draft-ietf-httpbis-unprompted-auth-06.txt		draft-ietf-httpbis-unprompted-auth-latest.txt
	HTTPBIS Working Group D. Schinazi		HTTPBIS Working Group D. Schinazi
	Internet-Draft Google LLC		Internet-Draft Google LLC
	Intended status: Standards Track D. Oliver		Intended status: Standards Track D. Oliver
	Expires: July 27, 2024 Guardian Project		Expires: November 29, 2024 Guardian Project
	J. Hoyland		J. Hoyland
	Cloudflare Inc.		Cloudflare Inc.
	January 24, 2024		May 28, 2024
	The Signature HTTP Authentication Scheme		The Concealed HTTP Authentication Scheme
	draft-ietf-httpbis-unprompted-auth-06		draft-ietf-httpbis-unprompted-auth-latest
	Abstract		Abstract
	Existing HTTP authentication schemes are probeable in the sense that		Most HTTP authentication schemes are probeable in the sense that it
	it is possible for an unauthenticated client to probe whether an		is possible for an unauthenticated client to probe whether an origin
	origin serves resources that require authentication. It is possible		serves resources that require authentication. It is possible for an
	for an origin to hide the fact that it requires authentication by not		origin to hide the fact that it requires authentication by not
	generating Unauthorized status codes, however that only works with		generating Unauthorized status codes, however that only works with
	non-cryptographic authentication schemes: cryptographic signatures		non-cryptographic authentication schemes: cryptographic signatures
	require a fresh nonce to be signed, and there is no existing way for		require a fresh nonce to be signed. At the time of writing, there
	the origin to share such a nonce without exposing the fact that it		was no existing way for the origin to share such a nonce without
	serves resources that require authentication. This document proposes		exposing the fact that it serves resources that require
	a new non-probeable cryptographic authentication scheme.		authentication. This document proposes a new non-probeable
			cryptographic authentication scheme.
	About This Document		About This Document
	This note is to be removed before publishing as an RFC.		This note is to be removed before publishing as an RFC.
	The latest revision of this draft can be found at		The latest revision of this draft can be found at
	<https://httpwg.org/http-extensions/draft-ietf-httpbis-unprompted-		<https://httpwg.org/http-extensions/draft-ietf-httpbis-unprompted-
	auth.html>. Status information for this document may be found at		auth.html>. Status information for this document may be found at
	<https://datatracker.ietf.org/doc/draft-ietf-httpbis-unprompted-		<https://datatracker.ietf.org/doc/draft-ietf-httpbis-unprompted-
	auth/>.		auth/>.
	skipping to change at page 2, line 15 ¶		skipping to change at page 2, line 20 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
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	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
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	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on July 27, 2024.		This Internet-Draft will expire on November 29, 2024.
	Copyright Notice		Copyright Notice
	Copyright (c) 2024 IETF Trust and the persons identified as the		Copyright (c) 2024 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
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	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3		1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
	2. The Signature Authentication Scheme . . . . . . . . . . . . . 4		2. The Concealed Authentication Scheme . . . . . . . . . . . . . 4
	3. TLS Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Client Handling . . . . . . . . . . . . . . . . . . . . . . . 4
	4. Computing the Authentication Proof . . . . . . . . . . . . . 4		3.1. Key Exporter Context . . . . . . . . . . . . . . . . . . 4
	4.1. Key Exporter Context . . . . . . . . . . . . . . . . . . 5		3.2. Key Exporter Output . . . . . . . . . . . . . . . . . . . 6
	4.2. Key Exporter Output . . . . . . . . . . . . . . . . . . . 6		3.3. Signature Computation . . . . . . . . . . . . . . . . . . 7
	4.3. Signature Computation . . . . . . . . . . . . . . . . . . 7		4. Authentication Parameters . . . . . . . . . . . . . . . . . . 7
	5. Authentication Parameters . . . . . . . . . . . . . . . . . . 7		4.1. The k Parameter . . . . . . . . . . . . . . . . . . . . . 8
	5.1. The k Parameter . . . . . . . . . . . . . . . . . . . . . 8		4.2. The a Parameter . . . . . . . . . . . . . . . . . . . . . 8
	5.2. The a Parameter . . . . . . . . . . . . . . . . . . . . . 8		4.3. The p Parameter . . . . . . . . . . . . . . . . . . . . . 8
	5.3. The p Parameter . . . . . . . . . . . . . . . . . . . . . 8		4.4. The s Parameter . . . . . . . . . . . . . . . . . . . . . 8
	5.4. The s Parameter . . . . . . . . . . . . . . . . . . . . . 8		4.5. The v Parameter . . . . . . . . . . . . . . . . . . . . . 8
	5.5. The v Parameter . . . . . . . . . . . . . . . . . . . . . 8		5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
	6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 9		6. Server Handling . . . . . . . . . . . . . . . . . . . . . . . 9
	7. Non-Probeable Server Handling . . . . . . . . . . . . . . . . 9		6.1. Frontend Handling . . . . . . . . . . . . . . . . . . . . 9
	8. Intermediary Considerations . . . . . . . . . . . . . . . . . 10		6.2. Communication between Frontend and Backend . . . . . . . 10
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 10		6.3. Backend Handling . . . . . . . . . . . . . . . . . . . . 10
	10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11		6.4. Non-Probeable Server Handling . . . . . . . . . . . . . . 11
	10.1. HTTP Authentication Schemes Registry . . . . . . . . . . 11		7. Requirements on TLS Usage . . . . . . . . . . . . . . . . . . 12
	10.2. TLS Keying Material Exporter Labels . . . . . . . . . . 11		8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
	11.1. Normative References . . . . . . . . . . . . . . . . . . 12		9.1. HTTP Authentication Schemes Registry . . . . . . . . . . 13
	11.2. Informative References . . . . . . . . . . . . . . . . . 13		9.2. TLS Keying Material Exporter Labels . . . . . . . . . . . 13
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14		9.3. HTTP Field Name . . . . . . . . . . . . . . . . . . . . . 13
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
			10.1. Normative References . . . . . . . . . . . . . . . . . . 14
			10.2. Informative References . . . . . . . . . . . . . . . . . 15
			Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
	1. Introduction		1. Introduction
	HTTP authentication schemes (see Section 11 of [HTTP]) allow origins		HTTP authentication schemes (see Section 11 of [HTTP]) allow origins
	to restrict access for some resources to only authenticated requests.		to restrict access for some resources to only authenticated requests.
