< draft-ietf-httpbis-digest-headers-05.txt		draft-ietf-httpbis-digest-headers-06.txt >
	HTTP R. Polli		HTTP R. Polli
	Internet-Draft Team Digitale, Italian Government		Internet-Draft Team Digitale, Italian Government
	Obsoletes: 3230 (if approved) L. Pardue		Obsoletes: 3230 (if approved) L. Pardue
	Intended status: Standards Track Cloudflare		Intended status: Standards Track Cloudflare
	Expires: 15 October 2021 13 April 2021		Expires: 31 March 2022 27 September 2021
	Digest Headers		Digest Fields
	draft-ietf-httpbis-digest-headers-05		draft-ietf-httpbis-digest-headers-06
	Abstract		Abstract
	This document defines the HTTP Digest and Want-Digest fields, thus		This document defines HTTP fields that support integrity checksums.
	allowing client and server to negotiate an integrity checksum of the		The Digest field can be used for the integrity of HTTP
	exchanged resource representation data.		representations. The Content-Digest field can be used for the
			integrity of HTTP message content. Want-Digest and Want-Content-
			Digest can be used to indicate a sender's desire to receive integrity
			fields respectively.
	This document obsoletes RFC 3230. It replaces the term "instance"		This document obsoletes RFC 3230.
	with "representation", which makes it consistent with the HTTP
	Semantic and Context defined in draft-ietf-httpbis-semantics.
	Note to Readers		Note to Readers
	_RFC EDITOR: please remove this section before publication_		_RFC EDITOR: please remove this section before publication_
	Discussion of this draft takes place on the HTTP working group		Discussion of this draft takes place on the HTTP working group
	mailing list (ietf-http-wg@w3.org), which is archived at		mailing list (ietf-http-wg@w3.org), which is archived at
	https://lists.w3.org/Archives/Public/ietf-http-wg/		https://lists.w3.org/Archives/Public/ietf-http-wg/
	(https://lists.w3.org/Archives/Public/ietf-http-wg/).		(https://lists.w3.org/Archives/Public/ietf-http-wg/).
	skipping to change at page 2, line 4 ¶		skipping to change at page 2, line 4 ¶
	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-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	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
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	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 15 October 2021.		This Internet-Draft will expire on 31 March 2022.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 IETF Trust and the persons identified as the		Copyright (c) 2021 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents (https://trustee.ietf.org/		Provisions Relating to IETF Documents (https://trustee.ietf.org/
	license-info) in effect on the date of publication of this document.		license-info) in effect on the date of publication of this document.
	Please review these documents carefully, as they describe your rights		Please review these documents carefully, as they describe your rights
	and restrictions with respect to this document. Code Components		and restrictions with respect to this document. Code Components
	extracted from this document must include Simplified BSD License text		extracted from this document must include Simplified BSD License text
	as described in Section 4.e of the Trust Legal Provisions and are		as described in Section 4.e of the Trust Legal Provisions and are
	provided without warranty as described in the Simplified BSD License.		provided without warranty as described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. A Brief History of HTTP Integrity Fields . . . . . . . . 4		1.1. Document Structure . . . . . . . . . . . . . . . . . . . 4
	1.2. This Proposal . . . . . . . . . . . . . . . . . . . . . . 4		1.2. Concept Overview . . . . . . . . . . . . . . . . . . . . 4
	1.3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5		1.3. Replacing RFC 3230 . . . . . . . . . . . . . . . . . . . 5
	1.4. Notational Conventions . . . . . . . . . . . . . . . . . 6		1.4. Notational Conventions . . . . . . . . . . . . . . . . . 6
	2. Representation Digest . . . . . . . . . . . . . . . . . . . . 6		2. Representation Digest . . . . . . . . . . . . . . . . . . . . 6
	3. The Digest Field . . . . . . . . . . . . . . . . . . . . . . 7		3. The Digest Field . . . . . . . . . . . . . . . . . . . . . . 7
	4. The Want-Digest Field . . . . . . . . . . . . . . . . . . . . 8		4. The Content-Digest Field . . . . . . . . . . . . . . . . . . 8
	5. Digest Algorithm Values . . . . . . . . . . . . . . . . . . . 8		5. Want-Digest and Want-Content-Digest Fields . . . . . . . . . 8
	6. Use of Digest when acting on resources . . . . . . . . . . . 11		6. Digest Algorithm Values . . . . . . . . . . . . . . . . . . . 9
	6.1. Digest and PATCH . . . . . . . . . . . . . . . . . . . . 11		7. Using Digest in State-Changing Requests . . . . . . . . . . . 13
	7. Deprecate Negotiation of Content-MD5 . . . . . . . . . . . . 11		7.1. Digest and Content-Location in Responses . . . . . . . . 13
	8. Obsolete Digest Field Parameters . . . . . . . . . . . . . . 12		8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
	9. Relationship to Subresource Integrity (SRI) . . . . . . . . . 12		8.1. Digest Does Not Protect the Full HTTP Message . . . . . . 13
	9.1. Supporting Both SRI and Representation Digest . . . . . . 13		8.2. Digest for End-to-End Integrity . . . . . . . . . . . . . 14
	10. Examples of Unsolicited Digest . . . . . . . . . . . . . . . 13		8.3. Usage in Signatures . . . . . . . . . . . . . . . . . . . 14
	10.1. Server Returns Full Representation Data . . . . . . . . 13		8.4. Usage in Trailer Fields . . . . . . . . . . . . . . . . . 15
	10.2. Server Returns No Representation Data . . . . . . . . . 14		8.5. Usage with Encryption . . . . . . . . . . . . . . . . . . 15
	10.3. Server Returns Partial Representation Data . . . . . . . 14		8.6. Algorithm Agility . . . . . . . . . . . . . . . . . . . . 15
	10.4. Client and Server Provide Full Representation Data . . . 15		8.7. Duplicate digest-algorithm in field value . . . . . . . . 16
	10.5. Client Provides Full Representation Data, Server Provides		8.8. Resource exhaustion . . . . . . . . . . . . . . . . . . . 16
	No Representation Data . . . . . . . . . . . . . . . . 15		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
	10.6. Client and Server Provide Full Representation Data, Client		9.1. Establish the HTTP Digest Algorithm Values Registry . . . 16
	Uses id-sha-256. . . . . . . . . . . . . . . . . . . . 16		9.2. Obsolete "contentMD5" token in Digest Algorithm . . . . . 17
	10.7. POST Response does not Reference the Request URI . . . . 17		9.3. Changes Compared to RFC3230 . . . . . . . . . . . . . . . 17
	10.8. POST Response Describes the Request Status . . . . . . . 18		9.4. Changes Compared to RFC5843 . . . . . . . . . . . . . . . 17
	10.9. Digest with PATCH . . . . . . . . . . . . . . . . . . . 18		9.5. Want-Digest Field Registration . . . . . . . . . . . . . 17
	10.10. Error responses . . . . . . . . . . . . . . . . . . . . 19		9.6. Digest Field Registration . . . . . . . . . . . . . . . . 18
	10.11. Use with Trailer Fields and Transfer Coding . . . . . . 20		9.7. Want-Content-Digest Field Registration . . . . . . . . . 18
	11. Examples of Want-Digest Solicited Digest . . . . . . . . . . 21		9.8. Content-Digest Field Registration . . . . . . . . . . . . 18
	11.1. Server Selects Client's Least Preferred Algorithm . . . 21
	11.2. Server Selects Algorithm Unsupported by Client . . . . . 22		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
	11.3. Server Does Not Support Client Algorithm and Returns an		10.1. Normative References . . . . . . . . . . . . . . . . . . 18
	Error . . . . . . . . . . . . . . . . . . . . . . . . . 22		10.2. Informative References . . . . . . . . . . . . . . . . . 20
	12. Security Considerations . . . . . . . . . . . . . . . . . . . 22		Appendix A. Resource Representation and Representation-Data . . 22
	12.1. Digest Does Not Protect the Full HTTP Message . . . . . 22		Appendix B. Examples of Unsolicited Digest . . . . . . . . . . . 24
	12.2. Broken Cryptographic Algorithms . . . . . . . . . . . . 23		B.1. Server Returns Full Representation Data . . . . . . . . . 24
	12.3. Other Deprecated Algorithms . . . . . . . . . . . . . . 23		B.2. Server Returns No Representation Data . . . . . . . . . . 24
	12.4. Digest for End-to-End Integrity . . . . . . . . . . . . 23		B.3. Server Returns Partial Representation Data . . . . . . . 25
	12.5. Digest and Content-Location in Responses . . . . . . . . 23		B.4. Client and Server Provide Full Representation Data . . . 25
	12.6. Usage in Signatures . . . . . . . . . . . . . . . . . . 24		B.5. Client Provides Full Representation Data, Server Provides
	12.7. Usage in Trailer Fields . . . . . . . . . . . . . . . . 24		No Representation Data . . . . . . . . . . . . . . . . . 26
	12.8. Usage with Encryption . . . . . . . . . . . . . . . . . 25		B.6. Client and Server Provide Full Representation Data, Client
	12.9. Algorithm Agility . . . . . . . . . . . . . . . . . . . 25		Uses id-sha-256. . . . . . . . . . . . . . . . . . . . . 27
	12.9.1. Duplicate digest-algorithm in field value . . . . . 25		B.7. POST Response does not Reference the Request URI . . . . 27
	12.10. Resource exhaustion . . . . . . . . . . . . . . . . . . 26		B.8. POST Response Describes the Request Status . . . . . . . 28
	13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26		B.9. Digest with PATCH . . . . . . . . . . . . . . . . . . . . 29
	13.1. Establish the HTTP Digest Algorithm Values Registry . . 26		B.10. Error responses . . . . . . . . . . . . . . . . . . . . . 30
	13.2. The "status" Field in the HTTP Digest Algorithm Values		B.11. Use with Trailer Fields and Transfer Coding . . . . . . . 30
	Registry . . . . . . . . . . . . . . . . . . . . . . . 26		Appendix C. Examples of Want-Digest Solicited Digest . . . . . . 31
	13.3. Deprecate "MD5" Digest Algorithm . . . . . . . . . . . . 26		C.1. Server Selects Client's Least Preferred Algorithm . . . . 31
	13.4. Update "UNIXsum" Digest Algorithm . . . . . . . . . . . 26		C.2. Server Selects Algorithm Unsupported by Client . . . . . 32
	13.5. Update "UNIXcksum" Digest Algorithm . . . . . . . . . . 27		C.3. Server Does Not Support Client Algorithm and Returns an
	13.6. Update "CRC32c" Digest Algorithm . . . . . . . . . . . . 27		Error . . . . . . . . . . . . . . . . . . . . . . . . . . 32
	13.7. Deprecate "SHA" Digest Algorithm . . . . . . . . . . . . 27		Appendix D. Changes from RFC3230 . . . . . . . . . . . . . . . . 32
	13.8. Obsolete "ADLER32" Digest Algorithm . . . . . . . . . . 28		D.1. Deprecate Negotiation of Content-MD5 . . . . . . . . . . 33
	13.9. Obsolete "contentMD5" token in Digest Algorithm . . . . 28		D.2. Obsolete Digest Field Parameters . . . . . . . . . . . . 33
	13.10. The "id-sha-256" Digest Algorithm . . . . . . . . . . . 28		Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 33
	13.11. The "id-sha-512" Digest Algorithm . . . . . . . . . . . 29		FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
	13.12. Changes Compared to RFC5843 . . . . . . . . . . . . . . 29		Code Samples . . . . . . . . . . . . . . . . . . . . . . . . . . 35
	13.13. Want-Digest Field Registration . . . . . . . . . . . . . 29		Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
	13.14. Digest Field Registration . . . . . . . . . . . . . . . 29		Since draft-ietf-httpbis-digest-headers-05 . . . . . . . . . . 36
	14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30		Since draft-ietf-httpbis-digest-headers-04 . . . . . . . . . . 37
	14.1. Normative References . . . . . . . . . . . . . . . . . . 30		Since draft-ietf-httpbis-digest-headers-03 . . . . . . . . . . 37
	14.2. Informative References . . . . . . . . . . . . . . . . . 31		Since draft-ietf-httpbis-digest-headers-02 . . . . . . . . . . 37
	Appendix A. Resource Representation and Representation-Data . . 33		Since draft-ietf-httpbis-digest-headers-01 . . . . . . . . . . 37
	Appendix B. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 35		Since draft-ietf-httpbis-digest-headers-00 . . . . . . . . . . 38
	Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 36		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
	Code Samples . . . . . . . . . . . . . . . . . . . . . . . . . . 36
	Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
	Since draft-ietf-httpbis-digest-headers-04 . . . . . . . . . . 38
	Since draft-ietf-httpbis-digest-headers-03 . . . . . . . . . . 38
	Since draft-ietf-httpbis-digest-headers-02 . . . . . . . . . . 38
	Since draft-ietf-httpbis-digest-headers-01 . . . . . . . . . . 38
	Since draft-ietf-httpbis-digest-headers-00 . . . . . . . . . . 39
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
	1. Introduction		1. Introduction
	The core specification of HTTP does not define a means to protect the		HTTP does not define a means to protect the integrity of
	integrity of resources. When HTTP messages are transferred between		representations. When HTTP messages are transferred between
	endpoints, the protocol might choose to make use of features of the		endpoints, the protocol might choose to make use of features of the
	lower layer in order to provide some integrity protection; for		lower layer in order to provide some integrity protection; for
	instance TCP checksums or TLS records [RFC2818].		instance, TCP checksums or TLS records [RFC2818].
