come First Served policy from Section 4.4 of [IANA-POLICY]. Each entry contains an optional "Reference" field.¶
It initially contains the following entries:¶
The reference for both is this document.¶
This document, if approved, requests IANA to register the following entries in the "TLS Exporter Labels" registry maintained at https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels¶
Both of these entries are listed with the following qualifiers:¶
The authors would like to thank many members of the IETF community, as this document is the fruit of many hallway conversations. Using the OID for the signature and HMAC algorithms was inspired by Signature Authentication in IKEv2.¶