Remove HMAC and switch to Authorization

draft-ietf-httpbis-unprompted-auth.md

@@ -1,5 +1,5 @@
 ---
-title: HTTP Unprompted Authentication
+title: The Signature HTTP Authentication Scheme
 docname: draft-ietf-httpbis-unprompted-auth-latest
 submissiontype: IETF
 number:
@@ -55,47 +55,47 @@ informative:
 
 --- abstract
 
-Existing HTTP authentication mechanisms are probeable in the sense that it is
+Existing HTTP authentication schemes 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.
+cryptographic signatures 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.
 
 --- middle
 
 # Introduction {#introduction}
 
-Existing HTTP authentication mechanisms (see {{Section 11 of !HTTP=RFC9110}})
-are probeable in the sense that it is possible for an unauthenticated client to
+Existing HTTP authentication schemes (see {{Section 11 of !HTTP=RFC9110}}) 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.
+non-cryptographic authentication schemes: cryptographic signatures 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.
+
+The Signature HTTP authentication scheme 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). The Signature HTTP authentication scheme 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
@@ -109,129 +109,74 @@ This document uses the following terminology from {{Section 3 of
 !STRUCTURED-FIELDS=RFC8941}} to specify syntax and parsing: Integer and Byte
 Sequence.
 
-# Computing the Authentication Proof {#compute-proof}
+# The Signature Authentication Scheme
 
-This document only defines the Signature and HMAC authentication schemes for
-uses of HTTP with TLS {{!TLS=RFC8446}}. This includes any use of HTTP over TLS
-as typically used for HTTP/2 {{H2}}, or HTTP/3 {{H3}} where the transport
-protocol uses TLS as its authentication and key exchange mechanism
-{{?QUIC-TLS=RFC9001}}.
+This document defines the "Signature" HTTP authentication scheme. It uses
+asymmetric cryptography. User agents possess a key ID and a public/private key
+pair, and origin servers maintain a mapping of authorized key IDs to their
+associated public keys.
 
-The user agent leverages a TLS keying material exporter {{!KEY-EXPORT=RFC5705}}
-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 {{schemes}} 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.
+This authentication scheme is only defined for uses of HTTP with TLS
+{{!TLS=RFC8446}}. This includes any use of HTTP over TLS as typically used for
+HTTP/2 {{H2}}, or HTTP/3 {{H3}} where the transport protocol uses TLS as its
+authentication and key exchange mechanism {{?QUIC-TLS=RFC9001}}.
 
 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:
+it is uniquely bound to the TLS session {{!RFC7627}}, the Signature
+authentication scheme requires 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.
+* The TLS version in use is 1.2 and the Extended Master Secret extension
+  {{RFC7627}} has been negotiated.
 
-# Header Field Definition {#header-definition}
+Clients MUST NOT use the Signature authentication scheme on connections that do
+not meet one of the two properties above. If a server receives a request that
+uses this authentication scheme on a connection that meets neither of the above
+properties, the server MUST treat the request as malformed.
 
-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.
+# Computing the Authentication Proof {#compute-proof}
+
+The user agent leverages a TLS keying material exporter {{!KEY-EXPORT=RFC5705}}
+with the label "EXPORTER-HTTP-Signature-Authentication" to generate a 32-byte
+symmetric key. That symmetric key is then used as nonce which can be signed
+using the client's chosen asymmetric private key. The resulting signature is
+then transmitted to the server using the Authorization field.
 
 # Authentication Parameters
 
-This specification defines the following authentication parameters, they can be
-used by the authentication schemes defined in {{schemes}}.
+This specification defines the following authentication parameters:
 
 ## The k Parameter {#parameter-k}
 
-The OPTIONAL "k" (key ID) parameter is a byte sequence that identifies which key
+The REQUIRED "k" (key ID) parameter is a byte sequence that identifies which key
 the user agent wishes to use to authenticate. This can for example be used to
 point to an entry into a server-side database of known keys.
 
 ## The p Parameter {#parameter-p}
 
-The OPTIONAL "p" (proof) parameter is a byte sequence that specifies the proof
+The REQUIRED "p" (proof) parameter is a byte sequence that specifies the proof
 that the user agent provides to attest to possessing the credential that matches
 its key ID.
 
 ## The s Parameter {#parameter-s}
 
-The OPTIONAL "s" (signature) parameter is an integer that specifies the
-signature algorithm used to compute the proof transmitted in the "p" directive.
-Its value is an integer between 0 and 255 inclusive from the IANA "TLS
-SignatureAlgorithm" registry maintained at
-<[](https://www.iana.org/assignments/tls-parameters#tls-parameters-16)>.
-
-## The h Parameter {#parameter-h}
-
-The OPTIONAL "h" (hash) parameter is an integer that specifies the hash
-algorithm used to compute the proof transmitted in the "p" directive. Its value
-is an integer between 0 and 255 inclusive from the IANA "TLS HashAlgorithm"
-registry maintained at
-<[](https://www.iana.org/assignments/tls-parameters#tls-parameters-18)>.
-
-# Authentication Schemes {#schemes}
-
-This document defines the "Signature" and "HMAC" HTTP authentication schemes.
-
-## Signature {#signature}
-
-The "Signature" HTTP Authentication Scheme uses asymmetric cryptography.
-User agents possess a key ID and a public/private key pair, and origin servers
-maintain a mapping of authorized key IDs to their associated public keys. When
-using this scheme, the "k", "p", and "s" parameters are REQUIRED. The TLS keying
-material export label for this scheme is
-"EXPORTER-HTTP-Unprompted-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.
+The REQUIRED "s" (signature) parameter is an integer that specifies the
+signature scheme used to compute the proof transmitted in the "p" directive.
+Its value is an integer between 0 and 65535 inclusive from the IANA "TLS
+SignatureScheme" registry maintained at
+<[](https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-signaturescheme)>.
 