	However, there are cases where relying on this alone is insufficient.
	An HTTP-level integrity mechanism that operates independent of
	transfer can be used to detect programming errors and/or corruption
	of data in flight or at rest, be used across multiple hops in order
	to provide end-to-end integrity guarantees, can aid fault diagnosis
	across hops and system boundaries, and can be used to validate
	integrity when reconstructing a resource fetched using different HTTP
	connections.
	This document defines a mechanism that acts on HTTP representation-		This document defines two digest integrity mechanisms for HTTP.
	data. It can be combined with other mechanisms that protect		First, representation data integrity, which acts on representation
	representation-metadata, such as digital signatures, in order to		data (Section 3.2 of [SEMANTICS]). Second, content digest integrity,
	protect the desired parts of an HTTP exchange in whole or in part.		which acts on conveyed content (Section 6.4 of [SEMANTICS]). Both
			mechanisms operate independent of transport integrity, offering the
			potential to detect programming errors and corruption of data in
			flight or at rest. They can be used across multiple hops in order to
			provide end-to-end integrity guarantees, which can aid fault
			diagnosis when resources are transferred across hops and system
			boundaries. Finally, they can be used to validate integrity when
			reconstructing a resource fetched using different HTTP connections.
	1.1. A Brief History of HTTP Integrity Fields		This document obsoletes [RFC3230].
	The Content-MD5 header field was originally introduced to provide		1.1. Document Structure
	integrity, but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it:
	The Content-MD5 header field has been removed because it was		This document is structured as follows:
	inconsistently implemented with respect to partial responses.
	[RFC3230] provided a more flexible solution introducing the concept		* Section 2 describes concepts related to representation digests,
	of "instance", and the fields "Digest" and "Want-Digest".
	1.2. This Proposal		* Section 3 defines the Digest request and response header and
			trailer field,
	The concept of "selected representation" defined in Section 3.2 of		* Section 4 defines the Content-Digest request and response header
	[SEMANTICS] makes [RFC3230] definitions inconsistent with current		and trailer field,
	HTTP semantics. This document updates the "Digest" and "Want-Digest"
	field definitions to align with [SEMANTICS] concepts.
	Basing "Digest" on the selected representation makes it		* Section 5 defines the Want-Digest and Want-Content-Digest request
	straightforward to apply it to use-cases where the transferred data		and response header and trailer field,
	does require some sort of manipulation to be considered a
	representation, or conveys a partial representation of a resource eg.
	Range Requests (see Section 14.2 of [SEMANTICS]).
	This document replaces [RFC3230] to better align with [SEMANTICS] and		* Section 6 and Appendix D.1 describe algorithms and their relation
	to provide more detailed description of "Digest" usage in request and		to Digest,
	response cases. Changes are intended to be semantically compatible
	with existing implementations but note that negotiation of "Content-
	MD5" is deprecated Section 7, "Digest" field parameters are obsoleted
	Section 8, "md5" and "sha" digest-algorithms are obsoleted
	Section 12.2 and the "adler32" algorithm is deprecated Section 12.3.
	The value of "Digest" is calculated on selected representation, which		* Section 7 details computing representation digests,
	is tied to the value contained in any "Content-Encoding" or "Content-
	Type" header fields. Therefore, a given resource may have multiple
	different digest values.
	To allow both parties to exchange a Digest of a representation with		* Appendix D.2 obsoletes Digest field parameters, and
	no content codings (see Section 8.4.1 of [SEMANTICS]) two more
	digest-algorithms are added ("id-sha-256" and "id-sha-512").
	1.3. Goals		* Appendix B and Appendix C provide examples of using Digest and
			Want-Digest.
	The goals of this proposal are:		1.2. Concept Overview
	1. Digest coverage for either the resource's "representation data"		This document defines the Digest request and response header and
	or "selected representation data" communicated via HTTP.		trailer field; see Section 3. At a high level, the value contains a
			checksum, computed over selected representation data (Section 3.2 of
			[SEMANTICS]), that the recipient can use to validate integrity.
			Basing Digest on the selected representation makes it straightforward
			to apply it to use-cases where the transferred data requires some
			sort of manipulation to be considered a representation or conveys a
			partial representation of a resource, such as Range Requests (see
			Section 14.2 of [SEMANTICS]).
	2. Support for multiple digest-algorithms.		To support use-cases where a simple checksum of the content bytes is
			required, this document introduces the Content-Digest request and
			response header and trailer field; see Section 4.
	3. Negotiation of the use of digests.		Digest and Content-Digest support algorithm agility. The Want-Digest
			and Want-Content-Digest fields allows endpoints to express interest
			in Digest and Content-Digest respectively, and preference of
			algorithms in either.
	The goals do not include:		Digest field calculations are tied to the Content-Encoding and
			Content-Type header fields. Therefore, a given resource may have
			multiple different checksum values when transferred with HTTP. To
			allow both parties to exchange a simple checksum with no content
			codings (see Section 8.4.1 of [SEMANTICS]), two more digest-
			algorithms are added ("id-sha-256" and "id-sha-512").
	HTTP message integrity: Digest mechanisms do not cover the full HTTP		Digest fields do not provide integrity for HTTP messages or fields.
	message nor its semantic, as representation metadata is not		However, they can be combined with other mechanisms that protect
	included in the checksum.		metadata, such as digital signatures, in order to protect the phases
			of an HTTP exchange in whole or in part.
	HTTP field integrity: Digest mechanisms cover only representation		This specification does not define means for authentication,
	and selected representation data, and do not protect the integrity		authorization or privacy.
	of associated representation metadata or other message fields.
	Authentication: Digest mechanisms do not support authentication of		1.3. Replacing RFC 3230
	the source of a digest, message or anything else. These
	mechanisms, therefore, are not a sufficient defense against many
	kinds of malicious attacks.
	Privacy: Digest mechanisms do not provide message privacy.		Historically, the Content-MD5 header field provided an HTTP integrity
			mechanism but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it due to
			inconsistent handling of partial responses. [RFC3230] defined the
			concept of "instance" digests and a more flexible integrity scheme to
			help address issues with Content-MD5. It first introduced the Digest
			and Want-Digest fields. HTTP terminology has evolved since [RFC3230]
			was published. The concept of "instance" has been superseded by
			selected representation.
	Authorization: Digest mechanisms do not support authorization or		This document replaces [RFC3230]. The Digest and Want-Digest field
	other kinds of access controls.		definitions are updated to align with the terms and notational
			conventions in [SEMANTICS]. Changes are intended to be semantically
			compatible with existing implementations but note that negotiation of
			Content-MD5 is deprecated Appendix D.1 and has been replaced by
			Content-Digest negotiation via Want-Content-Digest. Digest field
			parameters are obsoleted Appendix D.2 and the algorithm table has
			been updated to reflect the current state of the art.
	1.4. Notational Conventions		1.4. Notational Conventions
	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 Augmented BNF defined in [RFC5234] and updated		This document uses the Augmented BNF defined in [RFC5234] and updated
	by [RFC7405] along with the "#rule" extension defined in		by [RFC7405] along with the "#rule" extension defined in
	Section 5.6.1 of [SEMANTICS].		Section 5.6.1 of [SEMANTICS] and the "qvalue" rule defined in
			Section 12.4.2 of [SEMANTICS].
	The definitions "representation", "selected representation",		The definitions "representation", "selected representation",
	"representation data", "representation metadata", and "content" in		"representation data", "representation metadata", and "content" in
	this document are to be interpreted as described in [SEMANTICS].		this document are to be interpreted as described in [SEMANTICS].
	Algorithm names respect the casing used in their definition document		Algorithm names respect the casing used in their definition document
	(eg. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg.		(e.g. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted
	"sha", "crc32c").		(e.g. "sha", "crc32c").
	2. Representation Digest		2. Representation Digest
	The representation digest is an integrity mechanism for HTTP		The representation digest is an integrity mechanism for HTTP
	resources which uses a checksum that is calculated independently of		resources which uses a checksum that is calculated independently of
	the content (see Section 6.4 of [SEMANTICS]). It uses the		the content (see Section 6.4 of [SEMANTICS]). It uses the
	representation data (see Section 8.1 of [SEMANTICS]), that can be		representation data (see Section 8.1 of [SEMANTICS]), that can be
	fully or partially contained in the content, or not contained at all:		fully or partially contained in the content, or not contained at all.
	representation-data := Content-Encoding( Content-Type( bits ) )
	This takes into account the effect of the HTTP semantics on the		This takes into account the effect of the HTTP semantics on the
	messages; for example, the content can be affected by Range Requests		messages; for example, the content can be affected by Range Requests
	or methods such as HEAD, while the way the content is transferred "on		or methods such as HEAD, while the way the content is transferred "on
	the wire" is dependent on other transformations (e.g. transfer		the wire" is dependent on other transformations (e.g. transfer
	codings for HTTP/1.1 - see Section 6.1 of [HTTP11]). To help		codings for HTTP/1.1 - see Section 6.1 of [HTTP11]). To help
	illustrate how such things affect "Digest", several examples are		illustrate how such things affect Digest, several examples are
	provided in Appendix A.		provided in Appendix A.
	A representation digest consists of the value of a checksum computed		A representation digest consists of the value of a checksum computed
	on the entire selected "representation data" (see Section 8.1 of		on the entire selected representation data (see Section 8.1 of
	[SEMANTICS]) of a resource identified according to Section 6.4.2 of		[SEMANTICS]) of a resource identified according to Section 6.4.2 of
	[SEMANTICS] together with an indication of the algorithm used:		[SEMANTICS] together with an indication of the algorithm used:
	representation-data-digest = digest-algorithm "="		representation-data-digest = digest-algorithm "="
	<encoded digest output>		<encoded digest output>
	When a message has no representation data it is still possible to		When a message has no representation data it is still possible to
	assert that no representation data was sent computing the		assert that no representation data was sent computing the
	representation digest on an empty string (see Section 12.6).		representation digest on an empty string (see Section 8.3).
	The checksum is computed using one of the digest-algorithms listed in		The checksum is computed using one of the digest-algorithms listed in
	Section 5 and then encoded in the associated format.		the HTTP Digest Algorithm Values Registry (see Section 6) and then
			encoded in the associated format.
	The example below shows the "sha-256" digest-algorithm that uses
	base64 encoding.
	sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	3. The Digest Field		3. The Digest Field
	The "Digest" field contains a list of one or more representation		The Digest field contains a comma-separated list of one or more
	digest values as defined in Section 2. It can be used in both		representation digest values as defined in Section 2. It can be used
	requests and responses.		in both requests and responses.
	Digest = 1#representation-data-digest		Digest = 1#representation-data-digest
	For example:		For example:
	Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm		Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
	AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==		AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
	The relationship between "Content-Location" (see Section 8.7 of		A Digest field MAY contain multiple representation-data-digest
	[SEMANTICS]) and "Digest" is demonstrated in Section 10.7. A
	comprehensive set of examples showing the impacts of representation
	metadata, payload transformations and HTTP methods on Digest is
	provided in Section 10 and Section 11.
	A "Digest" field MAY contain multiple representation-data-digest
	values. For example, a server may provide representation-data-digest		values. For example, a server may provide representation-data-digest
	values using different algorithms, allowing it to support a		values using different algorithms, allowing it to support a
	population of clients with different evolving capabilities; this is		population of clients with different evolving capabilities; this is
	particularly useful in support of transitioning away from weaker		particularly useful in support of transitioning away from weaker
	algorithms should the need arise (see Section 12.9).		algorithms should the need arise (see Section 8.6).
	Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,		Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
	id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	A recipient MAY ignore any or all of the representation-data-digests		A recipient MAY ignore any or all of the representation-data-digests
	in a Digest field. This allows the recipient to choose which digest-		in a Digest field. This allows the recipient to choose which digest-
	algorithm(s) to use for validation instead of verifying every		algorithm(s) to use for validation instead of verifying every
	received representation-data-digest.		received representation-data-digest.
	A sender MAY send a representation-data-digest using a digest-		A sender MAY send a representation-data-digest using a digest-
	algorithm without knowing whether the recipient supports the digest-		algorithm without knowing whether the recipient supports the digest-
	algorithm, or even knowing that the recipient will ignore it.		algorithm, or even knowing that the recipient will ignore it.