	While these schemes commonly involve a challenge where the origin		While these schemes commonly involve a challenge where the origin
	asks the client to provide authentication information, it is possible		asks the client to provide authentication information, it is possible
	for clients to send such information unprompted. This is		for clients to send such information unprompted. This is
	particularly useful in cases where an origin wants to offer a service		particularly useful in cases where an origin wants to offer a service
	or capability only to "those who know" while all others are given no		or capability only to "those who know" while all others are given no
	indication the service or capability exists. Such designs rely on an		indication the service or capability exists. Such designs rely on an
	externally-defined mechanism by which keys are distributed. For		externally-defined mechanism by which keys are distributed. For
	example, a company might offer remote employee access to company		example, a company might offer remote employee access to company
	services directly via its website using their employee credentials,		services directly via its website using their employee credentials,
	or offer access to limited special capabilities for specific		or offer access to limited special capabilities for specific
	employees, while making discovering (probing for) such capabilities		employees, while making discovering (or probing for) such
	difficult. Members of less well-defined communities might use more		capabilities difficult. Members of less well-defined communities
	ephemeral keys to acquire access to geography- or capability-specific		might use more ephemeral keys to acquire access to geography- or
	resources, as issued by an entity whose user base is larger than the		capability-specific resources, as issued by an entity whose user base
	available resources can support (by having that entity metering the		is larger than the available resources can support (by having that
	availability of keys temporally or geographically).		entity metering the availability of keys temporally or
			geographically).
	While digital-signature-based HTTP authentication schemes already		While digital-signature-based HTTP authentication schemes already
	exist ([HOBA]), they rely on the origin explicitly sending a fresh		exist (e.g., [HOBA]), they rely on the origin explicitly sending a
	challenge to the client, to ensure that the signature input is fresh.		fresh challenge to the client, to ensure that the signature input is
	That makes the origin probeable as it send the challenge to		fresh. That makes the origin probeable as it sends the challenge to
	unauthenticated clients. This document defines a new signature-based		unauthenticated clients. This document defines a new signature-based
	authentication scheme that is not probeable.		authentication scheme that is not probeable.
	1.1. Conventions and Definitions		1.1. Conventions and Definitions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all		BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	This document uses the notation from Section 1.3 of [QUIC].		This document uses the notation from Section 1.3 of [QUIC].
	2. The Signature Authentication Scheme		2. The Concealed Authentication Scheme
	This document defines the "Signature" HTTP authentication scheme. It		This document defines the "Concealed" HTTP authentication scheme. It
	uses asymmetric cryptography. User agents possess a key ID and a		uses asymmetric cryptography. Clients possess a key ID and a public/
	public/private key pair, and origin servers maintain a mapping of		private key pair, and origin servers maintain a mapping of authorized
	authorized key IDs to their associated public keys.		key IDs to their associated public keys.
	The client uses a TLS keying material exporter to generate data to be		The client uses a TLS keying material exporter to generate data to be
	signed (see Section 4) then sends the signature using the		signed (see Section 3) then sends the signature using the
	Authorization or Proxy-Authorization header field. The signature and		Authorization or Proxy-Authorization header field. The signature and
	additional information are exchanged using authentication parameters		additional information are exchanged using authentication parameters
	(see Section 5).		(see Section 4).
	3. TLS Usage
	This authentication scheme is only defined for uses of HTTP with TLS
	[TLS]. 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].
	Because the TLS keying material exporter is only secure for
	authentication when it is uniquely bound to the TLS session
	[RFC7627], the Signature authentication scheme requires either one of
	the following properties:
	o The TLS version in use is greater or equal to 1.3 [TLS].
	o The TLS version in use is 1.2 and the Extended Master Secret
	extension [RFC7627] has been negotiated.
