HTTP Unprompted Authentication

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Table of Contents

1. Introduction

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.¶

Unprompted Authentication serves use cases in which a site wants to offer a service or capability only to "those who know" while all others are given no indication the service or capability exists. The conceptual model is that of a "speakeasy". "Knowing" is via an externally-defined mechanism by which keys are distributed. For example, a company might offer remote employee access to company services directly via its website using their employee credentials, or offer access to limited special capabilities for specific employees, while making discovering (probing for) such capabilities difficult. Members of less well-defined communities might use more ephemeral keys to acquire access to geography- or capability-specific resources, as issued by an entity whose user base is larger than the available resources can support (by having that entity metering the availability of keys temporally or geographically). Unprompted Authentication is also useful for cases where a service provider wants to distribute user-provisioning information for its resources without exposing the provisioning location to non-users.¶

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.¶

2. Computing the Authentication Proof

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 chosen 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.¶

Because the TLS keying material exporter is only secure for authentication when it is uniquely bound to the TLS session [RFC7627], the Signature and HMAC authentication schemes require either one of the following properties:¶

• The TLS version in use is greater or equal to 1.3 [TLS].¶
• The TLS version in use is greater or equal to 1.2 and the Extended Master Secret extension [RFC7627] has been negotiated.¶

Clients MUST NOT use the Signature and HMAC authentication schemes on connections that do not meet one of the two properties above. If a server receives a request that uses these authentication schemes on a connection that meets neither of the above properties, the server MUST treat the request as malformed.¶

3. Header Field Definition

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.¶

4. Authentication Parameters

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

5. Authentication Schemes

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

Unprompted-Authentication: Signature k=:YmFzZW1lbnQ=:;s=7;
p=:SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ==:

Unprompted-Authentication: HMAC k="YmFzZW1lbnQ=";h=6;
p="SW5zZXJ0IEhNQUMgb2Ygbm9uY2UgaGVyZSB3aGljaCB0YWtl
cyA1MTIgYml0cyBmb3IgU0hBLTUxMiEhISEhIQ=="

6. Server Handling

Servers that wish to introduce resources whose existence cannot be probed need to ensure that they do not reveal any information about those resources to unauthenticated clients. In particular, such servers MUST respond to authentication failures with the exact same response that they would have used for non-existent resources. For example, this can mean using HTTP status code 404 (Not Found) instead of 401 (Unauthorized). Such authentication failures can be caused for example by: * absence of the Unprompted-Authentication field * failure to parse the Unprompted-Authentication field * use of Unprompted Authentication with an unknown key ID * failure to validate the signature or MAC.¶

Such servers MUST also ensure that the timing of their request handling does not leak any information. This can be accomplished by delaying responses to all non-existent resources such that the timing of the authentication verification is not observable.¶

7. Intermediary Considerations

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 have two options:¶

• The intermediary can validate the authentication received from the client, then inform the upstream HTTP server of the presence of valid authentication.¶
• The intermediary can export the nonce (see Section 2}), and forward it to the upstream HTTP server, then the upstream server performs the validation.¶

The mechanism for the intermediary to communicate this information to the upstream HTTP server is out of scope for this document.¶

8. Security Considerations

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 key is used for authentication proofs 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.¶

Key material used for authentication in unprompted authentication, whether symmetric or asymmetric MUST NOT be reused in other protocols. Doing so can undermine the security guarantees of the authentication.¶

Sites offering Unprompted Authentication are able to link requests that use the same key for the Authentication Schemes provided. However, requests are not linkable across other sites if the keys used are private to the individual sites using Unprompted Authentication.¶

9. IANA Considerations

Acknowledgments

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.¶

Authors' Addresses