 For example, the key ID "basement" authenticating using Ed25519
 {{?ED25519=RFC8410}} could produce the following header field (lines are folded
 to fit):
 
 ~~~
-Unprompted-Authentication: Signature k=:YmFzZW1lbnQ=:;s=7;
+Authorization: Signature k=:YmFzZW1lbnQ=:;s=2055;
 p=:SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
 aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ==:
 ~~~
 
-## HMAC {#hmac}
-
-The "HMAC" HTTP Authentication Scheme uses symmetric cryptography. User
-agents possess a key ID and a secret key, and origin servers maintain a mapping
-of authorized key IDs to their associated secret key. When using this scheme,
-the "k", "p", and "h" parameters are REQUIRED. The TLS keying material export
-label for this scheme is "EXPORTER-HTTP-Unprompted-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 key ID "basement" authenticating using HMAC-SHA-512
-{{?SHA=RFC6234}} could produce the following header field (lines are folded to
-fit):
-
-~~~
-Unprompted-Authentication: HMAC k=:YmFzZW1lbnQ=:;h=6;
-p=:SW5zZXJ0IEhNQUMgb2Ygbm9uY2UgaGVyZSB3aGljaCB0YWtl
-cyA1MTIgYml0cyBmb3IgU0hBLTUxMiEhISEhIQ==:
-~~~
-
-## Other HTTP Authentication Schemes
-
-The HTTP Authentication Scheme registry maintained by IANA at
-<[](https://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml)>
-contains entries not defined in this document. Those entries MAY be used with
-Unprompted Authentication.
-
 # Server Handling
 
 Servers that wish to introduce resources whose existence cannot be probed need
@@ -241,10 +186,10 @@ 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.
+* absence of the Authorization field
+* failure to parse the Authorization field
+* use of the Signature authentication scheme with an unknown key ID
+* failure to validate the signature.
 
 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
@@ -253,10 +198,9 @@ verification is not observable.
 
 # Intermediary Considerations {#intermediary}
 
-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:
+Since the Signature HTTP authentication scheme leverages TLS keying material
+exporters, its 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.
@@ -270,87 +214,65 @@ upstream HTTP server is out of scope for this document.
 
 # Security Considerations {#security}
 
-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
+The Signature HTTP authentication scheme 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
+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.
-
-Origins offering Unprompted Authentication are able to link requests that use
-the same key for the Authentication Schemes provided. However, requests are
-not linkable across origins if the keys used are specific to the individual
-origins using Unprompted Authentication.
+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 on the same connection, 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 Authorization
+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 the Signature HTTP authentication scheme MUST NOT be
+reused in other protocols. Doing so can undermine the security guarantees of
+the authentication.
+
+Origins offering this scheme are able to link requests that use the same key.
+However, requests are not linkable across origins if the keys used are specific
+to the individual origins using this scheme.
 
 # IANA Considerations {#iana}
 
-## Unprompted-Authentication Header Field {#iana-header}
-
-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)>:
-
-Field Name:
-
-: Unprompted-Authentication
-
-Template:
+## HTTP Authentication Schemes Registry {#iana-schemes}
 
-: None
+This document, if approved, requests IANA to register the following entry in
+the "HTTP Authentication Schemes" Registry maintained at
+<[](https://www.iana.org/assignments/http-authschemes)>:
 
-Status:
+Authentication Scheme Name:
 
-: provisional (permanent if this document is approved)
+: Signature
 
 Reference:
 
 : This document
 
-Comments:
+Notes:
 
 : None
 {: spacing="compact"}
 
-## HTTP Authentication Schemes Registry {#iana-schemes}
-
-This document, if approved, requests IANA to add two new entries to the
-"HTTP Authentication Schemes" Registry maintained at
-<[](https://www.iana.org/assignments/http-authschemes)>.
-Both entries have the Reference set to this document, and the Notes empty.
-The Authentication Scheme Name of the entries are:
-
-* Signature
-
-* HMAC
-
 ## TLS Keying Material Exporter Labels {#iana-exporter-label}
 
-This document, if approved, requests IANA to register the following entries in
+This document, if approved, requests IANA to register the following entry in
 the "TLS Exporter Labels" registry maintained at
 <[](https://www.iana.org/assignments/tls-parameters#exporter-labels)>:
 
-* EXPORTER-HTTP-Unprompted-Authentication-Signature
-
-* EXPORTER-HTTP-Unprompted-Authentication-HMAC
+Value:
 
-Both of these entries are listed with the following qualifiers:
+: EXPORTER-HTTP-Signature-Authentication
 
 DTLS-OK: