wdiff draft-ietf-httpbis-digest-headers-05.txt draft-ietf-httpbis-digest-headers-06.txt

HTTP R. Polli
Internet-Draft Team Digitale, Italian Government
Obsoletes: 3230 (if approved) L. Pardue
Intended status: Standards Track Cloudflare
Expires: 15 October 2021 13 April 31 March 2022 27 September 2021

Digest Headers
draft-ietf-httpbis-digest-headers-05 Fields
draft-ietf-httpbis-digest-headers-06

Abstract

This document defines the HTTP fields that support integrity checksums.
The Digest and field can be used for the integrity of HTTP
representations. The Content-Digest field can be used for the
integrity of HTTP message content. Want-Digest fields, thus
allowing client and server Want-Content-
Digest can be used to negotiate an indicate a sender's desire to receive integrity checksum of the
exchanged resource representation data.
fields respectively.

This document obsoletes RFC 3230. It replaces the term "instance"
with "representation", which makes it consistent with the HTTP
Semantic and Context defined in draft-ietf-httpbis-semantics.

Note to Readers

_RFC EDITOR: please remove this section before publication_

Discussion of this draft takes place on the HTTP working group
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/).

The source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions (https://github.com/httpwg/
http-extensions).

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 15 October 2021. 31 March 2022.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 3
1.1. A Brief History of HTTP Integrity Fields Document Structure . . . . . . . . 4
1.2. This Proposal . . . . . . . . . . . 4
1.2. Concept Overview . . . . . . . . . . . . . 4
1.3. Goals . . . . . . . 4
1.3. Replacing RFC 3230 . . . . . . . . . . . . . . . . . . . 5
1.4. Notational Conventions . . . . . . . . . . . . . . . . . 6
2. Representation Digest . . . . . . . . . . . . . . . . . . . . 6
3. The Digest Field . . . . . . . . . . . . . . . . . . . . . . 7
4. The Want-Digest Content-Digest Field . . . . . . . . . . . . . . . . . . . . 8
5. Digest Algorithm Values . . . . . . . . . . Want-Digest and Want-Content-Digest Fields . . . . . . . . . 8
6. Use of Digest when acting on resources . . Algorithm Values . . . . . . . . . 11
6.1. Digest and PATCH . . . . . . . . . . . . . 9
7. Using Digest in State-Changing Requests . . . . . . . 11
7. Deprecate Negotiation of Content-MD5 . . . . 13
7.1. Digest and Content-Location in Responses . . . . . . . . 11 13
8. Obsolete Digest Field Parameters Security Considerations . . . . . . . . . . . . . . 12
9. Relationship to Subresource Integrity (SRI) . . . . . . . . . 12
9.1. Supporting Both SRI and Representation 13
8.1. Digest Does Not Protect the Full HTTP Message . . . . . . 13
10. Examples of Unsolicited
8.2. Digest for End-to-End Integrity . . . . . . . . . . . . . . . 13
10.1. Server Returns Full Representation Data . . . . . 14
8.3. Usage in Signatures . . . 13
10.2. Server Returns No Representation Data . . . . . . . . . 14
10.3. Server Returns Partial Representation Data . . . . . . . 14
10.4. Client and Server Provide Full Representation Data
8.4. Usage in Trailer Fields . . . 15
10.5. Client Provides Full Representation Data, Server Provides
No Representation Data . . . . . . . . . . . . . . . . 15
10.6. Client and Server Provide Full Representation Data, Client
Uses id-sha-256.
8.5. Usage with Encryption . . . . . . . . . . . . . . . . . . 15
8.6. Algorithm Agility . 16
10.7. POST Response does not Reference the Request URI . . . . 17
10.8. POST Response Describes the Request Status . . . . . . . 18
10.9. Digest with PATCH . . . . . . . . 15
8.7. Duplicate digest-algorithm in field value . . . . . . . . 16
8.8. Resource exhaustion . . . 18
10.10. Error responses . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . 19
10.11. Use with Trailer Fields and Transfer Coding . . . . . . 20
11. Examples of Want-Digest Solicited Digest . . . . . . . . . . 21
11.1. Server Selects Client's Least Preferred . 16
9.1. Establish the HTTP Digest Algorithm Values Registry . . . 21
11.2. Server Selects 16
9.2. Obsolete "contentMD5" token in Digest Algorithm Unsupported by Client . . . . . 22
11.3. Server Does Not Support Client Algorithm and Returns an
Error . 17
9.3. Changes Compared to RFC3230 . . . . . . . . . . . . . . . 17
9.4. Changes Compared to RFC5843 . . . . . . . . . 22
12. Security Considerations . . . . . . 17
9.5. Want-Digest Field Registration . . . . . . . . . . . . . 22
12.1. 17
9.6. Digest Does Not Protect the Full HTTP Message . . Field Registration . . . 22
12.2. Broken Cryptographic Algorithms . . . . . . . . . . . . 23
12.3. Other Deprecated Algorithms . 18
9.7. Want-Content-Digest Field Registration . . . . . . . . . 18
9.8. Content-Digest Field Registration . . . . 23
12.4. Digest for End-to-End Integrity . . . . . . . . 18

10. References . . . . 23
12.5. Digest and Content-Location in Responses . . . . . . . . 23
12.6. Usage in Signatures . . . . . . . . . . . . . 18
10.1. Normative References . . . . . 24
12.7. Usage in Trailer Fields . . . . . . . . . . . . . 18
10.2. Informative References . . . 24
12.8. Usage with Encryption . . . . . . . . . . . . . . 20
Appendix A. Resource Representation and Representation-Data . . 22
Appendix B. Examples of Unsolicited Digest . 25
12.9. Algorithm Agility . . . . . . . . . . 24
B.1. Server Returns Full Representation Data . . . . . . . . . 25
12.9.1. Duplicate digest-algorithm in field value 24
B.2. Server Returns No Representation Data . . . . . 25
12.10. Resource exhaustion . . . . . 24
B.3. Server Returns Partial Representation Data . . . . . . . 25
B.4. Client and Server Provide Full Representation Data . . . 25
B.5. Client Provides Full Representation Data, Server Provides
No Representation Data . . . 26
13. IANA Considerations . . . . . . . . . . . . . . 26
B.6. Client and Server Provide Full Representation Data, Client
Uses id-sha-256. . . . . . . . 26
13.1. Establish the HTTP Digest Algorithm Values Registry . . 26
13.2. The "status" Field in the HTTP Digest Algorithm Values
Registry . . . . . . . . . . . . 27
B.7. POST Response does not Reference the Request URI . . . . 27
B.8. POST Response Describes the Request Status . . . . . . . 26
13.3. Deprecate "MD5" 28
B.9. Digest Algorithm with PATCH . . . . . . . . . . . . 26
13.4. Update "UNIXsum" Digest Algorithm . . . . . . . . . . . 26
13.5. Update "UNIXcksum" Digest Algorithm 29
B.10. Error responses . . . . . . . . . . 27
13.6. Update "CRC32c" Digest Algorithm . . . . . . . . . . . 30
B.11. Use with Trailer Fields and Transfer Coding . 27
13.7. Deprecate "SHA" Digest Algorithm . . . . . . 30
Appendix C. Examples of Want-Digest Solicited Digest . . . . . . 27
13.8. Obsolete "ADLER32" Digest 31
C.1. Server Selects Client's Least Preferred Algorithm . . . . 31
C.2. Server Selects Algorithm Unsupported by Client . . . . . . 28
13.9. Obsolete "contentMD5" token in Digest 32
C.3. Server Does Not Support Client Algorithm and Returns an
Error . . . . 28
13.10. The "id-sha-256" Digest Algorithm . . . . . . . . . . . 28
13.11. The "id-sha-512" Digest Algorithm . . . . . . . . . . . 29
13.12. 32
Appendix D. Changes Compared to RFC5843 from RFC3230 . . . . . . . . . . . . . . 29
13.13. Want-Digest Field Registration . . . 32
D.1. Deprecate Negotiation of Content-MD5 . . . . . . . . . . 29
13.14. 33
D.2. Obsolete Digest Field Registration . . . . . . . . . . . . . . . 29
14. References . . . . . . . . . . . . . . . . . . . . . . . . Parameters . 30
14.1. Normative References . . . . . . . . . . . 33
Acknowledgements . . . . . . . 30
14.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Resource Representation and Representation-Data . . 33
Appendix B.
FAQ . . . . . . . . . . . . . . . . . . . . . . . . 35
Acknowledgements . . . . . . . . . . . 33
Code Samples . . . . . . . . . . . . . 36
Code Samples . . . . . . . . . . . . . . . 35
Changes . . . . . . . . . . . 36
Changes . . . . . . . . . . . . . . . . . . . 36
Since draft-ietf-httpbis-digest-headers-05 . . . . . . . . . . 37 36
Since draft-ietf-httpbis-digest-headers-04 . . . . . . . . . . 38 37
Since draft-ietf-httpbis-digest-headers-03 . . . . . . . . . . 38 37
Since draft-ietf-httpbis-digest-headers-02 . . . . . . . . . . 38 37
Since draft-ietf-httpbis-digest-headers-01 . . . . . . . . . . 38 37
Since draft-ietf-httpbis-digest-headers-00 . . . . . . . . . . 39 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 38

1. Introduction

The core specification of

HTTP does not define a means to protect the integrity of resources.
representations. When HTTP messages are transferred between
endpoints, the protocol might choose to make use of features of the
lower layer in order to provide some integrity protection; for
instance
instance, TCP checksums or TLS records [RFC2818].

However, there are cases where relying on this alone is insufficient.
An HTTP-level

This document defines two digest integrity mechanism that operates mechanisms for HTTP.
First, representation data integrity, which acts on representation
data (Section 3.2 of [SEMANTICS]). Second, content digest integrity,
which acts on conveyed content (Section 6.4 of [SEMANTICS]). Both
mechanisms operate independent of
transfer can be used transport integrity, offering the
potential to detect programming errors and/or and corruption of data in
flight or at rest, 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, and
boundaries. Finally, they 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-
data. It can be combined with other mechanisms that protect
representation-metadata, such obsoletes [RFC3230].

1.1. Document Structure

This document is structured as digital signatures, in order follows:

* Section 2 describes concepts related to
protect representation digests,

* Section 3 defines the desired parts of an HTTP exchange in whole or in part.

1.1. A Brief History of HTTP Integrity Fields

The Content-MD5 Digest request and response header field was originally introduced and
trailer field,

* Section 4 defines the Content-Digest request and response header
and trailer field,

* Section 5 defines the Want-Digest and Want-Content-Digest request
and response header and trailer field,

* Section 6 and Appendix D.1 describe algorithms and their relation
to provide
integrity, but HTTP/1.1 ([RFC7231], Digest,

* Section 7 details computing representation digests,

* Appendix B) obsoleted it:

The Content-MD5 header D.2 obsoletes Digest field has been removed because it was
inconsistently implemented with respect to partial responses.

[RFC3230] provided a more flexible solution introducing the concept
of "instance", parameters, and the fields "Digest"

* Appendix B and Appendix C provide examples of using Digest and "Want-Digest".
Want-Digest.

1.2. Concept Overview

This Proposal

The concept of "selected representation" defined in document defines the Digest request and response header and
trailer field; see Section 3. At a high level, the value contains a
checksum, computed over selected representation data (Section 3.2 of
[SEMANTICS] makes [RFC3230] definitions inconsistent with current
HTTP semantics. This document updates
[SEMANTICS]), that the "Digest" and "Want-Digest"
field definitions recipient can use to align with [SEMANTICS] concepts. validate integrity.
Basing "Digest" Digest on the selected representation makes it straightforward
to apply it to use-cases where the transferred data
does require requires some
sort of manipulation to be considered a
representation, representation or conveys a
partial representation of a resource eg. resource, such as Range Requests (see
Section 14.2 of [SEMANTICS]).

This document replaces [RFC3230] to better align with [SEMANTICS] and
to provide more detailed description

To support use-cases where a simple checksum of "Digest" usage in the content bytes is
required, this document introduces the Content-Digest request and
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" header and "sha" digest-algorithms are obsoleted trailer field; see Section 12.2 4.

Digest and the "adler32" Content-Digest support algorithm is deprecated Section 12.3. agility. The value Want-Digest
and Want-Content-Digest fields allows endpoints to express interest
in Digest and Content-Digest respectively, and preference of "Digest" is calculated on selected representation, which
is
algorithms in either.

Digest field calculations are tied to the value contained in any "Content-Encoding" or "Content-
Type" Content-Encoding and
Content-Type header fields. Therefore, a given resource may have
multiple different digest values. checksum values when transferred with HTTP. To
allow both parties to exchange a Digest of a representation simple checksum with no content
codings (see Section 8.4.1 of [SEMANTICS]) [SEMANTICS]), two more
digest-algorithms digest-
algorithms are added ("id-sha-256" and "id-sha-512").

1.3. Goals

The goals of this proposal are:

Digest coverage for either the resource's "representation data"
or "selected representation data" communicated via HTTP.

2. Support for multiple digest-algorithms.

3. Negotiation of the use of digests.

The goals fields do not include: provide integrity for HTTP message integrity: Digest messages or fields.
However, they can be combined with other mechanisms do not cover the full HTTP
message nor its semantic, that protect
metadata, such as representation metadata is not
included digital signatures, in the checksum.

HTTP field integrity: Digest mechanisms cover only representation
and selected representation data, and do not order to protect the integrity phases
of associated representation metadata an HTTP exchange in whole or other message fields.

Authentication: Digest mechanisms do in part.

This specification does not support authentication define means for authentication,
authorization or privacy.

1.3. Replacing RFC 3230

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 source
concept of "instance" digests and a digest, message or anything else. These
mechanisms, therefore, are not a sufficient defense against many
kinds 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 malicious attacks.

Privacy: "instance" has been superseded by
selected representation.

This document replaces [RFC3230]. The Digest mechanisms do not provide message privacy.

Authorization: and Want-Digest field
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 mechanisms do not support authorization or
other kinds field
parameters are obsoleted Appendix D.2 and the algorithm table has
been updated to reflect the current state of access controls. the art.

1.4. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405] along with the "#rule" extension defined in
Section 5.6.1 of [SEMANTICS] and the "qvalue" rule defined in
Section 12.4.2 of [SEMANTICS].

The definitions "representation", "selected representation",
"representation data", "representation metadata", and "content" in
this document are to be interpreted as described in [SEMANTICS].

Algorithm names respect the casing used in their definition document
(eg.
(e.g. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg.
(e.g. "sha", "crc32c").

2. Representation Digest

The representation digest is an integrity mechanism for HTTP
resources which uses a checksum that is calculated independently of
the content (see Section 6.4 of [SEMANTICS]). It uses the
representation data (see Section 8.1 of [SEMANTICS]), that can be
fully or partially contained in the content, or not contained at all:

representation-data := Content-Encoding( Content-Type( bits ) ) all.

This takes into account the effect of the HTTP semantics on the
messages; for example, the content can be affected by Range Requests
or methods such as HEAD, while the way the content is transferred "on
the wire" is dependent on other transformations (e.g. transfer
codings for HTTP/1.1 - see Section 6.1 of [HTTP11]). To help
illustrate how such things affect "Digest", Digest, several examples are
provided in Appendix A.

A representation digest consists of the value of a checksum computed
on the entire selected "representation data" representation data (see Section 8.1 of
[SEMANTICS]) of a resource identified according to Section 6.4.2 of
[SEMANTICS] together with an indication of the algorithm used:

representation-data-digest = digest-algorithm "="
<encoded digest output>

When a message has no representation data it is still possible to
assert that no representation data was sent computing the
representation digest on an empty string (see Section 12.6). 8.3).

The checksum is computed using one of the digest-algorithms listed in
the HTTP Digest Algorithm Values Registry (see Section 5 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

The "Digest" Digest field contains a comma-separated list of one or more
representation digest values as defined in Section 2. It can be used
in both requests and responses.

Digest = 1#representation-data-digest

For example:

Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

The relationship between "Content-Location" (see Section 8.7 of
[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 using different algorithms, allowing it to support a
population of clients with different evolving capabilities; this is
particularly useful in support of transitioning away from weaker
algorithms should the need arise (see Section 12.9). 8.6).

Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

A recipient MAY ignore any or all of the representation-data-digests
in a Digest field. This allows the recipient to choose which digest-
algorithm(s) to use for validation instead of verifying every
received representation-data-digest.

A sender MAY send a representation-data-digest using a digest-
algorithm without knowing whether the recipient supports the digest-
algorithm, or even knowing that the recipient will ignore it.

"Digest"

Digest can be sent in a trailer section. In this case, Digest MAY be
merged into the header section; see Section 6.5.1 of [SEMANTICS].

When an incremental
digest-algorithms digest-algorithm is used, the sender and the
receiver can dynamically compute the digest value while streaming the
content.

A non-comprehensive set of examples showing the impacts of
representation metadata, payload transformations and HTTP methods on
Digest is provided in Appendix B and Appendix C.

4. The Want-Digest Content-Digest Field

The "Want-Digest" 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.
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-Content-Digest = 1#want-digest-value
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
as if its associated "qvalue" were 1.0.

The sender is willing to accept a indicates the sender's digest-algorithm if and only if it
is listed in a "Want-Digest" field preferences.
Section 12.4.2 of a message, [SEMANTICS]) describes qvalue usage and its "qvalue" is
non-zero.

If semantics.

Senders can provide multiple acceptable digest-algorithm values are given, the
sender's preferred digest-algorithm is the one (or ones) items with the
highest "qvalue".

Two examples of its use are: same
qvalue.

Examples:

Want-Digest: sha-256
Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0

5.
Want-Content-Digest: sha-256
Want-Content-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0

6. Digest Algorithm Values

Digest-algorithm values are used to indicate a specific digest
computation.

digest-algorithm = token

All digest-algorithm token values are case-insensitive but lower case
is
preferred.

The Internet Assigned Numbers Authority (IANA) acts as preferred; digest-algorithm token values MUST be compared in a registry for
case-insensitive fashion.

Every digest-algorithm defines its computation procedure and encoding
output. Unless specified otherwise, comparison of encoded output is
case-sensitive.

The "HTTP Digest Algorithm Values Registry", maintained by IANA at
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:

* Digest algorithm: the token value. The registry contains can be used to
reserve token values

* Status: the tokens listed
below.

Some digest-algorithms, although registered, rely on vulnerable status of the algorithm. Use "standard" for
standardized algorithms without known problems; "experimental" or
some other appropriate value

- e.g. according to the type and MUST not be used: 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

* "md5", see [CMU-836068] Description: the description of the digest-algorithm and [NO-MD5]; its
encoding

* "sha", see [IACR-2020-014] and [NO-SHA1].

See Reference: a set of pointers to the references above primary documents defining the
digest-algorithm

The associated encoding for further information.

sha-256 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-256 SHA-512 algorithm [RFC6234]. The output of
this algorithm is encoded using the base64 encoding [RFC4648].

* Reference: [RFC6234], [RFC4648], this document.

* Status: standard

sha-512

sha-256
* Digest Algorithm: sha-256

* Description: The SHA-512 SHA-256 algorithm [RFC6234]. The output of
this algorithm is encoded using the base64 encoding [RFC4648].

* Reference: [RFC6234], [RFC4648], this document.

* Status: standard

md5
* Digest Algorithm: md5
* Description: The MD5 algorithm, as specified in [RFC1321]. The
output of this algorithm is encoded using the base64 encoding
[RFC4648]. This digest-algorithm MUST NOT be used as it's is now vulnerable to
collision attacks. See [NO-MD5] and [CMU-836068].

* Reference: [RFC1321], [RFC4648], this document.

* Status: deprecated

sha
* Digest Algorithm: sha

* Description: The SHA-1 algorithm [RFC3174]. The output of this
algorithm is encoded using the base64 encoding [RFC4648]. This
digest-algorithm MUST NOT be used as it's is now vulnerable to collision attacks. See
[NO-SHA1] and [IACR-2020-014].

* Reference: [RFC3174], [RFC6234], [RFC4648], this document.

* Status: deprecated

unixsum
* Digest Algorithm: unixsum

* Description: The algorithm computed by the UNIX "sum" command,
as defined by the Single UNIX Specification, Version 2 [UNIX].
The output of this algorithm is an ASCII decimal-digit string
representing the 16-bit checksum, which is the first word of
the output of the UNIX "sum" command.

* Reference: [UNIX], this document.

* Status: standard deprecated

unixcksum
* Digest Algorithm: unixcksum

* Description: The algorithm computed by the UNIX "cksum"
command, as defined by the Single UNIX Specification, Version 2
[UNIX]. The output of this algorithm is an ASCII digit string
representing the 32-bit CRC, which is the first word of the
output of the UNIX "cksum" command.

* Reference: [UNIX], this document.

* Status: standard 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
independent from "Content-Encoding", Content-Encoding, the following additional digest-
algorithms are defined:

id-sha-512
* Description: The sha-512 digest of the representation-data representation data of
the resource when no content coding is applied

* Reference: [RFC6234], [RFC4648], this document.

* Status: standard

id-sha-256
* Description: The sha-256 digest of the representation-data representation data of
the resource when no content coding is applied

* Reference: [RFC6234], [RFC4648], this document.

* Status: standard

If other digest-algorithm values are defined, the associated encoding
MUST either be represented as a quoted string or MUST NOT include ";"
or ","

7. Using Digest in State-Changing Requests

When 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 representation enclosed
representation, including its metadata, refers to that action.

In these requests in a state-changing request does not
describe the target resource, the representation digest MUST be
computed on the
representation-data of that action. representation-data. This is the only possible
choice because representation digest requires complete representation
metadata (see Section 2).

In responses,

* if the representation describes the status of the request,
"Digest" Digest
MUST be computed on the enclosed representation (see
Section 10.8 Appendix B.8
);

* if there is a referenced resource "Digest" Digest MUST be computed on the
selected representation of the referenced resource even if that is
different from the target resource. That might or might not
result in computing "Digest" Digest on the enclosed representation.

The latter case might be is done according to the HTTP semantics of the given
method, for example using the "Content-Location" Content-Location header field. field (see
Section 8.7 of [SEMANTICS]). In contrast, the "Location" Location header field
does not affect "Digest" Digest because it is not representation metadata.

6.1. Digest and PATCH

For example, in PATCH requests, the representation digest MUST 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 PATCH responses, instead, the representation
digest MUST will be computed on the selected representation of the patched
resource.

"Digest" usage with PATCH is thus very similar to POST, but with the
resource's own semantic partly implied by the method and by the patch
document.

7. Deprecate Negotiation of Content-MD5

This RFC deprecates the negotiation of Content-MD5 as it has been
obsoleted by [RFC7231]. The "contentMD5" token defined in Section 5
of [RFC3230] MUST NOT be used as a digest-algorithm.

8. Obsolete

7.1. Digest Field Parameters

Section 4.1.1 and 4.2 of [RFC3230] defined field parameters. This
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
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.

A digest-algorithm can still be parameterized by defining its own way
to encode parameters into the representation-data-digest, in such a
way as to mitigate security risks related to its computation.

9. Relationship to Subresource Integrity (SRI)

Subresource Integrity [SRI] is an integrity mechanism that shares
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
on a resource served by a first or third party authority. This is
done via the "integrity" attribute that can be added to "script" or
"link" HTML elements. Therefore, the integrity assertion is always
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 Content-Location 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 Responses

When a cryptographic hash algorithm
and digest value which is similar to "representation-data-digest"
(see Section 2). The major differences are in serialization format.

SRI does not specify handling of partial representation data (e.g.
Range requests). In contrast, this document specifies handling in
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
generation and validation of digests. In contrast, the requirements
in this document are weaker.

SRI defines no state-changing method for a client to declare an integrity assertion
on resources it transfers to a server. In contrast, the "Digest"
field can appear on requests.

9.1. Supporting Both SRI and Representation Digest

The SRI and Representation Digest mechanisms are different and
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
rules specified for each but since the two mechanisms are
independent, the ordering is not important. However, a user agent
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
find one validates successfully while the other fails. This document
specifies no requirements or guidance for user agents that experience
such cases.

10. Examples of Unsolicited Digest

The following examples demonstrate interactions where a server
responds with a "Digest" field even though the client did not solicit
one using "Want-Digest".

Some examples include JSON objects in the content. For presentation
purposes, objects that fit completely within the line-length limits
are presented on a single line using compact notation with no leading
space. Objects that would exceed line-length limits are presented
across multiple lines (one line per key-value pair) with 2 spaced of
leading indentation.

"Digest" is media-type agnostic and does not provide canonicalization
algorithms for specific formats. Examples of "Digest" are calculated
inclusive of any space.

10.1. Server Returns Full Representation Data

Request:

GET /items/123 HTTP/1.1
Host: foo.example

Response:

HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

10.2. Server Returns No Representation Data

In this example, a HEAD request is used to retrieve the checksum of a
resource.

The response "Digest" field-value is calculated over the JSON object
"{"hello": "world"}", which is not shown because there is no payload
data.

Request:

HEAD /items/123 HTTP/1.1
Host: foo.example

Response:

HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

10.3. Server Returns Partial Representation Data

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"}".

Request:

GET /items/123 HTTP/1.1
Host: foo.example
Range: bytes=1-7

Response:

HTTP/1.1 206 Partial Content
Content-Type: application/json
Content-Range: bytes 1-7/18
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

"hello"

10.4. Client and Server Provide Full Representation Data

The request contains a "Digest" field-value calculated on the
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
selected representation is brotli encoded. The "Digest" field-value
is therefore different compared to the request.

For presentation purposes, the response body is displayed as a
base64-encoded string because it contains non-printable characters.

Request:

PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

Response:

HTTP/1.1 200 Ok
Content-Type: application/json
Content-Location: /items/123
Content-Encoding: br
Content-Length: 22
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

10.5. Client Provides Full Representation Data, Server Provides No
Representation Data

The request "Digest" field-value is calculated on the enclosed
payload.

The response "Digest" field-value depends on returns the representation
metadata Content-Location header fields, including "Content-Encoding: br" even when
the response does not contain content.

Request:

PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Length: 18
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

Response:

HTTP/1.1 204 No Content
Content-Type: application/json
Content-Encoding: br
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

10.6. Client and Server Provide Full Representation Data, Client Uses
id-sha-256.

The response contains two digest values:

* one with no content coding applied, which in this case
accidentally matches the unencoded digest-value sent in the
request;

* one taking into account the "Content-Encoding".

As the response body contains non-printable characters, it is
displayed as a base64-encoded string.

Request:

PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

Response:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: br
Content-Location: /items/123
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

10.7. POST Response does not Reference the Request URI

The request "Digest" field-value is computed on
field, the enclosed representation (see Section 6).

The representation enclosed in the response refers to the resource identified
by "Content-Location" (see [SEMANTICS], Section 6.4.2).
"Digest" is thus computed on the enclosed representation.

Request:

POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=

{"title": "New Title"}

Response:

HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=

{
"id": "123",
"title": "New Title"
}

Note that a "204 No Content" response without content but with the
same "Digest" field-value would have been legitimate too.

10.8. POST Response Describes the Request Status

The request "Digest" field-value is computed on the enclosed
representation (see Section 6).

The representation enclosed in the response describes the status of
the request, so "Digest" is computed on that enclosed representation.

Response "Digest" has no explicit relation with the resource
referenced by "Location".

Request:

POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
Location: /books/123

{"title": "New Title"}

Response:

HTTP/1.1 201 Created
Content-Type: application/json
Digest: id-sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
Location: /books/123

{
"status": "created",
"id": "123",
"ts": 1569327729,
"instance": "/books/123"
}

10.9. Digest with PATCH

This case is analogous to a POST request where the target resource
reflects the effective request URI.

The PATCH request uses the "application/merge-patch+json" media type
defined in [RFC7396].

"Digest" is calculated on the enclosed payload, which corresponds to
the patch document.

The response "Digest" field-value is computed on the complete
representation of the patched resource.

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/1.1 200 OK
Content-Type: application/json
Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=

{
"id": "123",
"title": "New Title"
}

Note that a "204 No Content" response without content but with the
same "Digest" field-value would have been legitimate too.

10.10. Error responses

In error responses, the representation-data does not necessarily
refer to the target resource. Instead, it refers to the
representation of the error.

In the following example a client attempts to patch the resource
located at /books/123. However, the resource does not exist its value and the
server generates a 404 response with a body that describes the error
in accordance with [RFC7807].

The response "Digest" field-value Digest is computed on this enclosed
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
Content-Type: application/problem+json
Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=

{
"title": "Not Found",
"detail": "Cannot PATCH a non-existent resource",
"status": 404
}

10.11. Use with Trailer Fields and Transfer Coding accordingly. An origin server sends "Digest" as trailer field, so it can calculate
digest-value while streaming content and thus mitigate resource
consumption. The "Digest" field-value is the same as in Section 10.1
because "Digest" example is designed to be independent from the use of one or
more transfer codings (see Section 2).

Request:

GET /items/123 HTTP/1.1
Host: foo.example

Response:

HTTP/1.1 200 OK
Content-Type: application/json
Transfer-Encoding: chunked
Trailer: Digest

8\r\n
{"hello"\r\n
8
: "world\r\n
2\r\n
"}\r\n
0\r\n
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

11. Examples of Want-Digest Solicited Digest

The following examples demonstrate interactions where a client
solicits a "Digest" using "Want-Digest".

Some examples include JSON objects given
in the content. For presentation
purposes, objects that fit completely within the line-length limits
are presented on a single line using compact notation with no leading
space. Objects that would exceed line-length limits are presented
across multiple lines (one line per key-value pair) with 2 spaced of
leading indentation.

"Digest" is media-type agnostic and does not provide canonicalization
algorithms for specific formats. Examples of "Digest" are calculated
inclusive of any space.

11.1. Server Selects Client's Least Preferred Algorithm

The client requests a digest, preferring "sha". The server is free
to reply with "sha-256" anyway.

Request:

GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha-256;q=0.3, sha;q=1

Response:

HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

11.2. Server Selects Algorithm Unsupported by Client

The client requests a "sha" digest only. The server is currently
free to reply with a Digest containing an unsupported algorithm.

Request:

GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1

Response:

HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

{"hello": "world"}

11.3. Server Does Not Support Client Algorithm and Returns an Error

The client requests a "sha" Digest, the server advises "sha-256" and
"sha-512".

Request:

GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1

Response:

HTTP/1.1 400 Bad Request
Want-Digest: sha-256, sha-512

12. Appendix B.7.

8. Security Considerations

12.1.

8.1. Digest Does Not Protect the Full HTTP Message

This document specifies a data integrity mechanism that protects HTTP
"representation data",
representation data or content, but not HTTP "representation metadata" header and trailer
fields, from certain kinds of accidental corruption.

"Digest" is

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.

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.

8.2. Digest for End-to-End Integrity

"Digest" only covers the "representation data" and not the
"representation metadata". "Digest" could

Digest fields can help protect the
"representation data" from buggy manipulation, detect representation data or content
modification due to implementation errors, 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" 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" 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 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. 8.3.

8.3. 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" Digest is included in this set.

Since the "Digest" field is a hash digest fields are hashes of a resource representation, it representations, they
explicitly depends depend on the "representation metadata" (eg. representation metadata (e.g. the values of "Content-Type", "Content-Encoding"
Content-Type, Content-Encoding etc). A signature that protects "Digest"
Digest but not other "representation metadata" representation metadata can expose the
communication to tampering. For example, an actor could manipulate
the "Content-Type" 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" 7.1.

Digest fields SHOULD always be used over a connection that provides
integrity at the transport layer that protects HTTP fields.

A "Digest" Digest field using NOT RECOMMENDED digest-algorithms SHOULD NOT be
used in signatures.

Using signatures to protect the "Digest" checksum of an empty representation
allows receiving endpoints to detect if an eventual payload has been
stripped or added.

Any mangling of "Digest", digest fields, including de-duplication of representation-
data-digest
representation-data-digest values or combining different field values
(see Section 5.2 of [SEMANTICS]) might affect signature validation.

12.7.

8.4. Usage in Trailer Fields

Before sending digest fields in a trailer section, the sender should
consider that intermediaries are explicitly allowed to drop any
trailer (see Section 6.5.2 of [SEMANTICS]).

When "Digest" is digest fields are used in a trailer fields, the receiver gets section, the digest
value field-values
are received after the content. Eager processing of 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 trailer section (See Section 6.5.1 prevents digest validation, possibly leading to
processing of
[SEMANTICS]). invalid data.

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. (e.g. see [I-D.thomson-http-mice]).

12.8.

8.5. Usage with Encryption

"Digest"

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" "id-sha-256" digest-algorithm in conjunction with the
encrypted content-
coding content-coding [RFC8188].

The representation-data-digest 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.

12.9.

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 13.1. 9.1.

To help endpoints understand distinguish weaker algorithms from stronger ones,
this document adds to the IANA Digest Algorithm Values registry a new
"Status" field containing the most-recent most recent appraisal of the digest-
algorithm; the allowed values are specified in Section 13.2.
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" Want-Digest or Want-Content-Digest (see Section 4) 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).

12.9.1.

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 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.

8.8. Resource exhaustion

"Digest"

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.

13.

9. IANA Considerations

13.1.

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/http-dig-alg.xhtml)
http-dig-alg/) registry.

13.2. The "status" Field in the HTTP Digest Algorithm Values Registry

This memo adds the field "Status"

IANA is asked to update the HTTP Digest Algorithm Values
(https://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml)
registry. The allowed values "Reference" 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 this registry to the type and status of the primary refer
this 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 to inizialize the "SHA" digest-algorithm in registry with 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 tokens defined
in Section 5.

13.8. Obsolete "ADLER32" Digest Algorithm 6.

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 registry uses the 4-byte message "Wiki". This algorithm is obsoleted and SHOULD
NOT be used.

* Status: obsoleted

13.9. 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/http-dig-
alg.xhtml) (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 13.9 9.2 of this document, Section 5 of [RFC3230].

* Status: obsoleted

13.10.

9.3. Changes Compared to RFC3230

The "id-sha-256" Digest Algorithm

This memo registers the "id-sha-256" contentMD5 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 token defined in Section 5.

13.11. The "id-sha-512" Digest Algorithm

This memo registers the "id-sha-512" digest-algorithm in 5 of
[RFC3230] has been added to 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 Registry
with the "obsoleted" status.

All digest-algorithms defined in Section 5.

13.12. [RFC3230] are now "deprecated".

9.4. Changes Compared to RFC5843

The digest-algorithm values tokens for "MD5", "SHA", "SHA-256", "SHA-512",
"UNIXcksum", "UNIXsum", "ADLER32" and "CRC32c" "SHA-512"
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
their description states that this algorithm MUST NOT be used.

The status for "CRC2c", "UNIXsum" and "UNIXcksum" has been updated to
"standard". modified accordingly.

The "id-sha-256" and "id-sha-512" algorithms have been added to the
registry.

13.13.

9.5. Want-Digest Field Registration

This section registers the "Want-Digest" Want-Digest field in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].

Field name: "Want-Digest" Want-Digest

Status: permanent

Specification document(s): Section 4 5 of this document

13.14.

9.6. Digest Field Registration

This section registers the "Digest" Digest field in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" [SEMANTICS].

Field name: "Digest" Digest

Status: permanent

Specification document(s): Section 3 of this document

14.

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

14.1.

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>.

[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>.

[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-15, 30 March
httpbis-semantics-19, 12 September 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
15>.
<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.

14.2.

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-15, 30 March
httpbis-messaging-19, 12 September 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
15>.
<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://tools.ietf.org/html/draft-ietf-httpbis-header-
structure-19>.
<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://tools.ietf.org/html/draft-thomson-http-mice-03>.
<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>.

[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. (e.g. when it is
compressed) it is shown as base64-encoded string.

A request with a JSON object without any content coding.

Request: Base64-encoded string.

PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json

{"hello": "world"}

Here is

Figure 1: Request containing a gzip-compressed JSON object using a without any content coding.

Request: 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.

Request: 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.

Request:

GET /entries/1234 HTTP/1.1
Host: foo.example
Range: bytes=1-7

Response:

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==

Now the

Figure 5: Partial response from a gzip-encoded representation

The method too alters can also alter the content.

Request: 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

Response:

Figure 6: HEAD request

HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: gzip

Finally

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" Content-Location header field indicates
that the enclosed representation refers to the resource available at
"/authors/123".

Request:
/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"}

Response:

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
responds with a Digest or Content-Digest fields even though the
client did not solicit one using Want-Digest or Want-Content-Digest.

Checksum mechanisms defined in this document are media-type agnostic
and do not provide canonicalization algorithms for specific formats.
Examples are calculated inclusive of any space. While examples can
include both fields, Digest and Content-Digest can be returned
independently.

B.1. Server Returns Full Representation Data

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
Host: foo.example

Figure 10: GET request for an item

HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

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
resource.

The response Digest field-value is calculated over the JSON object
{"hello": "world"}, which is not shown because there is no payload
data. Content-Digest is computed on empty content.

HEAD /items/123 HTTP/1.1
Host: foo.example

Figure 12: HEAD request for an item

HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

Figure 13: Response with both Content-Digest and Digest; empty
content

B.3. Server Returns Partial Representation Data

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
Host: foo.example
Range: bytes=1-7

Figure 14: Request for partial content

HTTP/1.1 206 Partial Content
Content-Type: application/json
Content-Range: bytes 1-7/18
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Digest: sha-256=Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=

"hello"

Figure 15: Partial response with both Content-Digest and Digest

B.4. Client and Server Provide Full Representation Data

The request contains a Digest field-value calculated on the 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
selected representation is Brotli-encoded. The Digest field-value is
therefore different compared to the 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
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

Figure 16: PUT Request with Digest

HTTP/1.1 200 OK
Content-Type: application/json
Content-Location: /items/123
Content-Encoding: br
Content-Length: 22
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

Figure 17: Response with Digest of encoded response

B.5. Client Provides Full Representation Data, Server Provides No
Representation Data

The request Digest field-value is calculated on the enclosed payload.

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
Host: foo.example
Content-Type: application/json
Content-Length: 18
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

HTTP/1.1 204 No Content
Content-Type: application/json
Content-Encoding: br
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

Figure 18: Empty response with Digest

B.6. Client and Server Provide Full Representation Data, Client Uses
id-sha-256.

The response contains two digest values:

* one with no content coding applied, which in this case
accidentally matches the unencoded digest-value sent in the
request;

* one taking into account the Content-Encoding.

As the response body contains non-printable characters, it is
displayed as a base64-encoded string.

PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

Figure 19: PUT Request with Digest

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

Figure 20: Response with Digest of Encoded Content

B.7. POST Response does not Reference the Request URI

The request Digest field-value is computed on the enclosed
representation (see Section 7).

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
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=

{"title": "New Title"}

Figure 21: POST Request with Digest

HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=

{
"id": "123",
"title": "New Title"
}

Figure 22: Response with Digest of Resource

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.

B.8. POST Response Describes the Request Status

The request Digest field-value is computed on the enclosed
representation (see Section 7).

The representation enclosed in the response describes the status of
the request, so Digest is computed on that enclosed representation.

Response Digest has no explicit relation with the resource referenced
by Location.

POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=

{"title": "New Title"}
Figure 23: POST Request with Digest

HTTP/1.1 201 Created
Content-Type: application/json
Digest: id-sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
Location: /books/123

{
"status": "created",
"id": "123",
"ts": 1569327729,
"instance": "/books/123"
}

Figure 24: Response with Digest of Representation

B.9. Digest with PATCH

This case is analogous to a POST request where the target resource
reflects the effective request URI.

The PATCH request uses the application/merge-patch+json media type
defined in [RFC7396].

Digest is calculated on the enclosed payload, which corresponds to
the patch document.

The response Digest field-value is computed on the complete
representation of the patched resource.

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"}

Figure 25: PATCH Request with Digest

HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=

{
"id": "123",
"title": "New Title"
}

Figure 26: Response with Digest of Representation

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
refer to the target resource. Instead, it refers to the
representation of the error.

In the following example, a client sends the same request from
Figure 25 to patch the resource located at /books/123. However, the
resource does not exist and the server generates a 404 response with
a body that describes the error in accordance with [RFC7807].

The response Digest field-value is computed on this enclosed
representation.

HTTP/1.1 404 Not Found
Content-Type: application/problem+json
Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=

{
"title": "Not Found",
"detail": "Cannot PATCH a non-existent resource",
"status": 404
}

Figure 27: Response with Digest of Error Representation

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
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
more transfer codings (see Section 2).

GET /items/123 HTTP/1.1
Host: foo.example

Figure 28: GET Request

HTTP/1.1 200 OK
Content-Type: application/json
Transfer-Encoding: chunked
Trailer: Digest

8\r\n
{"hello"\r\n
8
: "world\r\n
2\r\n
"}\r\n
0\r\n
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

Figure 29: Chunked Response with Digest

Appendix C. Examples of Want-Digest Solicited Digest

The following examples demonstrate interactions where a client
solicits a Digest using Want-Digest. The behavior of Content-Digest
and Want-Content-Digest is identical.

Checksum mechanisms described in this document are media-type
agnostic and do not provide canonicalization algorithms for specific
formats. Examples are calculated inclusive of any space.

C.1. Server Selects Client's Least Preferred Algorithm

The client requests a digest, preferring "sha". The server is free
to reply with "sha-256" anyway.

GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha-256;q=0.3, sha;q=1

Figure 30: GET Request with Want-Digest

HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

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
free to reply with a Digest containing an unsupported algorithm.

GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1

Figure 32: GET Request with Want-Digest

HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

{"hello": "world"}

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
"sha-512".

GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1

Figure 34: GET Request with Want-Digest

HTTP/1.1 400 Bad Request
Want-Digest: sha-256, sha-512

Figure 35: Response with Want-Digest

Appendix D. Changes from RFC3230
D.1. Deprecate Negotiation of Content-MD5

This RFC deprecates the negotiation of Content-MD5 as it has been
obsoleted by [RFC7231].

See Section 4 for a new checksum negotiation mechanism for HTTP
message content.

D.2. Obsolete Digest Field Parameters

Sections 4.1.1 and 4.2 of [RFC3230] defined field parameters. This
document obsoletes the usage of parameters with Digest because this
feature has not been widely deployed and complicates field-value
processing.

A digest-algorithm can still be parameterized by defining its own way
to encode parameters into the representation-data-digest, in such a
way as to mitigate security risks related to its computation.

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.

Thanks to Julian Reschke for his valuable contributions to this
document, and to the following contributors that have helped improve
this specification by reporting bugs, asking smart questions,
drafting or reviewing text, and evaluating open issues: Mike Bishop,
Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean
Turner, and Erik Wilde.

FAQ

_RFC Editor: Please remove this section before publication._

1. Why remove all references to content-md5?

Those were unnecessary to understanding and using this
specification.

2. Why remove references to instance manipulation?

Those were unnecessary for correctly using and applying the
specification. An example with Range Request is more than
enough. This document uses the term "partial representation"
which should group all those cases.

3. How to use "Digest" Digest with "PATCH" PATCH method?

See Section 6. 7.

4. Why remove references to delta-encoding?

Unnecessary for a correct implementation of this specification.
The revised specification can be nicely adapted to "delta
encoding", but all the references here to delta encoding don't
add anything to this RFC. Another job would be to refresh delta
encoding.

5. Why remove references to Digest Authentication?

This specification seems to me completely unrelated to Digest
Authentication but for the word "Digest".

6. What changes in "Want-Digest"? Want-Digest?

The contentMD5 token defined in Section 5 of [RFC3230] is
deprecated by Section 7. Appendix D.1.

To clarify that "Digest" Digest and "Want-Digest" Want-Digest can be used in both
requests and responses - [RFC3230] carefully uses "sender" sender and
"receiver"
receiver in their definition - we added examples on using
"Want-Digest" Want-
Digest in responses to advertise the supported digest-
algorithms digest-algorithms
and the inability to accept requests with unsupported
digest-algorithms. digest-
algorithms.

7. Does this specification change supported algorithms?

Yes. This RFC updates [RFC5843] which is still delegated for all
algorithms updates, and adds two more algorithms: "id-sha-256"
and "id-sha-512" which allows to send a checksum of a resource
representation with no content codings applied. To simplify a
future transition to Structured Fields
[I-D.ietf-httpbis-header-structure] we suggest to use lowercase
for digest-algorithms.

8. What about mid-stream trailer fields?
While mid-stream trailer fields (https://github.com/httpwg/http-
core/issues/313#issuecomment-584389706) are interesting, since
this specification is a rewrite of [RFC3230] we do not think we
should face that. As a first thought, nothing in this document
precludes future work that would find a use for mid-stream
trailers, for example an incremental digest-algorithm. A
document defining such a digest-algorithm is best positioned to
describe how it is used.

Acknowledgements

Code Samples

_RFC Editor: Please remove this section before publication._

How can I generate and validate the "Digest" Digest values shown in the
examples throughout this document?

The following python3 code can be used to generate digests for JSON
objects using SHA algorithms for a range of encodings. Note that
these are formatted as base64. This function could be adapted to
other algorithms and should take into account their specific
formatting rules.

import base64, json, hashlib, brotli, logging
log = logging.getLogger()

def encode_item(item, encoding=lambda x: x):
indent = 2 if isinstance(item, dict) and len(item) > 1 else None
json_bytes = json.dumps(item, indent=indent).encode()
return encoding(json_bytes)

def digest_bytes(bytes_, algorithm=hashlib.sha256):
checksum_bytes = algorithm(bytes_).digest()
log.warning("Log bytes: \n[%r]", bytes_)
return base64.encodebytes(checksum_bytes).strip()

def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256):
content_encoded = encode_item(item, encoding)
return digest_bytes(content_encoded, algorithm)

item = {"hello": "world"}

Changes

_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

* Improve SRI section #1354

* About duplicate digest-algorithms #1221

* Improve security considerations #852

* md5 and sha deprecation references #1392

* Obsolete 3230 #1395

* Editorial #1362

Since draft-ietf-httpbis-digest-headers-03

* Reference semantics-12

* Detail encryption quirks

* Details on Algorithm agility #1250

* Obsolete parameters #850

Since draft-ietf-httpbis-digest-headers-02

* Deprecate SHA-1 #1154

* Avoid id-* with encrypted content

* Digest is independent from MESSAGING and HTTP/1.1 is not normative
#1215

* Identity is not a valid field value for content-encoding #1223

* Mention trailers #1157

* Reference httpbis-semantics #1156

* Add contentMD5 as an obsoleted digest-algorithm #1249

* Use lowercase digest-algorithms names in the doc and in the
digest-algorithm IANA table.

Since draft-ietf-httpbis-digest-headers-01
* Digest of error responses is computed on the error representation-
data #1004

* Effect of HTTP semantics on payload and message body moved to
appendix #1122

* Editorial refactoring, moving headers sections up. #1109-#1112,
#1116, #1117, #1122-#1124

Since draft-ietf-httpbis-digest-headers-00

* Align title with document name

* Add id-sha-* algorithm examples #880

* Reference [RFC6234] and [RFC3174] instead of FIPS-1

* Deprecate MD5

* Obsolete ADLER-32 but don't forbid it #828

* Update CRC32C value in IANA table #828

* Use when acting on resources (POST, PATCH) #853

* Added Relationship with SRI, draft Use Cases #868, #971

* Warn about the implications of "Content-Location" Content-Location

Authors' Addresses

Roberto Polli
Team Digitale, Italian Government
Italy

Email: robipolli@gmail.com

Lucas Pardue
Cloudflare

Email: lucaspardue.24.7@gmail.com