	"Digest" can be sent in a trailer section. When an incremental		Digest can be sent in a trailer section. In this case, Digest MAY be
	digest-algorithms is used, the sender and the receiver can		merged into the header section; see Section 6.5.1 of [SEMANTICS].
	dynamically compute the digest value while streaming the content.
	4. The Want-Digest Field		When an incremental digest-algorithm is used, the sender and the
			receiver can dynamically compute the digest value while streaming the
			content.
	The "Want-Digest" field indicates the sender's desire to receive a		A non-comprehensive set of examples showing the impacts of
	representation digest on messages associated with the request URI and		representation metadata, payload transformations and HTTP methods on
	representation metadata.		Digest is provided in Appendix B and Appendix C.
			4. The Content-Digest Field
			The Content-Digest field contains a comma-separated list of one or
			more content digest values. A content digest value is computed by
			applying a digest-algorithm to the actual message content (see
			Section 6.4 of [SEMANTICS]). It can be used in both requests and
			responses.
			Content-Digest = 1#content-digest
			content-digest = digest-algorithm "="
			<encoded digest output>
			For example:
			Content-Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
			AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
			A Content-Digest field MAY contain multiple content-digest values,
			similarly to Digest (see Section 3)
			Content-Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
			id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
			A recipient MAY ignore any or all of the content-digests in a
			Content-Digest field. This allows the recipient to choose which
			digest-algorithm(s) to use for validation instead of verifying every
			received content-digest.
			A sender MAY send a content-digest using a digest-algorithm without
			knowing whether the recipient supports the digest-algorithm, or even
			knowing that the recipient will ignore it.
			Content-Digest can be sent in a trailer section. In this case,
			Content-Digest MAY be merged into the header section; see
			Section 6.5.1 of [SEMANTICS].
			When an incremental digest-algorithm is used, the sender and the
			receiver can dynamically compute the digest value while streaming the
			content.
			5. Want-Digest and Want-Content-Digest Fields
			Senders can indicate their integrity checksum preferences using the
			Want-Digest or Want-Content-Digest fields. These can be used in both
			requests and responses.
			Want-Digest indicates the sender's desire to receive a representation
			digest on messages associated with the request URI and representation
			metadata, using the Digest field.
			Want-Content-Digest indicates the sender's desire to receive a
			content digest on messages associated with the request URI and
			representation metadata, using the Content-Digest field.
	Want-Digest = 1#want-digest-value		Want-Digest = 1#want-digest-value
			Want-Content-Digest = 1#want-digest-value
	want-digest-value = digest-algorithm [ ";" "q" "=" qvalue]		want-digest-value = digest-algorithm [ ";" "q" "=" qvalue]
	qvalue = ( "0"  [ "."  0*1DIGIT ] ) /
	( "1"  [ "."  0*1( "0" ) ] )
	If a digest-algorithm is not accompanied by a "qvalue", it is treated		qvalue indicates the sender's digest-algorithm preferences.
	as if its associated "qvalue" were 1.0.		Section 12.4.2 of [SEMANTICS]) describes qvalue usage and semantics.
	The sender is willing to accept a digest-algorithm if and only if it
	is listed in a "Want-Digest" field of a message, and its "qvalue" is
	non-zero.
	If multiple acceptable digest-algorithm values are given, the		Senders can provide multiple digest-algorithm items with the same
	sender's preferred digest-algorithm is the one (or ones) with the		qvalue.
	highest "qvalue".
	Two examples of its use are:		Examples:
	Want-Digest: sha-256		Want-Digest: sha-256
	Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0		Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0
			Want-Content-Digest: sha-256
			Want-Content-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0
	5. Digest Algorithm Values		6. Digest Algorithm Values
	Digest-algorithm values are used to indicate a specific digest		Digest-algorithm values are used to indicate a specific digest
	computation.		computation.
	digest-algorithm = token		digest-algorithm = token
	All digest-algorithm values are case-insensitive but lower case is		All digest-algorithm token values are case-insensitive but lower case
	preferred.		is preferred; digest-algorithm token values MUST be compared in a
			case-insensitive fashion.
	The Internet Assigned Numbers Authority (IANA) acts as a registry for		Every digest-algorithm defines its computation procedure and encoding
	digest-algorithm values. The registry contains the tokens listed		output. Unless specified otherwise, comparison of encoded output is
	below.		case-sensitive.
	Some digest-algorithms, although registered, rely on vulnerable		The "HTTP Digest Algorithm Values Registry", maintained by IANA at
	algorithms and MUST not be used:		https://www.iana.org/assignments/http-dig-alg/
			(https://www.iana.org/assignments/http-dig-alg/) registers digest-
			algorithm values. Registrations MUST include the following fields:
	* "md5", see [CMU-836068] and [NO-MD5];		* Digest algorithm: the token value. The registry can be used to
			reserve token values
	* "sha", see [IACR-2020-014] and [NO-SHA1].		* Status: the status of the algorithm. Use "standard" for
			standardized algorithms without known problems; "experimental" or
			some other appropriate value
	See the references above for further information.		- e.g. according to the type and status of the primary document
			in which the algorithm is defined; "deprecated" when the
			algorithm is insecure or otherwise undesirable; "reserved" when
			Digest algorithm references a reserved token value
	sha-256		* Description: the description of the digest-algorithm and its
	* Description: The SHA-256 algorithm [RFC6234]. The output of		encoding
			* Reference: a set of pointers to the primary documents defining the
			digest-algorithm
			The associated encoding for new digest-algorithms MUST either be
			represented as a quoted string or MUST NOT include ";" or "," in the
			character sets used for the encoding.
			Deprecated digest algorithms MUST NOT be used.
			The registry is initialized with the tokens listed below.
			sha-512
			* Digest Algorithm: sha-512
			* Description: The SHA-512 algorithm [RFC6234]. The output of
	this algorithm is encoded using the base64 encoding [RFC4648].		this algorithm is encoded using the base64 encoding [RFC4648].
	* Reference: [RFC6234], [RFC4648], this document.		* Reference: [RFC6234], [RFC4648], this document.
	* Status: standard		* Status: standard
	sha-512		sha-256
	* Description: The SHA-512 algorithm [RFC6234]. The output of		* Digest Algorithm: sha-256
			* Description: The SHA-256 algorithm [RFC6234]. The output of
	this algorithm is encoded using the base64 encoding [RFC4648].		this algorithm is encoded using the base64 encoding [RFC4648].
	* Reference: [RFC6234], [RFC4648], this document.		* Reference: [RFC6234], [RFC4648], this document.
	* Status: standard		* Status: standard
	md5		md5
			* Digest Algorithm: md5
	* Description: The MD5 algorithm, as specified in [RFC1321]. The		* Description: The MD5 algorithm, as specified in [RFC1321]. The
	output of this algorithm is encoded using the base64 encoding		output of this algorithm is encoded using the base64 encoding
	[RFC4648]. This digest-algorithm MUST NOT be used as it's now		[RFC4648]. This digest-algorithm is now vulnerable to
	vulnerable to collision attacks. See [NO-MD5] and		collision attacks. See [NO-MD5] and [CMU-836068].
	[CMU-836068].
	* Reference: [RFC1321], [RFC4648], this document.		* Reference: [RFC1321], [RFC4648], this document.
	* Status: deprecated		* Status: deprecated
	sha		sha
			* Digest Algorithm: sha
	* Description: The SHA-1 algorithm [RFC3174]. The output of this		* Description: The SHA-1 algorithm [RFC3174]. The output of this
	algorithm is encoded using the base64 encoding [RFC4648]. This		algorithm is encoded using the base64 encoding [RFC4648]. This
	digest-algorithm MUST NOT be used as it's now vulnerable to		digest-algorithm is now vulnerable to collision attacks. See
	collision attacks. See [NO-SHA1] and [IACR-2020-014].		[NO-SHA1] and [IACR-2020-014].
	* Reference: [RFC3174], [RFC6234], [RFC4648], this document.		* Reference: [RFC3174], [RFC6234], [RFC4648], this document.
	* Status: deprecated		* Status: deprecated
	unixsum		unixsum
			* Digest Algorithm: unixsum
	* Description: The algorithm computed by the UNIX "sum" command,		* Description: The algorithm computed by the UNIX "sum" command,
	as defined by the Single UNIX Specification, Version 2 [UNIX].		as defined by the Single UNIX Specification, Version 2 [UNIX].
	The output of this algorithm is an ASCII decimal-digit string		The output of this algorithm is an ASCII decimal-digit string
	representing the 16-bit checksum, which is the first word of		representing the 16-bit checksum, which is the first word of
	the output of the UNIX "sum" command.		the output of the UNIX "sum" command.
	* Reference: [UNIX], this document.		* Reference: [UNIX], this document.
	* Status: standard		* Status: deprecated
	unixcksum		unixcksum
			* Digest Algorithm: unixcksum
	* Description: The algorithm computed by the UNIX "cksum"		* Description: The algorithm computed by the UNIX "cksum"
	command, as defined by the Single UNIX Specification, Version 2		command, as defined by the Single UNIX Specification, Version 2
	[UNIX]. The output of this algorithm is an ASCII digit string		[UNIX]. The output of this algorithm is an ASCII digit string
	representing the 32-bit CRC, which is the first word of the		representing the 32-bit CRC, which is the first word of the
	output of the UNIX "cksum" command.		output of the UNIX "cksum" command.
	* Reference: [UNIX], this document.		* Reference: [UNIX], this document.
	* Status: standard		* Status: deprecated
			adler32
			* Digest Algorithm: adler32
			* Description: The ADLER32 algorithm is a checksum specified in
			[RFC1950] "ZLIB Compressed Data Format". The 32-bit output is
			encoded in hexadecimal (using between 1 and 8 ASCII characters
			from 0-9, A-F, and a-f; leading 0's are allowed). For example,
			adler32=03da0195 and adler32=3DA0195 are both valid checksums
			for the 4-byte message "Wiki". This algorithm is obsoleted and
			SHOULD NOT be used.
			* Reference: [RFC1950], this document.
			* Status: deprecated
			crc32c
			* Digest Algorithm: crc32c
			* Description: The CRC32c algorithm is a 32-bit cyclic redundancy
			check. It achieves a better hamming distance (for better
			error-detection performance) than many other 32-bit CRC
			functions. Other places it is used include iSCSI and SCTP.
			The 32-bit output is encoded in hexadecimal (using between 1
			and 8 ASCII characters from 0-9, A-F, and a-f; leading 0's are
			allowed). For example, crc32c=0a72a4df and crc32c=A72A4DF are
			both valid checksums for the 3-byte message "dog".
			* Reference: [RFC4960] appendix B, this document.
			* Status: deprecated.
	To allow sender and recipient to provide a checksum which is		To allow sender and recipient to provide a checksum which is
	independent from "Content-Encoding", the following additional digest-		independent from Content-Encoding, the following additional digest-
	algorithms are defined:		algorithms are defined:
	id-sha-512		id-sha-512
	* Description: The sha-512 digest of the representation-data of		* Description: The sha-512 digest of the representation data of
	the resource when no content coding is applied		the resource when no content coding is applied
	* Reference: [RFC6234], [RFC4648], this document.		* Reference: [RFC6234], [RFC4648], this document.
	* Status: standard		* Status: standard
	id-sha-256		id-sha-256
	* Description: The sha-256 digest of the representation-data of		* Description: The sha-256 digest of the representation data of
	the resource when no content coding is applied		the resource when no content coding is applied
	* Reference: [RFC6234], [RFC4648], this document.		* Reference: [RFC6234], [RFC4648], this document.
	* Status: standard		* Status: standard
	If other digest-algorithm values are defined, the associated encoding		7. Using Digest in State-Changing Requests
	MUST either be represented as a quoted string or MUST NOT include ";"
	or "," in the character sets used for the encoding.
	6. Use of Digest when acting on resources
	POST and PATCH requests can appear to convey partial representations
	but are semantically acting on resources. The enclosed
	representation, including its metadata, refers to that action.
	In these requests the representation digest MUST be computed on the		When the representation enclosed in a state-changing request does not
	representation-data of that action. This is the only possible choice		describe the target resource, the representation digest MUST be
	because representation digest requires complete representation		computed on the representation-data. This is the only possible
			choice because representation digest requires complete representation
	metadata (see Section 2).		metadata (see Section 2).
	In responses,		In responses,
	* if the representation describes the status of the request,		* if the representation describes the status of the request, Digest
	"Digest" MUST be computed on the enclosed representation (see		MUST be computed on the enclosed representation (see Appendix B.8
	Section 10.8 );		);
	* if there is a referenced resource "Digest" MUST be computed on the		* if there is a referenced resource Digest MUST be computed on the
	selected representation of the referenced resource even if that is		selected representation of the referenced resource even if that is
	different from the target resource. That might or might not		different from the target resource. That might or might not
	result in computing "Digest" on the enclosed representation.		result in computing Digest on the enclosed representation.
	The latter case might be done according to the HTTP semantics of the		The latter case is done according to the HTTP semantics of the given
	given method, for example using the "Content-Location" header field.		method, for example using the Content-Location header field (see
	In contrast, the "Location" header field does not affect "Digest"		Section 8.7 of [SEMANTICS]). In contrast, the Location header field
	because it is not representation metadata.		does not affect Digest because it is not representation metadata.
	6.1. Digest and PATCH		For example, in PATCH requests, the representation digest will be
			computed on the patch document because the representation metadata
			refers to the patch document and not to the target resource (see
			Section 2 of [PATCH]). In responses, instead, the representation
			digest will be computed on the selected representation of the patched
			resource.
	In PATCH requests, the representation digest MUST be computed on the		7.1. Digest and Content-Location in Responses
	patch document because the representation metadata refers to the
	patch document and not to the target resource (see Section 2 of
	[PATCH]).
	In PATCH responses, the representation digest MUST be computed on the		When a state-changing method returns the Content-Location header
	selected representation of the patched resource.		field, the enclosed representation refers to the resource identified
			by its value and Digest is computed accordingly. An example is given
			in Appendix B.7.
	"Digest" usage with PATCH is thus very similar to POST, but with the		8. Security Considerations
	resource's own semantic partly implied by the method and by the patch
	document.
	7. Deprecate Negotiation of Content-MD5		8.1. Digest Does Not Protect the Full HTTP Message
	This RFC deprecates the negotiation of Content-MD5 as it has been		This document specifies a data integrity mechanism that protects HTTP
	obsoleted by [RFC7231]. The "contentMD5" token defined in Section 5		representation data or content, but not HTTP header and trailer
	of [RFC3230] MUST NOT be used as a digest-algorithm.		fields, from certain kinds of accidental corruption.
	8. Obsolete Digest Field Parameters		Digest fields are not intended to be a general protection against
			malicious tampering with HTTP messages. This can be achieved by
			combining it with other approaches such as transport-layer security
			or digital signatures.
	Section 4.1.1 and 4.2 of [RFC3230] defined field parameters. This		8.2. Digest for End-to-End Integrity
	document obsoletes the usage of parameters with "Digest" because this
	feature has not been widely deployed and complicates field-value
	processing.
	[RFC3230] intended field parameters to provide a common way to attach		Digest fields can help detect representation data or content
	additional information to a representation-data-digest. However, if		modification due to implementation errors, undesired "transforming
	parameters are used as an input to validate the checksum, an attacker		proxies" (see Section 7.7 of [SEMANTICS]) or other actions as the
	could alter them to steer the validation behavior.		data passes across multiple hops or system boundaries. Even a simple
			mechanism for end-to-end representation data integrity is valuable
			because user-agent can validate that resource retrieval succeeded
			before handing off to a HTML parser, video player etc. for parsing.
	A digest-algorithm can still be parameterized by defining its own way		Identity digest-algorithms (e.g. "id-sha-256" and "id-sha-512") are
	to encode parameters into the representation-data-digest, in such a		particularly useful for end-to-end integrity because they allow
	way as to mitigate security risks related to its computation.		piecing together a resource from different sources with different
			HTTP messaging characteristics. For example, different servers that
			apply different content codings.
	9. Relationship to Subresource Integrity (SRI)		Note that using digest fields alone does not provide end-to-end
			integrity of HTTP messages over multiple hops, since metadata could
			be manipulated at any stage. Methods to protect metadata are
			discussed in Section 8.3.
	Subresource Integrity [SRI] is an integrity mechanism that shares		8.3. Usage in Signatures
	some similarities to the present document's mechanism. However,
	there are differences in motivating factors, threat model and
	specification of integrity digest generation, signalling and
	validation.
	SRI allows a first-party authority to declare an integrity assertion		Digital signatures are widely used together with checksums to provide
	on a resource served by a first or third party authority. This is		the certain identification of the origin of a message [NIST800-32].
	done via the "integrity" attribute that can be added to "script" or		Such signatures can protect one or more HTTP fields and there are
	"link" HTML elements. Therefore, the integrity assertion is always		additional considerations when Digest is included in this set.
	made out-of-band to the resource fetch. In contrast, the "Digest"
	field is supplied in-band alongside the selected representation,
	meaning that an authority can only declare an integrity assertion for
	itself. Methods to improve the security properties of representation
	digests are presented in Section 12. This contrast is interesting
	because on one hand self-assertion is less likely to be affected by
	coordination problems such as the first-party holding stale
	information about the third party, but on the other hand the self-
	assertion is only as trustworthy as the authority that provided it.
	The SRI "integrity" attribute contains a cryptographic hash algorithm		Since digest fields are hashes of resource representations, they
	and digest value which is similar to "representation-data-digest"		explicitly depend on the representation metadata (e.g. the values of
	(see Section 2). The major differences are in serialization format.		Content-Type, Content-Encoding etc). A signature that protects
			Digest but not other representation metadata can expose the
			communication to tampering. For example, an actor could manipulate
			the Content-Type field-value and cause a digest validation failure at
			the recipient, preventing the application from accessing the
			representation. Such an attack consumes the resources of both
			endpoints. See also Section 7.1.
	SRI does not specify handling of partial representation data (e.g.		Digest fields SHOULD always be used over a connection that provides
	Range requests). In contrast, this document specifies handling in		integrity at the transport layer that protects HTTP fields.
	terms that are fully compatible with core HTTP concepts (an example
	is provided in Section 10.3).
	SRI specifies strong requirements on the selection of algorithm for		A Digest field using NOT RECOMMENDED digest-algorithms SHOULD NOT be
	generation and validation of digests. In contrast, the requirements		used in signatures.
	in this document are weaker.
	SRI defines no method for a client to declare an integrity assertion		Using signatures to protect the checksum of an empty representation
	on resources it transfers to a server. In contrast, the "Digest"		allows receiving endpoints to detect if an eventual payload has been
	field can appear on requests.		stripped or added.
	9.1. Supporting Both SRI and Representation Digest		Any mangling of digest fields, including de-duplication of
			representation-data-digest values or combining different field values
			(see Section 5.2 of [SEMANTICS]) might affect signature validation.
	The SRI and Representation Digest mechanisms are different and		8.4. Usage in Trailer Fields
	complementary but one is not capable of replacing the other because
	they have different threat, security and implementation properties.
	A user agent that supports both mechanisms is expected to apply the		Before sending digest fields in a trailer section, the sender should
	rules specified for each but since the two mechanisms are		consider that intermediaries are explicitly allowed to drop any
	independent, the ordering is not important. However, a user agent		trailer (see Section 6.5.2 of [SEMANTICS]).
	supporting both could benefit from performing representation digest
	validation first because it does not always require a conversion into
	identity encoding.
	There is a chance that a user agent supporting both mechanisms may		When digest fields are used in a trailer section, the field-values
	find one validates successfully while the other fails. This document		are received after the content. Eager processing of content before
	specifies no requirements or guidance for user agents that experience		the trailer section prevents digest validation, possibly leading to
	such cases.		processing of invalid data.
	10. Examples of Unsolicited Digest		Not every digest-algorithm is suitable for use in the trailer
			section, some may require to pre-process the whole payload before
			sending a message (e.g. see [I-D.thomson-http-mice]).
			8.5. Usage with Encryption
			Digest fields may expose details of encrypted payload when the
			checksum is computed on the unencrypted data. For example, the use
			of the "id-sha-256" digest-algorithm in conjunction with the
			encrypted content-coding [RFC8188].
			The checksum of an encrypted payload can change between different
			messages depending on the encryption algorithm used; in those cases
			its value could not be used to provide a proof of integrity "at rest"
			unless the whole (e.g. encoded) content is persisted.
			8.6. Algorithm Agility
			The security properties of digest-algorithms are not fixed.
			Algorithm Agility (see [RFC7696]) is achieved by providing
			implementations with flexibility choose digest-algorithms from the
			IANA Digest Algorithm Values registry in Section 9.1.
			To help endpoints distinguish weaker algorithms from stronger ones,
			this document adds to the IANA Digest Algorithm Values registry a new
			"Status" field containing the most recent appraisal of the digest-
			algorithm.
			An endpoint might have a preference for algorithms, such as
			preferring "standard" algorithms over "deprecated" ones. Transition
			from weak algorithms is supported by negotiation of digest-algorithm
			using Want-Digest or Want-Content-Digest (see Section 5) or by
			sending multiple representation-data-digest values from which the
			receiver chooses. Endpoints are advised that sending multiple values
			consumes resources, which may be wasted if the receiver ignores them
			(see Section 3).
			8.7. Duplicate digest-algorithm in field value
			An endpoint might receive multiple representation-data-digest values
			(see Section 3) that use the same digest-algorithm with different or
			identical digest-values. For example:
			Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=,
			sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
			A receiver is permitted to ignore any representation-data-digest
			value, so validation of duplicates is left as an implementation
			decision. Endpoints might select all, some, or none of the values
			for checksum comparison and, based on the intersection of those
			results, conditionally pass or fail digest validation.
			8.8. Resource exhaustion
			Digest fields validation consumes computational resources. In order
			to avoid resource exhaustion, implementations can restrict validation
			of the algorithm types, number of validations, or the size of
			content.
			9. IANA Considerations
			9.1. Establish the HTTP Digest Algorithm Values Registry
			This memo sets this specification to be the establishing document for
			the HTTP Digest Algorithm Values (https://www.iana.org/assignments/
			http-dig-alg/) registry.
			IANA is asked to update the "Reference" for this registry to refer
			this document and to inizialize the registry with the tokens defined
			in Section 6.
			This registry uses the Specification Required policy (Section 4.6 of
			[RFC8126]).
			9.2. Obsolete "contentMD5" token in Digest Algorithm
			This memo adds the "contentMD5" token in the HTTP Digest Algorithm
			Values (https://www.iana.org/assignments/http-dig-alg/) registry:
			* Digest Algorithm: contentMD5
			* Description: Section 5 of [RFC3230] defined the "contentMD5" token
			to be used only in Want-Digest. This token is obsoleted and MUST
			NOT be used.
			* Reference: Section 9.2 of this document, Section 5 of [RFC3230].
			* Status: obsoleted
			9.3. Changes Compared to RFC3230
			The contentMD5 digest-algorithm token defined in Section 5 of
			[RFC3230] has been added to the HTTP Digest Algorithm Values Registry
			with the "obsoleted" status.
			All digest-algorithms defined in [RFC3230] are now "deprecated".
			9.4. Changes Compared to RFC5843
			The digest-algorithm tokens for "MD5", "SHA", "SHA-256", "SHA-512"
			have been updated to lowercase.
			The status of "MD5" and "SHA" has been updated to "deprecated", and
			their description has been modified accordingly.
			The "id-sha-256" and "id-sha-512" algorithms have been added to the
			registry.
			9.5. Want-Digest Field Registration
			This section registers the Want-Digest field in the "Hypertext
			Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
			Field name: Want-Digest
			Status: permanent
			Specification document(s): Section 5 of this document
			9.6. Digest Field Registration
			This section registers the Digest field in the "Hypertext Transfer
			Protocol (HTTP) Field Name Registry" [SEMANTICS].
			Field name: Digest
			Status: permanent
			Specification document(s): Section 3 of this document
			9.7. Want-Content-Digest Field Registration
			This section registers the Want-Content-Digest field in the
			"Hypertext Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
			Field name: Want-Content-Digest
			Status: permanent
			Specification document(s): Section 5 of this document
			9.8. Content-Digest Field Registration
			This section registers the Content-Digest field in the "Hypertext
			Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
			Field name: Content-Digest
			Status: permanent
			Specification document(s): Section 4 of this document
			10. References
			10.1. Normative References
			[CMU-836068]
			Carnagie Mellon University, Software Engineering
			Institute, "MD5 Vulnerable to collision attacks", 31
			December 2008, <https://www.kb.cert.org/vuls/id/836068/>.
			[IACR-2020-014]
			Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", 5
			January 2020, <https://eprint.iacr.org/2020/014.pdf>.
			[NIST800-32]
			National Institute of Standards and Technology, U.S.
			Department of Commerce, "Introduction to Public Key
			Technology and the Federal PKI Infrastructure", February
			2001, <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
			nistspecialpublication800-32.pdf>.
			[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
			DOI 10.17487/RFC1321, April 1992,
			<https://www.rfc-editor.org/rfc/rfc1321>.
			[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
			Specification version 3.3", RFC 1950,
			DOI 10.17487/RFC1950, May 1996,
			<https://www.rfc-editor.org/rfc/rfc1950>.
			[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
			Requirement Levels", BCP 14, RFC 2119,
			DOI 10.17487/RFC2119, March 1997,
			<https://www.rfc-editor.org/rfc/rfc2119>.
			[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
			(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
			<https://www.rfc-editor.org/rfc/rfc3174>.
			[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
			RFC 3230, DOI 10.17487/RFC3230, January 2002,
			<https://www.rfc-editor.org/rfc/rfc3230>.
			[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
			Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
			<https://www.rfc-editor.org/rfc/rfc4648>.
			[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
			RFC 4960, DOI 10.17487/RFC4960, September 2007,
			<https://www.rfc-editor.org/rfc/rfc4960>.
			[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
			Specifications: ABNF", STD 68, RFC 5234,
			DOI 10.17487/RFC5234, January 2008,
			<https://www.rfc-editor.org/rfc/rfc5234>.
			[RFC5843] Bryan, A., "Additional Hash Algorithms for HTTP Instance
			Digests", RFC 5843, DOI 10.17487/RFC5843, April 2010,
			<https://www.rfc-editor.org/rfc/rfc5843>.
			[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
			(SHA and SHA-based HMAC and HKDF)", RFC 6234,
			DOI 10.17487/RFC6234, May 2011,
			<https://www.rfc-editor.org/rfc/rfc6234>.
			[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
			RFC 7405, DOI 10.17487/RFC7405, December 2014,
			<https://www.rfc-editor.org/rfc/rfc7405>.
			[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
			Writing an IANA Considerations Section in RFCs", BCP 26,
			RFC 8126, DOI 10.17487/RFC8126, June 2017,
			<https://www.rfc-editor.org/rfc/rfc8126>.
			[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
			2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
			May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
			[SEMANTICS]
			Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
			Semantics", Work in Progress, Internet-Draft, draft-ietf-
			httpbis-semantics-19, 12 September 2021,
			<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
			semantics-19>.
			[UNIX] The Open Group, "The Single UNIX Specification, Version 2
			- 6 Vol Set for UNIX 98", February 1997.
			10.2. Informative References
			[HTTP11] Fielding, R. T., Nottingham, M., and J. Reschke,
			"HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
			httpbis-messaging-19, 12 September 2021,
			<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
			messaging-19>.
			[I-D.ietf-httpbis-header-structure]
			Nottingham, M. and P. Kamp, "Structured Field Values for
			HTTP", Work in Progress, Internet-Draft, draft-ietf-
			httpbis-header-structure-19, 3 June 2020,
			<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
			header-structure-19>.
			[I-D.thomson-http-mice]
			Thomson, M. and J. Yasskin, "Merkle Integrity Content
			Encoding", Work in Progress, Internet-Draft, draft-
			thomson-http-mice-03, 13 August 2018,
			<https://datatracker.ietf.org/doc/html/draft-thomson-http-
			mice-03>.
			[NO-MD5] Turner, S. and L. Chen, "Updated Security Considerations
			for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
			RFC 6151, DOI 10.17487/RFC6151, March 2011,
			<https://www.rfc-editor.org/rfc/rfc6151>.
			[NO-SHA1] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
			Considerations for the SHA-0 and SHA-1 Message-Digest
			Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
			<https://www.rfc-editor.org/rfc/rfc6194>.
			[PATCH] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
			RFC 5789, DOI 10.17487/RFC5789, March 2010,
			<https://www.rfc-editor.org/rfc/rfc5789>.
			[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
			DOI 10.17487/RFC2818, May 2000,
			<https://www.rfc-editor.org/rfc/rfc2818>.
			[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
			Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
			DOI 10.17487/RFC7231, June 2014,
			<https://www.rfc-editor.org/rfc/rfc7231>.
			[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
			DOI 10.17487/RFC7396, October 2014,
			<https://www.rfc-editor.org/rfc/rfc7396>.
			[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
			Agility and Selecting Mandatory-to-Implement Algorithms",
			BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
			<https://www.rfc-editor.org/rfc/rfc7696>.
			[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
			APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
			<https://www.rfc-editor.org/rfc/rfc7807>.
			[RFC8188] Thomson, M., "Encrypted Content-Encoding for HTTP",
			RFC 8188, DOI 10.17487/RFC8188, June 2017,
			<https://www.rfc-editor.org/rfc/rfc8188>.
			Appendix A. Resource Representation and Representation-Data
			The following examples show how representation metadata, payload
			transformations and method impacts on the message and content. When
			the content contains non-printable characters (e.g. when it is
			compressed) it is shown as a Base64-encoded string.
			PUT /entries/1234 HTTP/1.1
			Host: foo.example
			Content-Type: application/json
			{"hello": "world"}
			Figure 1: Request containing a JSON object without any content coding
			PUT /entries/1234 HTTP/1.1
			Host: foo.example
			Content-Type: application/json
			Content-Encoding: gzip
			H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
			Figure 2: Request containing a gzip-encoded JSON object
			Now the same content conveys a malformed JSON object, because the
			request does not indicate a content coding.
			PUT /entries/1234 HTTP/1.1
			Host: foo.example
			Content-Type: application/json
			H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
			Figure 3: Request containing malformed JSON
			A Range-Request alters the content, conveying a partial
			representation.
			GET /entries/1234 HTTP/1.1
			Host: foo.example
			Range: bytes=1-7
			Figure 4: Request for partial content
			HTTP/1.1 206 Partial Content
			Content-Encoding: gzip
			Content-Type: application/json
			Content-Range: bytes 1-7/18
			iwgAla3RXA==
			Figure 5: Partial response from a gzip-encoded representation
			The method can also alter the content. For example, the response to
			a HEAD request does not carry content.
			HEAD /entries/1234 HTTP/1.1
			Host: foo.example
			Accept: application/json
			Accept-Encoding: gzip
			Figure 6: HEAD request
			HTTP/1.1 200 OK
			Content-Type: application/json
			Content-Encoding: gzip
			Figure 7: Response to HEAD request (empty content)
			Finally, the semantics of an HTTP response might decouple the
			effective request URI from the enclosed representation. In the
			example response below, the Content-Location header field indicates
			that the enclosed representation refers to the resource available at
			/authors/123, even though the request is directed to /authors/.
			POST /authors/ HTTP/1.1
			Host: foo.example
			Accept: application/json
			Content-Type: application/json
			{"author": "Camilleri"}
			Figure 8: POST request
			HTTP/1.1 201 Created
			Content-Type: application/json
			Content-Location: /authors/123
			Location: /authors/123
			{"id": "123", "author": "Camilleri"}
			Figure 9: Response with Content-Location header
			Appendix B. Examples of Unsolicited Digest
	The following examples demonstrate interactions where a server		The following examples demonstrate interactions where a server
	responds with a "Digest" field even though the client did not solicit		responds with a Digest or Content-Digest fields even though the
	one using "Want-Digest".		client did not solicit one using Want-Digest or Want-Content-Digest.
	Some examples include JSON objects in the content. For presentation		Some examples include JSON objects in the content. For presentation
	purposes, objects that fit completely within the line-length limits		purposes, objects that fit completely within the line-length limits
	are presented on a single line using compact notation with no leading		are presented on a single line using compact notation with no leading
	space. Objects that would exceed line-length limits are presented		space. Objects that would exceed line-length limits are presented
	across multiple lines (one line per key-value pair) with 2 spaced of		across multiple lines (one line per key-value pair) with 2 spaced of
	leading indentation.		leading indentation.
	"Digest" is media-type agnostic and does not provide canonicalization		Checksum mechanisms defined in this document are media-type agnostic
	algorithms for specific formats. Examples of "Digest" are calculated		and do not provide canonicalization algorithms for specific formats.
	inclusive of any space.		Examples are calculated inclusive of any space. While examples can
			include both fields, Digest and Content-Digest can be returned
			independently.
	10.1. Server Returns Full Representation Data		B.1. Server Returns Full Representation Data
	Request:		In this example, the message content conveys complete representation
			data, so Digest and Content-Digest have the same value.
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Response:		Figure 10: GET request for an item
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
			Content-Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	{"hello": "world"}		{"hello": "world"}
	10.2. Server Returns No Representation Data		Figure 11: Response with Content-Digest
			B.2. Server Returns No Representation Data
	In this example, a HEAD request is used to retrieve the checksum of a		In this example, a HEAD request is used to retrieve the checksum of a
	resource.		resource.
	The response "Digest" field-value is calculated over the JSON object		The response Digest field-value is calculated over the JSON object
	"{"hello": "world"}", which is not shown because there is no payload		{"hello": "world"}, which is not shown because there is no payload
	data.		data. Content-Digest is computed on empty content.
	Request:
	HEAD /items/123 HTTP/1.1		HEAD /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Response:		Figure 12: HEAD request for an item
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
			Content-Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
	10.3. Server Returns Partial Representation Data		Figure 13: Response with both Content-Digest and Digest; empty
			content
	In this example, the client makes a range request and the server		B.3. Server Returns Partial Representation Data
	responds with partial content. The "Digest" field-value represents
	the entire JSON object "{"hello": "world"}".
	Request:		In this example, the client makes a range request and the server
			responds with partial content. The Digest field-value represents the
			entire JSON object {"hello": "world"}, while the Content-Digest
			field-value is computed on the message content "hello".
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Range: bytes=1-7		Range: bytes=1-7
	Response:		Figure 14: Request for partial content
	HTTP/1.1 206 Partial Content		HTTP/1.1 206 Partial Content
	Content-Type: application/json		Content-Type: application/json
	Content-Range: bytes 1-7/18		Content-Range: bytes 1-7/18
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
			Content-Digest: sha-256=Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=
	"hello"		"hello"
	10.4. Client and Server Provide Full Representation Data		Figure 15: Partial response with both Content-Digest and Digest
	The request contains a "Digest" field-value calculated on the		B.4. Client and Server Provide Full Representation Data
	enclosed representation. It also includes an "Accept-Encoding: br"
	header field that advertises the client supports brotli encoding.
	The response includes a "Content-Encoding: br" that indicates the		The request contains a Digest field-value calculated on the enclosed
	selected representation is brotli encoded. The "Digest" field-value		representation. It also includes an Accept-Encoding: br header field
	is therefore different compared to the request.		that advertises the client supports Brotli encoding.
	For presentation purposes, the response body is displayed as a		The response includes a Content-Encoding: br that indicates the
	base64-encoded string because it contains non-printable characters.		selected representation is Brotli-encoded. The Digest field-value is
			therefore different compared to the request.
	Request:		For presentation purposes, the response body is displayed as a
			Base64-encoded string because it contains non-printable characters.
	PUT /items/123 HTTP/1.1		PUT /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Content-Type: application/json		Content-Type: application/json
	Accept-Encoding: br		Accept-Encoding: br
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	{"hello": "world"}		{"hello": "world"}
	Response:		Figure 16: PUT Request with Digest
	HTTP/1.1 200 Ok		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Content-Location: /items/123		Content-Location: /items/123
	Content-Encoding: br		Content-Encoding: br
	Content-Length: 22		Content-Length: 22
	Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=		Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
	iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==		iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
	10.5. Client Provides Full Representation Data, Server Provides No		Figure 17: Response with Digest of encoded response
	Representation Data
	The request "Digest" field-value is calculated on the enclosed		B.5. Client Provides Full Representation Data, Server Provides No
	payload.		Representation Data
	The response "Digest" field-value depends on the representation		The request Digest field-value is calculated on the enclosed payload.
	metadata header fields, including "Content-Encoding: br" even when
	the response does not contain content.
	Request:		The response Digest field-value depends on the representation
			metadata header fields, including Content-Encoding: br even when the
			response does not contain content.
	PUT /items/123 HTTP/1.1		PUT /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Content-Type: application/json		Content-Type: application/json
	Content-Length: 18		Content-Length: 18
	Accept-Encoding: br		Accept-Encoding: br
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	{"hello": "world"}		{"hello": "world"}
	Response:
	HTTP/1.1 204 No Content		HTTP/1.1 204 No Content
	Content-Type: application/json		Content-Type: application/json
	Content-Encoding: br		Content-Encoding: br
	Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=		Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
	10.6. Client and Server Provide Full Representation Data, Client Uses		Figure 18: Empty response with Digest
	id-sha-256.
			B.6. Client and Server Provide Full Representation Data, Client Uses
			id-sha-256.
	The response contains two digest values:		The response contains two digest values:
	* one with no content coding applied, which in this case		* one with no content coding applied, which in this case
	accidentally matches the unencoded digest-value sent in the		accidentally matches the unencoded digest-value sent in the
	request;		request;
	* one taking into account the "Content-Encoding".		* one taking into account the Content-Encoding.
	As the response body contains non-printable characters, it is		As the response body contains non-printable characters, it is
	displayed as a base64-encoded string.		displayed as a base64-encoded string.
	Request:
	PUT /items/123 HTTP/1.1		PUT /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Content-Type: application/json		Content-Type: application/json
	Accept-Encoding: br		Accept-Encoding: br
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	{"hello": "world"}		{"hello": "world"}
	Response:		Figure 19: PUT Request with Digest
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Content-Encoding: br		Content-Encoding: br
	Content-Location: /items/123		Content-Location: /items/123
	Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,		Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
	id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==		iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
	10.7. POST Response does not Reference the Request URI		Figure 20: Response with Digest of Encoded Content
	The request "Digest" field-value is computed on the enclosed		B.7. POST Response does not Reference the Request URI
	representation (see Section 6).
	The representation enclosed in the response refers to the resource		The request Digest field-value is computed on the enclosed
	identified by "Content-Location" (see [SEMANTICS], Section 6.4.2).		representation (see Section 7).
	"Digest" is thus computed on the enclosed representation.
	Request:		The representation enclosed in the response refers to the resource
			identified by Content-Location (see Section 6.4.2 of [SEMANTICS]).
			Digest is thus computed on the enclosed representation.
	POST /books HTTP/1.1		POST /books HTTP/1.1
	Host: foo.example		Host: foo.example
	Content-Type: application/json		Content-Type: application/json
	Accept: application/json		Accept: application/json
	Accept-Encoding: identity		Accept-Encoding: identity
	Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=		Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
	{"title": "New Title"}		{"title": "New Title"}
	Response:		Figure 21: POST Request with Digest
	HTTP/1.1 201 Created		HTTP/1.1 201 Created
	Content-Type: application/json		Content-Type: application/json
	Content-Location: /books/123		Content-Location: /books/123
	Location: /books/123		Location: /books/123
	Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=		Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=
	{		{
	"id": "123",		"id": "123",
	"title": "New Title"		"title": "New Title"
	}		}
	Note that a "204 No Content" response without content but with the		Figure 22: Response with Digest of Resource
	same "Digest" field-value would have been legitimate too.
	10.8. POST Response Describes the Request Status		Note that a 204 No Content response without content but with the same
			Digest field-value would have been legitimate too. In that case,
			Content-Digest would have been computed on an empty content.
	The request "Digest" field-value is computed on the enclosed		B.8. POST Response Describes the Request Status
	representation (see Section 6).
	The representation enclosed in the response describes the status of		The request Digest field-value is computed on the enclosed
	the request, so "Digest" is computed on that enclosed representation.		representation (see Section 7).
	Response "Digest" has no explicit relation with the resource		The representation enclosed in the response describes the status of
	referenced by "Location".		the request, so Digest is computed on that enclosed representation.
	Request:		Response Digest has no explicit relation with the resource referenced
			by Location.
	POST /books HTTP/1.1		POST /books HTTP/1.1
	Host: foo.example		Host: foo.example
	Content-Type: application/json		Content-Type: application/json
	Accept: application/json		Accept: application/json
	Accept-Encoding: identity		Accept-Encoding: identity
	Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=		Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
	Location: /books/123
	{"title": "New Title"}		{"title": "New Title"}
			Figure 23: POST Request with Digest
	Response:
	HTTP/1.1 201 Created		HTTP/1.1 201 Created
	Content-Type: application/json		Content-Type: application/json
	Digest: id-sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=		Digest: id-sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
	Location: /books/123		Location: /books/123
	{		{
	"status": "created",		"status": "created",
	"id": "123",		"id": "123",
	"ts": 1569327729,		"ts": 1569327729,
	"instance": "/books/123"		"instance": "/books/123"
	}		}
	10.9. Digest with PATCH		Figure 24: Response with Digest of Representation
			B.9. Digest with PATCH
	This case is analogous to a POST request where the target resource		This case is analogous to a POST request where the target resource
	reflects the effective request URI.		reflects the effective request URI.
	The PATCH request uses the "application/merge-patch+json" media type		The PATCH request uses the application/merge-patch+json media type
	defined in [RFC7396].		defined in [RFC7396].
	"Digest" is calculated on the enclosed payload, which corresponds to		Digest is calculated on the enclosed payload, which corresponds to
	the patch document.		the patch document.
	The response "Digest" field-value is computed on the complete		The response Digest field-value is computed on the complete
	representation of the patched resource.		representation of the patched resource.
	Request:
	PATCH /books/123 HTTP/1.1		PATCH /books/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Content-Type: application/merge-patch+json		Content-Type: application/merge-patch+json
	Accept: application/json		Accept: application/json
	Accept-Encoding: identity		Accept-Encoding: identity
	Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=		Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
	{"title": "New Title"}		{"title": "New Title"}
	Response:		Figure 25: PATCH Request with Digest
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=		Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=
	{		{
	"id": "123",		"id": "123",
	"title": "New Title"		"title": "New Title"
	}		}
	Note that a "204 No Content" response without content but with the		Figure 26: Response with Digest of Representation
	same "Digest" field-value would have been legitimate too.
	10.10. Error responses		Note that a 204 No Content response without content but with the same
			Digest field-value would have been legitimate too.
			B.10. Error responses
	In error responses, the representation-data does not necessarily		In error responses, the representation-data does not necessarily
	refer to the target resource. Instead, it refers to the		refer to the target resource. Instead, it refers to the
	representation of the error.		representation of the error.
	In the following example a client attempts to patch the resource		In the following example, a client sends the same request from
	located at /books/123. However, the resource does not exist and the		Figure 25 to patch the resource located at /books/123. However, the
	server generates a 404 response with a body that describes the error		resource does not exist and the server generates a 404 response with
	in accordance with [RFC7807].		a body that describes the error in accordance with [RFC7807].
	The response "Digest" field-value is computed on this enclosed		The response Digest field-value is computed on this enclosed
	representation.		representation.
	Request:
	PATCH /books/123 HTTP/1.1
	Host: foo.example
	Content-Type: application/merge-patch+json
	Accept: application/json
	Accept-Encoding: identity
	Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
	{"title": "New Title"}
	Response:
	HTTP/1.1 404 Not Found		HTTP/1.1 404 Not Found
	Content-Type: application/problem+json		Content-Type: application/problem+json
	Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=		Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=
	{		{
	"title": "Not Found",		"title": "Not Found",
	"detail": "Cannot PATCH a non-existent resource",		"detail": "Cannot PATCH a non-existent resource",
	"status": 404		"status": 404
	}		}
	10.11. Use with Trailer Fields and Transfer Coding		Figure 27: Response with Digest of Error Representation
	An origin server sends "Digest" as trailer field, so it can calculate		B.11. Use with Trailer Fields and Transfer Coding
			An origin server sends Digest as trailer field, so it can calculate
	digest-value while streaming content and thus mitigate resource		digest-value while streaming content and thus mitigate resource
	consumption. The "Digest" field-value is the same as in Section 10.1		consumption. The Digest field-value is the same as in Appendix B.1
	because "Digest" is designed to be independent from the use of one or		because Digest is designed to be independent from the use of one or
	more transfer codings (see Section 2).		more transfer codings (see Section 2).
	Request:
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Response:		Figure 28: GET Request
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Transfer-Encoding: chunked		Transfer-Encoding: chunked
	Trailer: Digest		Trailer: Digest
	8\r\n		8\r\n
	{"hello"\r\n		{"hello"\r\n
	8		8
	: "world\r\n		: "world\r\n
	2\r\n		2\r\n
	"}\r\n		"}\r\n
	0\r\n		0\r\n
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	11. Examples of Want-Digest Solicited Digest		Figure 29: Chunked Response with Digest
			Appendix C. Examples of Want-Digest Solicited Digest
	The following examples demonstrate interactions where a client		The following examples demonstrate interactions where a client
	solicits a "Digest" using "Want-Digest".		solicits a Digest using Want-Digest. The behavior of Content-Digest
			and Want-Content-Digest is identical.
	Some examples include JSON objects in the content. For presentation		Some examples include JSON objects in the content. For presentation
	purposes, objects that fit completely within the line-length limits		purposes, objects that fit completely within the line-length limits
	are presented on a single line using compact notation with no leading		are presented on a single line using compact notation with no leading
	space. Objects that would exceed line-length limits are presented		space. Objects that would exceed line-length limits are presented
	across multiple lines (one line per key-value pair) with 2 spaced of		across multiple lines (one line per key-value pair) with 2 spaced of
	leading indentation.		leading indentation.
	"Digest" is media-type agnostic and does not provide canonicalization		Checksum mechanisms described in this document are media-type
	algorithms for specific formats. Examples of "Digest" are calculated		agnostic and do not provide canonicalization algorithms for specific
	inclusive of any space.		formats. Examples are calculated inclusive of any space.
	11.1. Server Selects Client's Least Preferred Algorithm		C.1. Server Selects Client's Least Preferred Algorithm
	The client requests a digest, preferring "sha". The server is free		The client requests a digest, preferring "sha". The server is free
	to reply with "sha-256" anyway.		to reply with "sha-256" anyway.
	Request:
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Want-Digest: sha-256;q=0.3, sha;q=1		Want-Digest: sha-256;q=0.3, sha;q=1
	Response:		Figure 30: GET Request with Want-Digest
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=		Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
	{"hello": "world"}		{"hello": "world"}
	11.2. Server Selects Algorithm Unsupported by Client		Figure 31: Response with Different Algorithm
			C.2. Server Selects Algorithm Unsupported by Client
	The client requests a "sha" digest only. The server is currently		The client requests a "sha" digest only. The server is currently
	free to reply with a Digest containing an unsupported algorithm.		free to reply with a Digest containing an unsupported algorithm.
	Request:
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Want-Digest: sha;q=1		Want-Digest: sha;q=1
	Response:		Figure 32: GET Request with Want-Digest
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm		Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
	+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==		+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
	{"hello": "world"}		{"hello": "world"}
	11.3. Server Does Not Support Client Algorithm and Returns an Error		Figure 33: Response with Unsupported Algorithm
			C.3. Server Does Not Support Client Algorithm and Returns an Error
	The client requests a "sha" Digest, the server advises "sha-256" and		The client requests a "sha" Digest, the server advises "sha-256" and
	"sha-512".		"sha-512".
	Request:
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: foo.example		Host: foo.example
	Want-Digest: sha;q=1		Want-Digest: sha;q=1
	Response:		Figure 34: GET Request with Want-Digest
	HTTP/1.1 400 Bad Request		HTTP/1.1 400 Bad Request
	Want-Digest: sha-256, sha-512		Want-Digest: sha-256, sha-512
	12. Security Considerations		Figure 35: Response with Want-Digest
	12.1. Digest Does Not Protect the Full HTTP Message
	This document specifies a data integrity mechanism that protects HTTP
	"representation data", but not HTTP "representation metadata" fields,
	from certain kinds of accidental corruption.
	"Digest" is not intended to be a general protection against malicious
	tampering with HTTP messages. This can be achieved by combining it
	with other approaches such as transport-layer security or digital
	signatures.
	12.2. Broken Cryptographic Algorithms
	Cryptographic algorithms are intended to provide a proof of integrity
	suited towards cryptographic constructions such as signatures.
	However, these rely on collision-resistance for their security proofs
	[CMU-836068]. The "md5" and "sha" digest-algorithms are vulnerable
	to collisions attacks, so they MUST NOT be used with "Digest".
	12.3. Other Deprecated Algorithms
	The ADLER32 algorithm defined in [RFC1950] has been deprecated by
	[RFC3309] because, under certain conditions, it provides weak
	detection of errors. It is now NOT RECOMMENDED for use with
	"Digest".
	12.4. Digest for End-to-End Integrity
	"Digest" only covers the "representation data" and not the
	"representation metadata". "Digest" could help protect the
	"representation data" from buggy manipulation, undesired
	"transforming proxies" (see Section 7.7 of [SEMANTICS]) or other
	actions as the data passes across multiple hops or system boundaries.
	Even a simple mechanism for end-to-end "representation data"
	integrity is valuable because user-agent can validate that resource
	retrieval succeeded before handing off to a HTML parser, video player
	etc. for parsing.
	Identity digest-algorithms (e.g. "id-sha-256" and "id-sha-512") are
	particularly useful for end-to-end integrity because they allow
	piecing together a resource from different sources with different
	HTTP messaging characteristics. For example, different servers that
	apply different content codings.
	Note that using "Digest" alone does not provide end-to-end integrity
	of HTTP messages over multiple hops, since metadata could be
	manipulated at any stage. Methods to protect metadata are discussed
	in Section 12.6.
	12.5. Digest and Content-Location in Responses
	When a state-changing method returns the "Content-Location" header
	field, the enclosed representation refers to the resource identified
	by its value and "Digest" is computed accordingly.
	12.6. Usage in Signatures
	Digital signatures are widely used together with checksums to provide
	the certain identification of the origin of a message [NIST800-32].
	Such signatures can protect one or more HTTP fields and there are
	additional considerations when "Digest" is included in this set.
	Since the "Digest" field is a hash of a resource representation, it
	explicitly depends on the "representation metadata" (eg. the values
	of "Content-Type", "Content-Encoding" etc). A signature that
	protects "Digest" but not other "representation metadata" can expose
	the communication to tampering. For example, an actor could
	manipulate the "Content-Type" field-value and cause a digest
	validation failure at the recipient, preventing the application from
	accessing the representation. Such an attack consumes the resources
	of both endpoints. See also Section 12.5.
	"Digest" SHOULD always be used over a connection that provides
	integrity at the transport layer that protects HTTP fields.
	A "Digest" field using NOT RECOMMENDED digest-algorithms SHOULD NOT
	be used in signatures.
	Using signatures to protect the "Digest" of an empty representation
	allows receiving endpoints to detect if an eventual payload has been
	stripped or added.
	Any mangling of "Digest", including de-duplication of representation-
	data-digest values or combining different field values (see
	Section 5.2 of [SEMANTICS]) might affect signature validation.
	12.7. Usage in Trailer Fields
	When "Digest" is used in trailer fields, the receiver gets the digest
	value after the content and may thus be tempted to process the data
	before validating the digest value. It is prefereable that data is
	only be processed after validating the Digest.
	If received in trailers, "Digest" MUST NOT be discarded; instead, it
	MAY be merged in the header section (See Section 6.5.1 of
	[SEMANTICS]).
	Not every digest-algorithm is suitable for use in the trailer
	section, some may require to pre-process the whole payload before
	sending a message (eg. see [I-D.thomson-http-mice]).
	12.8. Usage with Encryption
	"Digest" may expose details of encrypted payload when the checksum is
	computed on the unencrypted data. For example, the use of the "id-
	sha-256" digest-algorithm in conjunction with the encrypted content-
	coding [RFC8188].
	The representation-data-digest of an encrypted payload can change
	between different messages depending on the encryption algorithm
	used; in those cases its value could not be used to provide a proof
	of integrity "at rest" unless the whole (e.g. encoded) content is
	persisted.
	12.9. Algorithm Agility
	The security properties of digest-algorithms are not fixed.
	Algorithm Agility (see [RFC7696]) is achieved by providing
	implementations with flexibility choose digest-algorithms from the
	IANA Digest Algorithm Values registry in Section 13.1.
	To help endpoints understand weaker algorithms from stronger ones,
	this document adds to the IANA Digest Algorithm Values registry a new
	"Status" field containing the most-recent appraisal of the digest-
	algorithm; the allowed values are specified in Section 13.2.
	An endpoint might have a preference for algorithms, such as
	preferring "standard" algorithms over "deprecated" ones. Transition
	from weak algorithms is supported by negotiation of digest-algorithm
	using "Want-Digest" (see Section 4) or by sending multiple
	representation-data-digest values from which the receiver chooses.
	Endpoints are advised that sending multiple values consumes
	resources, which may be wasted if the receiver ignores them (see
	Section 3).
	12.9.1. Duplicate digest-algorithm in field value
	An endpoint might receive multiple representation-data-digest values
	(see Section 3) that use the same digest-algorithm with different or
	identical digest-values. For example:
	Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=,
	sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
	A receiver is permitted to ignore any representation-data-digest
	value, so validation of duplicates is left as an implementation
	decision. Endpoints might select all, some or none of the values for
	checksum comparison and, based on the intersection of those results,
	conditionally pass or fail digest validation.
	12.10. Resource exhaustion
	"Digest" validation consumes computational resources. In order to
	avoid resource exhaustion, implementations can restrict validation of
	the algorithm types, number of validations, or the size of content.
	13. IANA Considerations
	13.1. Establish the HTTP Digest Algorithm Values Registry
	This memo sets this specification to be the establishing document for
	the HTTP Digest Algorithm Values (https://www.iana.org/assignments/
	http-dig-alg/http-dig-alg.xhtml) registry.
	13.2. The "status" Field in the HTTP Digest Algorithm Values Registry
	This memo adds the field "Status" to the HTTP Digest Algorithm Values
	(https://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml)
	registry. The allowed values for the "Status" fields are described
	below.
	Status
	* "standard" for standardized algorithms without known problems;
	* "experimental", "obsoleted" or some other appropriate value -
	e.g. according to the type and status of the primary document
	in which the algorithm is defined;
	* "deprecated" when the algorithm is insecure or otherwise
	undesirable.
	13.3. Deprecate "MD5" Digest Algorithm
	This memo updates the "MD5" digest-algorithm in the HTTP Digest
	Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
	dig-alg.xhtml) registry:
	* Digest Algorithm: md5
	* Description: As specified in Section 5.
	* Status: As specified in Section 5.
	13.4. Update "UNIXsum" Digest Algorithm
	This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest
	Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
	dig-alg.xhtml) registry:
	* Digest Algorithm: As specified in Section 5.
	* Description: As specified in Section 5.
	* Status: As specified in Section 5.
	13.5. Update "UNIXcksum" Digest Algorithm
	This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest
	Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
	dig-alg.xhtml) registry:
	* Digest Algorithm: As specified in Section 5.
	* Description: As specified in Section 5.
	* Status: As specified in Section 5.
	13.6. Update "CRC32c" Digest Algorithm
	This memo updates the "CRC32c" digest-algorithm in the HTTP Digest
	Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
	dig-alg.xhtml) registry:
	* Digest Algorithm: crc32c
	* Description: The CRC32c algorithm is a 32-bit cyclic redundancy
	check. It achieves a better hamming distance (for better error-
	detection performance) than many other 32-bit CRC functions.
	Other places it is used include iSCSI and SCTP. The 32-bit output
	is encoded in hexadecimal (using between 1 and 8 ASCII characters
	from 0-9, A-F, and a-f; leading 0's are allowed). For example,
	crc32c=0a72a4df and crc32c=A72A4DF are both valid checksums for
	the 3-byte message "dog".
	* Reference: [RFC4960] appendix B, this document.
	* Status: standard.
	13.7. Deprecate "SHA" Digest Algorithm
	This memo updates the "SHA" digest-algorithm in the HTTP Digest
	Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
	dig-alg.xhtml) registry:
	* Digest Algorithm: sha
	* Description: As specified in Section 5.
	* Status: As specified in Section 5.
	13.8. Obsolete "ADLER32" Digest Algorithm
	This memo updates the "ADLER32" digest-algorithm in the HTTP Digest
	Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
	dig-alg.xhtml) registry:
	* Digest Algorithm: adler32
	* Description: The ADLER32 algorithm is a checksum specified in
	[RFC1950] "ZLIB Compressed Data Format". The 32-bit output is
	encoded in hexadecimal (using between 1 and 8 ASCII characters
	from 0-9, A-F, and a-f; leading 0's are allowed). For example,
	adler32=03da0195 and adler32=3DA0195 are both valid checksums for
	the 4-byte message "Wiki". This algorithm is obsoleted and SHOULD
	NOT be used.
	* Status: obsoleted
	13.9. Obsolete "contentMD5" token in Digest Algorithm
	This memo adds the "contentMD5" token in the HTTP Digest Algorithm
	Values (https://www.iana.org/assignments/http-dig-alg/http-dig-
	alg.xhtml) registry:
	* Digest Algorithm: contentMD5
	* Description: Section 5 of [RFC3230] defined the "contentMD5" token
	to be used only in Want-Digest. This token is obsoleted and MUST
	NOT be used.
	* Reference: Section 13.9 of this document, Section 5 of [RFC3230].
	* Status: obsoleted
	13.10. The "id-sha-256" Digest Algorithm
	This memo registers the "id-sha-256" digest-algorithm in the HTTP
	Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
	alg/http-dig-alg.xhtml) registry:
	* Digest Algorithm: id-sha-256
	* Description: As specified in Section 5.
	* Status: As specified in Section 5.
	13.11. The "id-sha-512" Digest Algorithm
	This memo registers the "id-sha-512" digest-algorithm in the HTTP
	Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
	alg/http-dig-alg.xhtml) registry:
	* Digest Algorithm: id-sha-512
	* Description: As specified in Section 5.
	* Status: As specified in Section 5.
	13.12. Changes Compared to RFC5843
	The digest-algorithm values for "MD5", "SHA", "SHA-256", "SHA-512",
	"UNIXcksum", "UNIXsum", "ADLER32" and "CRC32c" have been updated to
	lowercase.
	The status of "MD5" has been updated to "deprecated", and its
	description states that this algorithm MUST NOT be used.
	The status of "SHA" has been updated to "deprecated", and its
	description states that this algorithm MUST NOT be used.
	The status for "CRC2c", "UNIXsum" and "UNIXcksum" has been updated to
	"standard".
	The "id-sha-256" and "id-sha-512" algorithms have been added to the
	registry.
	13.13. Want-Digest Field Registration
	This section registers the "Want-Digest" field in the "Hypertext
	Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
	Field name: "Want-Digest"
	Status: permanent
	Specification document(s): Section 4 of this document
	13.14. Digest Field Registration
	This section registers the "Digest" field in the "Hypertext Transfer
	Protocol (HTTP) Field Name Registry" [SEMANTICS].
	Field name: "Digest"
	Status: permanent
	Specification document(s): Section 3 of this document
	14. References
	14.1. Normative References
	[CMU-836068]
	Carnagie Mellon University, Software Engineering
	Institute, "MD5 Vulnerable to collision attacks", 31
	December 2008, <https://www.kb.cert.org/vuls/id/836068/>.
	[IACR-2020-014]
	Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", 5
	January 2020, <https://eprint.iacr.org/2020/014.pdf>.
	[NIST800-32]
	National Institute of Standards and Technology, U.S.
	Department of Commerce, "Introduction to Public Key
	Technology and the Federal PKI Infrastructure", February
	2001, <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
	nistspecialpublication800-32.pdf>.
	[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
	DOI 10.17487/RFC1321, April 1992,
	<https://www.rfc-editor.org/rfc/rfc1321>.
	[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
	Specification version 3.3", RFC 1950,
	DOI 10.17487/RFC1950, May 1996,
	<https://www.rfc-editor.org/rfc/rfc1950>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/rfc/rfc2119>.
	[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
	(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
	<https://www.rfc-editor.org/rfc/rfc3174>.
	[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
	RFC 3230, DOI 10.17487/RFC3230, January 2002,
	<https://www.rfc-editor.org/rfc/rfc3230>.
	[RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
	Transmission Protocol (SCTP) Checksum Change", RFC 3309,
	DOI 10.17487/RFC3309, September 2002,
	<https://www.rfc-editor.org/rfc/rfc3309>.
	[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
	Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
	<https://www.rfc-editor.org/rfc/rfc4648>.
	[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
	RFC 4960, DOI 10.17487/RFC4960, September 2007,
	<https://www.rfc-editor.org/rfc/rfc4960>.
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234,
	DOI 10.17487/RFC5234, January 2008,
	<https://www.rfc-editor.org/rfc/rfc5234>.
	[RFC5843] Bryan, A., "Additional Hash Algorithms for HTTP Instance
	Digests", RFC 5843, DOI 10.17487/RFC5843, April 2010,
	<https://www.rfc-editor.org/rfc/rfc5843>.
	[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
	(SHA and SHA-based HMAC and HKDF)", RFC 6234,
	DOI 10.17487/RFC6234, May 2011,
	<https://www.rfc-editor.org/rfc/rfc6234>.
	[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
	RFC 7405, DOI 10.17487/RFC7405, December 2014,
	<https://www.rfc-editor.org/rfc/rfc7405>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
	[SEMANTICS]
	Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
	Semantics", Work in Progress, Internet-Draft, draft-ietf-
	httpbis-semantics-15, 30 March 2021,
	<https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
	15>.
	[UNIX] The Open Group, "The Single UNIX Specification, Version 2
	- 6 Vol Set for UNIX 98", February 1997.
	14.2. Informative References
	[HTTP11] Fielding, R. T., Nottingham, M., and J. Reschke,
	"HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
	httpbis-messaging-15, 30 March 2021,
	<https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
	15>.
	[I-D.ietf-httpbis-header-structure]
	Nottingham, M. and P. Kamp, "Structured Field Values for
	HTTP", Work in Progress, Internet-Draft, draft-ietf-
	httpbis-header-structure-19, 3 June 2020,
	<https://tools.ietf.org/html/draft-ietf-httpbis-header-
	structure-19>.
	[I-D.thomson-http-mice]
	Thomson, M. and J. Yasskin, "Merkle Integrity Content
	Encoding", Work in Progress, Internet-Draft, draft-
	thomson-http-mice-03, 13 August 2018,
	<https://tools.ietf.org/html/draft-thomson-http-mice-03>.
	[NO-MD5] Turner, S. and L. Chen, "Updated Security Considerations
	for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
	RFC 6151, DOI 10.17487/RFC6151, March 2011,
	<https://www.rfc-editor.org/rfc/rfc6151>.