	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.
	4. Computing the Authentication Proof		3. Client Handling
	The user agent computes the authentication proof using a TLS keying		When a client wishes to uses the Concealed HTTP authentication scheme
	material exporter [KEY-EXPORT] with the following parameters:		with a request, it SHALL compute the authentication proof using a TLS
			keying material exporter [KEY-EXPORT] with the following parameters:
	o the label is set to "EXPORTER-HTTP-Signature-Authentication"		o the label is set to "EXPORTER-HTTP-Concealed-Authentication"
	o the context is set to the structure described in Section 4.1		o the context is set to the structure described in Section 3.1
	o the exporter output length is set to 48 bytes (see Section 4.2)		o the exporter output length is set to 48 bytes (see Section 3.2)
	4.1. Key Exporter Context		3.1. Key Exporter Context
	The TLS key exporter context is described in Figure 1:		The TLS key exporter context is described in Figure 1:
	Signature Algorithm (16),		Signature Algorithm (16),
	Key ID Length (i),		Key ID Length (i),
	Key ID (..),		Key ID (..),
	Public Key Length (i),		Public Key Length (i),
	Public Key (..),		Public Key (..),
	Scheme Length (i),		Scheme Length (i),
	Scheme (..),		Scheme (..),
	skipping to change at page 5, line 27 ¶		skipping to change at page 5, line 23 ¶
	Host (..),		Host (..),
	Port (16),		Port (16),
	Realm Length (i),		Realm Length (i),
	Realm (..),		Realm (..),
	Figure 1: Key Exporter Context Format		Figure 1: Key Exporter Context Format
	The key exporter context contains the following fields:		The key exporter context contains the following fields:
	Signature Algorithm: The signature scheme sent in the "s" Parameter		Signature Algorithm: The signature scheme sent in the "s" Parameter
	(see Section 5.4).		(see Section 4.4).
	Key ID: The key ID sent in the "k" Parameter (see Section 5.1).		Key ID: The key ID sent in the "k" Parameter (see Section 4.1).
	Public Key: The public key used by the server to validate the		Public Key: The public key used by the server to validate the
	signature provided by the client (the encoding is described		signature provided by the client (the encoding is described
	below).		below).
	Scheme: The scheme for this request, encoded using the format of the		Scheme: The scheme for this request, encoded using the format of the
	scheme portion of a URI as defined in Section 3.1 of [URI].		scheme portion of a URI as defined in Section 3.1 of [URI].
	Host: The host for this request, encoded using the format of the		Host: The host for this request, encoded using the format of the
	host portion of a URI as defined in Section 3.2.2 of [URI].		host portion of a URI as defined in Section 3.2.2 of [URI].
	Port: The port for this request, encoded in network byte order.		Port: The port for this request, encoded in network byte order.
	Note that the port is either included in the URI, or is the		Note that the port is either included in the URI, or is the
	default port for the scheme in use; see Section 3.2.3 of [URI].		default port for the scheme in use; see Section 3.2.3 of [URI].
	Realm: The real of authentication that is sent in the realm		Realm: The realm of authentication that is sent in the realm
	authentication parameter (Section 11.5 of [HTTP]). If the realm		authentication parameter (Section 11.5 of [HTTP]). If the realm
	authentication parameter is not present, this SHALL be empty.		authentication parameter is not present, this SHALL be empty.
	This document does not define a means for the origin to		This document does not define a means for the origin to
	communicate a realm to the client. If a client is not configured		communicate a realm to the client. If a client is not configured
	to use a specific realm, it SHALL use an empty realm and SHALL NOT		to use a specific realm, it SHALL use an empty realm and SHALL NOT
	send the realm authentication parameter.		send the realm authentication parameter.
	The Signature Algorithm and Port fields are encoded as unsigned		The Signature Algorithm and Port fields are encoded as unsigned
	16-bit integers in network byte order. The Key ID, Public Key,		16-bit integers in network byte order. The Key ID, Public Key,
	Scheme, Host, and Real fields are length prefixed strings; they are		Scheme, Host, and Realm fields are length prefixed strings; they are
	preceded by a Length field that represents their length in bytes.		preceded by a Length field that represents their length in bytes.
	These length fields are encoded using the variable-length integer		These length fields are encoded using the variable-length integer
	encoding from Section 16 of [QUIC] and MUST be encoded in the minimum		encoding from Section 16 of [QUIC] and MUST be encoded in the minimum
	number of bytes necessary.		number of bytes necessary.