	[NO-SHA1] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
	Considerations for the SHA-0 and SHA-1 Message-Digest
	Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
	<https://www.rfc-editor.org/rfc/rfc6194>.
	[PATCH] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
	RFC 5789, DOI 10.17487/RFC5789, March 2010,
	<https://www.rfc-editor.org/rfc/rfc5789>.
	[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
	DOI 10.17487/RFC2818, May 2000,
	<https://www.rfc-editor.org/rfc/rfc2818>.
	[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
	Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
	DOI 10.17487/RFC7231, June 2014,
	<https://www.rfc-editor.org/rfc/rfc7231>.
	[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
	DOI 10.17487/RFC7396, October 2014,
	<https://www.rfc-editor.org/rfc/rfc7396>.
	[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
	Agility and Selecting Mandatory-to-Implement Algorithms",
	BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
	<https://www.rfc-editor.org/rfc/rfc7696>.
	[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
	APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
	<https://www.rfc-editor.org/rfc/rfc7807>.
	[RFC8188] Thomson, M., "Encrypted Content-Encoding for HTTP",
	RFC 8188, DOI 10.17487/RFC8188, June 2017,
	<https://www.rfc-editor.org/rfc/rfc8188>.
	[SRI] Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
	"Subresource Integrity", W3C Recommendation REC-SRI-
	20160623, 23 June 2016,
	<https://www.w3.org/TR/2016/REC-SRI-20160623/>.
	Appendix A. Resource Representation and Representation-Data
	The following examples show how representation metadata, payload
	transformations and method impacts on the message and content. When
	the content contains non-printable characters (eg. when it is
	compressed) it is shown as base64-encoded string.
	A request with a JSON object without any content coding.
	Request:
	PUT /entries/1234 HTTP/1.1
	Host: foo.example
	Content-Type: application/json
	{"hello": "world"}
	Here is a gzip-compressed JSON object using a content coding.
	Request:
	PUT /entries/1234 HTTP/1.1
	Host: foo.example
	Content-Type: application/json
	Content-Encoding: gzip
	H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
	Now the same content conveys a malformed JSON object.
	Request:
	PUT /entries/1234 HTTP/1.1
	Host: foo.example
	Content-Type: application/json
	H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
	A Range-Request alters the content, conveying a partial
	representation.
	Request:
	GET /entries/1234 HTTP/1.1
	Host: foo.example
	Range: bytes=1-7
	Response:
	HTTP/1.1 206 Partial Content
	Content-Encoding: gzip
	Content-Type: application/json
	Content-Range: bytes 1-7/18
	iwgAla3RXA==
	Now the method too alters the content.
	Request:		Appendix D. Changes from RFC3230
			D.1. Deprecate Negotiation of Content-MD5
	HEAD /entries/1234 HTTP/1.1		This RFC deprecates the negotiation of Content-MD5 as it has been
	Host: foo.example		obsoleted by [RFC7231].
	Accept: application/json
	Accept-Encoding: gzip
	Response:		See Section 4 for a new checksum negotiation mechanism for HTTP
			message content.
	HTTP/1.1 200 OK		D.2. Obsolete Digest Field Parameters
	Content-Type: application/json
	Content-Encoding: gzip
	Finally the semantics of an HTTP response might decouple the		Sections 4.1.1 and 4.2 of [RFC3230] defined field parameters. This
	effective request URI from the enclosed representation. In the		document obsoletes the usage of parameters with Digest because this
	example response below, the "Content-Location" header field indicates		feature has not been widely deployed and complicates field-value
	that the enclosed representation refers to the resource available at		processing.
	"/authors/123".
	Request:		[RFC3230] intended field parameters to provide a common way to attach
			additional information to a representation-data-digest. However, if
			parameters are used as an input to validate the checksum, an attacker
			could alter them to steer the validation behavior.
	POST /authors/ HTTP/1.1		A digest-algorithm can still be parameterized by defining its own way
	Host: foo.example		to encode parameters into the representation-data-digest, in such a
	Accept: application/json		way as to mitigate security risks related to its computation.
	Content-Type: application/json
	{"author": "Camilleri"}		Acknowledgements
	Response:		The vast majority of this document is inherited from [RFC3230], so
			thanks to J. Mogul and A. Van Hoff for their great work. The
			original idea of refreshing this document arose from an interesting
			discussion with M. Nottingham, J. Yasskin and M. Thomson when
			reviewing the MICE content coding.
	HTTP/1.1 201 Created		Thanks to Julian Reschke for his valuable contributions to this
	Content-Type: application/json		document, and to the following contributors that have helped improve
	Content-Location: /authors/123		this specification by reporting bugs, asking smart questions,
	Location: /authors/123		drafting or reviewing text, and evaluating open issues: Mike Bishop,
			Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean
			Turner, and Erik Wilde.
	{"id": "123", "author": "Camilleri"}		FAQ
	Appendix B. FAQ		_RFC Editor: Please remove this section before publication._
	1. Why remove all references to content-md5?		1. Why remove all references to content-md5?
	Those were unnecessary to understanding and using this		Those were unnecessary to understanding and using this
	specification.		specification.
	2. Why remove references to instance manipulation?		2. Why remove references to instance manipulation?
	Those were unnecessary for correctly using and applying the		Those were unnecessary for correctly using and applying the
	specification. An example with Range Request is more than		specification. An example with Range Request is more than
	enough. This document uses the term "partial representation"		enough. This document uses the term "partial representation"
	which should group all those cases.		which should group all those cases.
	3. How to use "Digest" with "PATCH" method?		3. How to use Digest with PATCH method?
	See Section 6.		See Section 7.
	4. Why remove references to delta-encoding?		4. Why remove references to delta-encoding?
	Unnecessary for a correct implementation of this specification.		Unnecessary for a correct implementation of this specification.
	The revised specification can be nicely adapted to "delta		The revised specification can be nicely adapted to "delta
	encoding", but all the references here to delta encoding don't		encoding", but all the references here to delta encoding don't
	add anything to this RFC. Another job would be to refresh delta		add anything to this RFC. Another job would be to refresh delta
	encoding.		encoding.
	5. Why remove references to Digest Authentication?		5. Why remove references to Digest Authentication?
	This specification seems to me completely unrelated to Digest		This specification seems to me completely unrelated to Digest
	Authentication but for the word "Digest".		Authentication but for the word "Digest".
	6. What changes in "Want-Digest"?		6. What changes in Want-Digest?
	The contentMD5 token defined in Section 5 of [RFC3230] is		The contentMD5 token defined in Section 5 of [RFC3230] is
	deprecated by Section 7.		deprecated by Appendix D.1.
	To clarify that "Digest" and "Want-Digest" can be used in both		To clarify that Digest and Want-Digest can be used in both
	requests and responses - [RFC3230] carefully uses "sender" and		requests and responses - [RFC3230] carefully uses sender and
	"receiver" in their definition - we added examples on using		receiver in their definition - we added examples on using Want-
	"Want-Digest" in responses to advertise the supported digest-		Digest in responses to advertise the supported digest-algorithms
	algorithms and the inability to accept requests with unsupported		and the inability to accept requests with unsupported digest-
	digest-algorithms.		algorithms.
	7. Does this specification change supported algorithms?		7. Does this specification change supported algorithms?
	Yes. This RFC updates [RFC5843] which is still delegated for all		Yes. This RFC updates [RFC5843] which is still delegated for all
	algorithms updates, and adds two more algorithms: "id-sha-256"		algorithms updates, and adds two more algorithms: "id-sha-256"
	and "id-sha-512" which allows to send a checksum of a resource		and "id-sha-512" which allows to send a checksum of a resource
	representation with no content codings applied. To simplify a		representation with no content codings applied. To simplify a
	future transition to Structured Fields		future transition to Structured Fields
	[I-D.ietf-httpbis-header-structure] we suggest to use lowercase		[I-D.ietf-httpbis-header-structure] we suggest to use lowercase
	for digest-algorithms.		for digest-algorithms.
	skipping to change at page 36, line 25 ¶		skipping to change at page 35, line 4 ¶
	Yes. This RFC updates [RFC5843] which is still delegated for all		Yes. This RFC updates [RFC5843] which is still delegated for all
	algorithms updates, and adds two more algorithms: "id-sha-256"		algorithms updates, and adds two more algorithms: "id-sha-256"
	and "id-sha-512" which allows to send a checksum of a resource		and "id-sha-512" which allows to send a checksum of a resource
	representation with no content codings applied. To simplify a		representation with no content codings applied. To simplify a
	future transition to Structured Fields		future transition to Structured Fields
	[I-D.ietf-httpbis-header-structure] we suggest to use lowercase		[I-D.ietf-httpbis-header-structure] we suggest to use lowercase
	for digest-algorithms.		for digest-algorithms.
	8. What about mid-stream trailer fields?		8. What about mid-stream trailer fields?
	While mid-stream trailer fields (https://github.com/httpwg/http-		While mid-stream trailer fields (https://github.com/httpwg/http-
	core/issues/313#issuecomment-584389706) are interesting, since		core/issues/313#issuecomment-584389706) are interesting, since
	this specification is a rewrite of [RFC3230] we do not think we		this specification is a rewrite of [RFC3230] we do not think we
	should face that. As a first thought, nothing in this document		should face that. As a first thought, nothing in this document
	precludes future work that would find a use for mid-stream		precludes future work that would find a use for mid-stream
	trailers, for example an incremental digest-algorithm. A		trailers, for example an incremental digest-algorithm. A
	document defining such a digest-algorithm is best positioned to		document defining such a digest-algorithm is best positioned to
	describe how it is used.		describe how it is used.
	Acknowledgements
	The vast majority of this document is inherited from [RFC3230], so
	thanks to J. Mogul and A. Van Hoff for their great work. The
	original idea of refreshing this document arose from an interesting
	discussion with M. Nottingham, J. Yasskin and M. Thomson when
	reviewing the MICE content coding.
	Code Samples		Code Samples
	_RFC Editor: Please remove this section before publication._		_RFC Editor: Please remove this section before publication._
	How can I generate and validate the "Digest" values shown in the		How can I generate and validate the Digest values shown in the
	examples throughout this document?		examples throughout this document?
	The following python3 code can be used to generate digests for JSON		The following python3 code can be used to generate digests for JSON
	objects using SHA algorithms for a range of encodings. Note that		objects using SHA algorithms for a range of encodings. Note that
	these are formatted as base64. This function could be adapted to		these are formatted as base64. This function could be adapted to
	other algorithms and should take into account their specific		other algorithms and should take into account their specific
	formatting rules.		formatting rules.
	import base64, json, hashlib, brotli, logging		import base64, json, hashlib, brotli, logging
	log = logging.getLogger()		log = logging.getLogger()
	def encode_item(item, encoding=lambda x: x):		def encode_item(item, encoding=lambda x: x):
	skipping to change at page 38, line 5 ¶		skipping to change at page 36, line 44 ¶
	print("Encoding | digest-algorithm | digest-value")		print("Encoding | digest-algorithm | digest-value")
	print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512))		print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512))
	# Encoding | digest-algorithm | digest-value		# Encoding | digest-algorithm | digest-value
	# Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'		# Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
	# '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='		# '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='
	Changes		Changes
	_RFC Editor: Please remove this section before publication._		_RFC Editor: Please remove this section before publication._
			Since draft-ietf-httpbis-digest-headers-05
			* Reboot digest-algorithm values registry #1567
			* Add Content-Digest #1542
			* Remove SRI section #1478
	Since draft-ietf-httpbis-digest-headers-04		Since draft-ietf-httpbis-digest-headers-04
	* Improve SRI section #1354		* Improve SRI section #1354
	* About duplicate digest-algorithms #1221		* About duplicate digest-algorithms #1221
	* Improve security considerations #852		* Improve security considerations #852
	* md5 and sha deprecation references #1392		* md5 and sha deprecation references #1392
	skipping to change at page 39, line 29 ¶		skipping to change at page 38, line 31 ¶
	* Deprecate MD5		* Deprecate MD5
	* Obsolete ADLER-32 but don't forbid it #828		* Obsolete ADLER-32 but don't forbid it #828
	* Update CRC32C value in IANA table #828		* Update CRC32C value in IANA table #828
	* Use when acting on resources (POST, PATCH) #853		* Use when acting on resources (POST, PATCH) #853
	* Added Relationship with SRI, draft Use Cases #868, #971		* Added Relationship with SRI, draft Use Cases #868, #971
	* Warn about the implications of "Content-Location"		* Warn about the implications of Content-Location
	Authors' Addresses		Authors' Addresses
	Roberto Polli		Roberto Polli
	Team Digitale, Italian Government		Team Digitale, Italian Government
	Italy		Italy
	Email: robipolli@gmail.com		Email: robipolli@gmail.com
	Lucas Pardue		Lucas Pardue
End of changes. 194 change blocks.
	1044 lines changed or deleted		974 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/