	The encoding of the public key is determined by the Signature		The encoding of the public key is determined by the Signature
	Algorithm in use as follows:		Algorithm in use as follows:
	RSASSA-PSS algorithms: The public key is an RSAPublicKey structure		RSASSA-PSS algorithms: The public key is an RSAPublicKey structure
	[PKCS1] encoded in DER [X.690]. BER encodings which are not DER		[PKCS1] encoded in DER [X.690]. BER encodings which are not DER
	MUST be rejected.		MUST be rejected.
	skipping to change at page 6, line 30 ¶		skipping to change at page 6, line 26 ¶
	ECDSA algorithms: The public key is a		ECDSA algorithms: The public key is a
	UncompressedPointRepresentation structure defined in		UncompressedPointRepresentation structure defined in
	Section 4.2.8.2 of [TLS], using the curve specified by the		Section 4.2.8.2 of [TLS], using the curve specified by the
	SignatureScheme.		SignatureScheme.
	EdDSA algorithms: The public key is the byte string encoding defined		EdDSA algorithms: The public key is the byte string encoding defined
	in [EdDSA].		in [EdDSA].
	This document does not define the public key encodings for other		This document does not define the public key encodings for other
	algorithms. In order for a SignatureScheme to be usable with the		algorithms. In order for a SignatureScheme to be usable with the
	Signature HTTP authentication scheme, its public key encoding needs		Concealed HTTP authentication scheme, its public key encoding needs
	to be defined in a corresponding document.		to be defined in a corresponding document.
	4.2. Key Exporter Output		3.2. Key Exporter Output
	The key exporter output is 48 bytes long. Of those, the first 32		The key exporter output is 48 bytes long. Of those, the first 32
	bytes are part of the input to the signature and the next 16 bytes		bytes are part of the input to the signature and the next 16 bytes
	are sent alongside the signature. This allows the recipient to		are sent alongside the signature. This allows the recipient to
	confirm that the exporter produces the right values. This is		confirm that the exporter produces the right values. This is
	described in Figure 2:		described in Figure 2:
	Signature Input (256),		Signature Input (256),
	Verification (128),		Verification (128),
	Figure 2: Key Exporter Output Format		Figure 2: Key Exporter Output Format
	The key exporter context contains the following fields:		The key exporter context contains the following fields:
	Signature Input: This is part of the data signed using the client's		Signature Input: This is part of the data signed using the client's
	chosen asymmetric private key (see Section 4.3).		chosen asymmetric private key (see Section 3.3).
	Verification: The verification is transmitted to the server using		Verification: The verification is transmitted to the server using
	the v Parameter (see Section 5.5).		the v Parameter (see Section 4.5).
	4.3. Signature Computation		3.3. Signature Computation
	Once the Signature Input has been extracted from the key exporter		Once the Signature Input has been extracted from the key exporter
	output (see Section 4.2), it is prefixed with static data before		output (see Section 3.2), it is prefixed with static data before
	being signed to mitigate issues caused by key reuse. The signature		being signed to mitigate issues caused by key reuse. The signature
	is computed over the concatenation of:		is computed over the concatenation of:
	o A string that consists of octet 32 (0x20) repeated 64 times		o A string that consists of octet 32 (0x20) repeated 64 times
	o The context string "HTTP Signature Authentication"		o The context string "HTTP Concealed Authentication"
	o A single 0 byte which serves as a separator		o A single 0 byte which serves as a separator
	o The Signature Input extracted from the key exporter output (see		o The Signature Input extracted from the key exporter output (see
	Section 4.2)		Section 3.2)
	For example, if the Signature Input has all its 32 bytes set to 01,		For example, if the Signature Input has all its 32 bytes set to 01,
	the content covered by the signature (in hexadecimal format) would		the content covered by the signature (in hexadecimal format) would
	be:		be:
	2020202020202020202020202020202020202020202020202020202020202020		2020202020202020202020202020202020202020202020202020202020202020
	2020202020202020202020202020202020202020202020202020202020202020		2020202020202020202020202020202020202020202020202020202020202020
	48545450205369676E61747572652041757468656E7469636174696F6E		48545450205369676E61747572652041757468656E7469636174696F6E
	00		00
	0101010101010101010101010101010101010101010101010101010101010101		0101010101010101010101010101010101010101010101010101010101010101
	Figure 3: Example Content Covered by Signature		Figure 3: Example Content Covered by Signature
	This constructions mirrors that of the TLS 1.3 CertificateVerify		This constructions mirrors that of the TLS 1.3 CertificateVerify
	message defined in Section 4.4.3 of [TLS].		message defined in Section 4.4.3 of [TLS].
	The resulting signature is then transmitted to the server using the		The resulting signature is then transmitted to the server using the
	"p" Parameter (see Section 5.3).		"p" Parameter (see Section 4.3).
	5. Authentication Parameters		4. Authentication Parameters
	This specification defines the following authentication parameters.		This specification defines the following authentication parameters.
	All of the byte sequences below are encoded using base64url (see		All of the byte sequences below are encoded using base64url (see
	Section 5 of [BASE64]) without quotes and without padding. In other		Section 5 of [BASE64]) without quotes and without padding. In other
	words, these byte sequence authentication parameters values MUST NOT		words, these byte sequence authentication parameters values MUST NOT
	include any characters other then ASCII letters, digits, dash and		include any characters other then ASCII letters, digits, dash and
	underscore.		underscore.
	The integer below is encoded without a minus and without leading		The integer below is encoded without a minus and without leading
	zeroes. In other words, the integer authentication parameters value		zeroes. In other words, the integer authentication parameters value
	MUST NOT include any characters other than digits, and MUST NOT start		MUST NOT include any characters other than digits, and MUST NOT start
	with a zero unless the full value is "0".		with a zero unless the full value is "0".
	Using the syntax from [ABNF]:		Using the syntax from [ABNF]:
	signature-byte-sequence-param-value = *( ALPHA / DIGIT / "-" / "_" )		concealed-byte-sequence-param-value = *( ALPHA / DIGIT / "-" / "_" )
	signature-integer-param-value = %x31-39 1*4( DIGIT ) / "0"		concealed-integer-param-value = %x31-39 1*4( DIGIT ) / "0"
	Figure 4: Authentication Parameter Value ABNF		Figure 4: Authentication Parameter Value ABNF
	5.1. The k Parameter		4.1. The k Parameter
	The REQUIRED "k" (key ID) parameter is a byte sequence that		The REQUIRED "k" (key ID) parameter is a byte sequence that
	identifies which key the user agent wishes to use to authenticate.		identifies which key the client wishes to use to authenticate. This
	This can for example be used to point to an entry into a server-side		can for example be used to point to an entry into a server-side
	database of known keys.		database of known keys.
	5.2. The a Parameter		4.2. The a Parameter
	The REQUIRED "a" (public key) parameter is a byte sequence that		The REQUIRED "a" (public key) parameter is a byte sequence that
	contains the public key used by the server to validate the signature		contains the public key used by the server to validate the signature
	provided by the client. This avoids key confusion issues (see		provided by the client. This avoids key confusion issues (see
	[SEEMS-LEGIT]). The encoding of the public key is described in		[SEEMS-LEGIT]). The encoding of the public key is described in
	Section 4.1.		Section 3.1.
	5.3. The p Parameter		4.3. The p Parameter
	The REQUIRED "p" (proof) parameter is a byte sequence that specifies		The REQUIRED "p" (proof) parameter is a byte sequence that specifies
	the proof that the user agent provides to attest to possessing the		the proof that the client provides to attest to possessing the
	credential that matches its key ID.		credential that matches its key ID.
	5.4. The s Parameter		4.4. The s Parameter
	The REQUIRED "s" (signature) parameter is an integer that specifies		The REQUIRED "s" (signature) parameter is an integer that specifies
	the signature scheme used to compute the proof transmitted in the "p"		the signature scheme used to compute the proof transmitted in the "p"
	directive. Its value is an integer between 0 and 65535 inclusive		directive. Its value is an integer between 0 and 65535 inclusive
	from the IANA "TLS SignatureScheme" registry maintained at		from the IANA "TLS SignatureScheme" registry maintained at
	<<https://www.iana.org/assignments/tls-parameters/tls-		<<https://www.iana.org/assignments/tls-parameters/tls-
	parameters.xhtml#tls-signaturescheme>>.		parameters.xhtml#tls-signaturescheme>>.
	5.5. The v Parameter		4.5. The v Parameter
	The REQUIRED "v" (verification) parameter is a byte sequence that		The REQUIRED "v" (verification) parameter is a byte sequence that
	specifies the verification that the user agent provides to attest to		specifies the verification that the client provides to attest to
	possessing the key exporter output (see Section 4.2 for details).		possessing the key exporter output (see Section 3.2 for details).
	This avoids issues with signature schemes where certain keys can		This avoids issues with signature schemes where certain keys can
	generate signatures that are valid for multiple inputs (see		generate signatures that are valid for multiple inputs (see
	[SEEMS-LEGIT]).		[SEEMS-LEGIT]).
	6. Example		5. Example
	For example, the key ID "basement" authenticating using Ed25519		For example, the key ID "basement" authenticating using Ed25519
	[ED25519] could produce the following header field:		[ED25519] could produce the following header field:
	NOTE: '\' line wrapping per RFC 8792		NOTE: '\' line wrapping per RFC 8792
	Authorization: Signature \		Authorization: Concealed \
	k=YmFzZW1lbnQ, \		k=YmFzZW1lbnQ, \
	a=VGhpcyBpcyBh-HB1YmxpYyBrZXkgaW4gdXNl_GhlcmU, \		a=VGhpcyBpcyBh-HB1YmxpYyBrZXkgaW4gdXNl_GhlcmU, \
	s=2055, \		s=2055, \
	v=dmVyaWZpY2F0aW9u_zE2Qg, \		v=dmVyaWZpY2F0aW9u_zE2Qg, \
	p=SW5zZXJ0_HNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo\		p=SW5zZXJ0_HNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo\
	aWNoIHRha2VzIDUxMiBiaXRz-GZvciBFZDI1NTE5IQ		aWNoIHRha2VzIDUxMiBiaXRz-GZvciBFZDI1NTE5IQ
	Figure 5: Example Header Field		Figure 5: Example Header Field
	7. Non-Probeable Server Handling		6. Server Handling
	Servers that wish to introduce resources whose existence cannot be		In this section, we subdivide the server role in two:
	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:
	o absence of the Authorization (or Proxy-Authorization) field		o the "frontend" runs in the HTTP server that terminates the TLS or
			QUIC connection created by the client.
	o failure to parse that field		o the "backend" runs in the HTTP server that has access to the
			database of accepted key identifiers and public keys.
	o use of the Signature authentication scheme with an unknown key ID		In most deployments, we expect the frontend and backend roles to both
			be implemented in a single HTTP origin server (as defined in
			Section 3.6 of [HTTP]). However, these roles can be split such that
			the frontend is an HTTP gateway (as defined in Section 3.7 of [HTTP])
			and the backend is an HTTP origin server.
	o mismatch between key ID and provided public key		6.1. Frontend Handling
	o failure to validate the verification parameter		If a frontend is configured to check the Concealed authentication
			scheme, it will parse the Authorization (or Proxy-Authorization)
			header field. If the authentication scheme is set to "Concealed",
			the frontend MUST validate that all the required authentication
			parameters are present and can be parsed correctly as defined in
			Section 4. If any parameter is missing or fails to parse, the
			frontend MUST ignore the entire Authorization (or Proxy-
			Authorization) header field.
	o failure to validate the signature.		The frontend then uses the data from these authentication parameters
			to compute the key exporter output, as defined in Section 3.2. The
			frontend then shares the header field and the key exporter output
			with the backend.
	In order to validate the signature, the server needs to first parse		6.2. Communication between Frontend and Backend
	the field containing the signature, then look up the key ID in its
	database of public keys, and finally perform the cryptographic		If the frontend and backend roles are implemented in the same
	validation. These steps can take time, and an attacker could detect		machine, this can be handled by a simple function call.
	use of this mechanism if that time is observable by comparing the
	timing of a request for a known non-existent resource to the timing		If the roles are split between two separate HTTP servers, then the
	of a request for a potentially authenticated resource. Servers can		backend won't be able to directly access the TLS keying material
	mitigate this observability by slightly delaying responses to some		exporter from the TLS connection between the client and frontend, so
	non-existent resources such that the timing of the authentication		the frontend needs to explictly send it. This document defines the
	verification is not observable. This delay needs to be carefully		"Concealed-Auth-Export" request header field for this purpose. The
	considered to avoid having the delay itself leak the fact that this		Concealed-Auth-Export header field's value is a Structured Field Byte
	origin uses this mechanism at all.		Sequence (see Section 3.3.5 of [STRUCTURED-FIELDS]) that contains the
			48-byte key exporter output (see Section 3.2), without any
			parameters. For example:
			NOTE: '\' line wrapping per RFC 8792
			Concealed-Auth-Export: :VGhpcyBleGFtcGxlIFRMUyBleHBvcn\
			RlciBvdXRwdXQgaXMgNDggYnl0ZXMgI/+h:
			Figure 6: Example Concealed-Auth-Export Header Field
			The frontend SHALL forward the HTTP request to the backend, including
			the original unmodified Authorization (or Proxy-Authorization) header
			field and the newly added Concealed-Auth-Export header field.
			Note that, since the security of this mechanism requires the key
			exporter output to be correct, backends need to trust frontends to
			send it truthfully. This trust relationship is common because the
			frontend already needs access to the TLS certificate private key in
			order to respond to requests. HTTP servers that parse the Concealed-
			Auth-Export header field MUST ignore it unless they have already
			established that they trust the sender. Similarly, frontends that
			send the Concealed-Auth-Export header field MUST ensure that they do
			not forward any Concealed-Auth-Export header field received from the
			client.
			6.3. Backend Handling
			Once the backend receives the Authorization (or Proxy-Authorization)
			header field and the key exporter output, it looks up the key ID in
			its database of public keys. The backend SHALL then perform the
			following checks:
			o validate that all the required authentication parameters are
			present and can be parsed correctly as defined in Section 4
			o ensure the key ID is present in the backend's database and maps to
			a corresponding public key
			o validate that the public key from the database is equal to the one
			in the Authorization (or Proxy-Authorization) header field
			o validate that the verification field from the Authorization (or
			Proxy-Authorization) header field matches the one extracted from
			the key exporter output
			o verify the cryptographic signature as defined in Section 3.3
			If all of these checks succeed, the backend can consider the request
			to be properly authenticated, and can reply accordingly (the backend
			can also forward the request to another HTTP server).
			If any of the above checks fail, the backend MUST treat it as if the
			Authorization (or Proxy-Authorization) header field was missing.
			6.4. Non-Probeable 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).
			The authentication checks described above can take time to compute,
			and an attacker could detect use of this mechanism if that time is
			observable by comparing the timing of a request for a known non-
			existent resource to the timing of a request for a potentially
			authenticated resource. Servers can mitigate this observability by
			slightly delaying responses to some non-existent resources such that
			the timing of the authentication verification is not observable.
			This delay needs to be carefully considered to avoid having the delay
			itself leak the fact that this origin uses this mechanism at all.
	Non-probeable resources also need to be non-discoverable for		Non-probeable resources also need to be non-discoverable for
	unauthenticated users. For example, if a server operator wishes to		unauthenticated users. For example, if a server operator wishes to
	hide an authenticated resource by pretending it does not exist to		hide an authenticated resource by pretending it does not exist to
	unauthenticated users, then the server operator needs to ensure there		unauthenticated users, then the server operator needs to ensure there
	are no unauthenticated pages with links to that resource, and no		are no unauthenticated pages with links to that resource, and no
	other out-of-band ways for unauthenticated users to discover this		other out-of-band ways for unauthenticated users to discover this
	resource.		resource.
	8. Intermediary Considerations		7. Requirements on TLS Usage
	Since the Signature HTTP authentication scheme leverages TLS keying		This authentication scheme is only defined for uses of HTTP with TLS
	material exporters, its output cannot be transparently forwarded by		[TLS]. This includes any use of HTTP over TLS as typically used for
	HTTP intermediaries. HTTP intermediaries that support this		HTTP/2 [H2], or HTTP/3 [H3] where the transport protocol uses TLS as
	specification have two options:		its authentication and key exchange mechanism [QUIC-TLS].
	o The intermediary can validate the authentication received from the		Because the TLS keying material exporter is only secure for
	client, then inform the upstream HTTP server of the presence of		authentication when it is uniquely bound to the TLS session
	valid authentication.		[RFC7627], the Concealed authentication scheme requires either one of
			the following properties:
	o The intermediary can export the Signature Input and Verification		o The TLS version in use is greater or equal to 1.3 [TLS].
	(see Section 4.2}), 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		o The TLS version in use is 1.2 and the Extended Master Secret
	the upstream HTTP server is out of scope for this document.		extension [RFC7627] has been negotiated.
	Note that both of these mechanisms require the upstream HTTP server		Clients MUST NOT use the Concealed authentication scheme on
	to trust the intermediary. This is usually the case because the		connections that do not meet one of the two properties above. If a
	intermediary already needs access to the TLS certificate private key		server receives a request that uses this authentication scheme on a
	in order to respond to requests.		connection that meets neither of the above properties, the server
			MUST treat the request as if the authentication were not present.
	9. Security Considerations		8. Security Considerations
	The Signature HTTP authentication scheme allows a user agent to		The Concealed HTTP authentication scheme allows a client to
	authenticate to an origin server while guaranteeing freshness and		authenticate to an origin server while guaranteeing freshness and
	without the need for the server to transmit a nonce to the user		without the need for the server to transmit a nonce to the client.
	agent. This allows the server to accept authenticated clients		This allows the server to accept authenticated clients without
	without revealing that it supports or expects authentication for some		revealing that it supports or expects authentication for some
	resources. It also allows authentication without the user agent		resources. It also allows authentication without the client leaking
	leaking the presence of authentication to observers due to clear-text		the presence of authentication to observers due to clear-text TLS
	TLS Client Hello extensions.		Client Hello extensions.
	The authentication proofs described in this document are not bound to		The authentication proofs described in this document are not bound to
	individual HTTP requests; if the key is used for authentication		individual HTTP requests; if the key is used for authentication
	proofs on multiple requests on the same connection, they will all be		proofs on multiple requests on the same connection, they will all be
	identical. This allows for better compression when sending over the		identical. This allows for better compression when sending over the
	wire, but implies that client implementations that multiplex		wire, but implies that client implementations that multiplex
	different security contexts over a single HTTP connection need to		different security contexts over a single HTTP connection need to
	ensure that those contexts cannot read each other's header fields.		ensure that those contexts cannot read each other's header fields.
	Otherwise, one context would be able to replay the Authorization		Otherwise, one context would be able to replay the Authorization
	header field of another. This constraint is met by modern Web		header field of another. This constraint is met by modern Web
	browsers. If an attacker were to compromise the browser such that it		browsers. If an attacker were to compromise the browser such that it
	could access another context's memory, the attacker might also be		could access another context's memory, the attacker might also be
	able to access the corresponding key, so binding authentication to		able to access the corresponding key, so binding authentication to
	requests would not provide much benefit in practice.		requests would not provide much benefit in practice.
	Key material used for the Signature HTTP authentication scheme MUST		Key material used for the Concealed HTTP authentication scheme MUST
	NOT be reused in other protocols. Doing so can undermine the		NOT be reused in other protocols. Doing so can undermine the
	security guarantees of the authentication.		security guarantees of the authentication.
	Origins offering this scheme can link requests that use the same key.		Origins offering this scheme can link requests that use the same key.
	However, requests are not linkable across origins if the keys used		However, requests are not linkable across origins if the keys used
	are specific to the individual origins using this scheme.		are specific to the individual origins using this scheme.
	10. IANA Considerations		9. IANA Considerations
	10.1. HTTP Authentication Schemes Registry		9.1. HTTP Authentication Schemes Registry
	This document, if approved, requests IANA to register the following		This document, if approved, requests IANA to register the following
	entry in the "HTTP Authentication Schemes" Registry maintained at		entry in the "HTTP Authentication Schemes" Registry maintained at
	<<https://www.iana.org/assignments/http-authschemes>>:		<<https://www.iana.org/assignments/http-authschemes>>:
	Authentication Scheme Name: Signature		Authentication Scheme Name: Concealed
	Reference: This document		Reference: This document
	Notes: None		Notes: None
	10.2. TLS Keying Material Exporter Labels		9.2. TLS Keying Material Exporter Labels
	This document, if approved, requests IANA to register the following		This document, if approved, requests IANA to register the following
	entry in the "TLS Exporter Labels" registry maintained at		entry in the "TLS Exporter Labels" registry maintained at
	<<https://www.iana.org/assignments/tls-parameters#exporter-labels>>:		<<https://www.iana.org/assignments/tls-parameters#exporter-labels>>:
	Value: EXPORTER-HTTP-Signature-Authentication		Value: EXPORTER-HTTP-Concealed-Authentication
	DTLS-OK: N		DTLS-OK: N
	Recommended: Y		Recommended: Y
	Reference: This document		Reference: This document
	11. References		9.3. HTTP Field Name
	11.1. Normative References		This document, if approved, requests IANA to register the following
			entry in the "Hypertext Transfer Protocol (HTTP) Field Name" registry
			maintained at <<https://www.iana.org/assignments/http-fields/http-
			fields.xhtml>>:
			Field Name: Concealed-Auth-Export
			Template: None
			Status: permanent
			Reference: This document
			Comments: None
			10. References
			10.1. Normative References
	[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax		[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234,		Specifications: ABNF", STD 68, RFC 5234,
	DOI 10.17487/RFC5234, January 2008,		DOI 10.17487/RFC5234, January 2008,
	<https://www.rfc-editor.org/info/rfc5234>.		<https://www.rfc-editor.org/info/rfc5234>.
	[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data		[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
	Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,		Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
	<https://www.rfc-editor.org/info/rfc4648>.		<https://www.rfc-editor.org/info/rfc4648>.
	skipping to change at page 13, line 15 ¶		skipping to change at page 15, line 15 ¶
	[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,		[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
	Langley, A., and M. Ray, "Transport Layer Security (TLS)		Langley, A., and M. Ray, "Transport Layer Security (TLS)
	Session Hash and Extended Master Secret Extension",		Session Hash and Extended Master Secret Extension",
	RFC 7627, DOI 10.17487/RFC7627, September 2015,		RFC 7627, DOI 10.17487/RFC7627, September 2015,
	<https://www.rfc-editor.org/info/rfc7627>.		<https://www.rfc-editor.org/info/rfc7627>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
			[STRUCTURED-FIELDS]
			Nottingham, M. and P. Kamp, "Structured Field Values for
			HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
			<https://www.rfc-editor.org/info/rfc8941>.
	[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol		[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
	Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,		Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
	<https://www.rfc-editor.org/info/rfc8446>.		<https://www.rfc-editor.org/info/rfc8446>.
	[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform		[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
	Resource Identifier (URI): Generic Syntax", STD 66,		Resource Identifier (URI): Generic Syntax", STD 66,
	RFC 3986, DOI 10.17487/RFC3986, January 2005,		RFC 3986, DOI 10.17487/RFC3986, January 2005,
	<https://www.rfc-editor.org/info/rfc3986>.		<https://www.rfc-editor.org/info/rfc3986>.
	[X.690] ITU-T, "Information technology - ASN.1 encoding Rules:		[X.690] ITU-T, "Information technology - ASN.1 encoding Rules:
	Specification of Basic Encoding Rules (BER), Canonical		Specification of Basic Encoding Rules (BER), Canonical
	Encoding Rules (CER) and Distinguished Encoding Rules		Encoding Rules (CER) and Distinguished Encoding Rules
	(DER)", ISO/IEC 8824-1:2021 , February 2021.		(DER)", ISO/IEC 8824-1:2021 , February 2021.
	11.2. Informative References		10.2. Informative References
	[ED25519] Josefsson, S. and J. Schaad, "Algorithm Identifiers for		[ED25519] Josefsson, S. and J. Schaad, "Algorithm Identifiers for
	Ed25519, Ed448, X25519, and X448 for Use in the Internet		Ed25519, Ed448, X25519, and X448 for Use in the Internet
	X.509 Public Key Infrastructure", RFC 8410,		X.509 Public Key Infrastructure", RFC 8410,
	DOI 10.17487/RFC8410, August 2018,		DOI 10.17487/RFC8410, August 2018,
	<https://www.rfc-editor.org/info/rfc8410>.		<https://www.rfc-editor.org/info/rfc8410>.
	[H2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,		[H2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
	DOI 10.17487/RFC9113, June 2022,		DOI 10.17487/RFC9113, June 2022,
	<https://www.rfc-editor.org/info/rfc9113>.		<https://www.rfc-editor.org/info/rfc9113>.
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