title: Digest Headers abbrev: docname: draft-ietf-httpbis-digest-headers-latest category: std obsoletes: 3230

ipr: trust200902 area: Applications and Real-Time workgroup: HTTP keyword: Digest

stand_alone: yes smart_quotes: no pi: [toc, tocindent, sortrefs, symrefs, strict, compact, comments, inline, docmapping]

author:

ins: R. Polli
name: Roberto Polli
org: Team Digitale, Italian Government
email: robipolli@gmail.com
country: Italy

ins: L. Pardue name: Lucas Pardue org: Cloudflare email: lucaspardue.24.7@gmail.com

normative: RFC1321: RFC3174: RFC1950: RFC3230: RFC3309: RFC2119: RFC5843: RFC4648: RFC5234: RFC6234: RFC7405: RFC8174: UNIX: title: The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98 author: org: The Open Group date: 1997-02 NIST800-32: title: Introduction to Public Key Technology and the Federal PKI Infrastructure author: org: National Institute of Standards and Technology, U.S. Department of Commerce date: 2001-02 target: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-32.pdf CMU-836068: title: MD5 Vulnerable to collision attacks author: org: Carnagie Mellon University, Software Engineering Institute date: 2008-12-31 target: https://www.kb.cert.org/vuls/id/836068/ IACR-2020-014: title: SHA-1 is a Shambles author: - ins: G. Leurent org: Inria, France - ins: T. Peyrin org: Nanyang Technological University, Singapore; Temasek Laboratories, Singapore date: 2020-01-05 target: https://eprint.iacr.org/2020/014.pdf

informative: RFC2818: RFC7231: RFC7396: SRI: title: "Subresource Integrity" date: 2016-06-23 author: - ins: D. Akhawe - ins: F. Braun - ins: F. Marier - ins: J. Weinberger seriesinfo: W3C Recommendation: REC-SRI-20160623 target: https://www.w3.org/TR/2016/REC-SRI-20160623/

--- abstract

This document defines the HTTP Digest and Want-Digest fields, thus allowing client and server to negotiate an integrity checksum of the exchanged resource representation data.

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_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/.

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

--- middle

Introduction

The core specification of HTTP does not define a means to protect the integrity of resources. 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 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-data. It can be combined with other mechanisms that protect representation-metadata, such as digital signatures, in order to protect the desired parts of an HTTP exchange in whole or in part.

A Brief History of HTTP Integrity Fields {#history}

The Content-MD5 header field was originally introduced to provide integrity, but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it:

The Content-MD5 header 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", and the fields Digest and Want-Digest.

This Proposal

The concept of selected representation defined in Section 3.2 of {{!SEMANTICS=I-D.ietf-httpbis-semantics}} makes [RFC3230] definitions inconsistent with current 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 straightforward to apply it to use-cases where the transferred data 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 to provide more detailed description of Digest usage in request and response cases. Changes are intended to be semantically compatible with existing implementations but note that negotiation of Content-MD5 is deprecated {{deprecate-contentMD5}}, Digest field parameters are obsoleted {{obsolete-parameters}}, "md5" and "sha" digest-algorithms are obsoleted {{broken-algorithms}} and the "adler32" algorithm is deprecated {{deprecated-algorithms}}.

The value of Digest is calculated on selected representation, which 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 no content codings (see Section 8.4.1 of {{SEMANTICS}}) two more digest-algorithms are added ("id-sha-256" and "id-sha-512").

Goals

The goals of this proposal are:

Digest coverage for either the resource's representation data or selected representation data communicated via HTTP.
Support for multiple digest-algorithms.
Negotiation of the use of digests.

The goals do not include:

HTTP message integrity: : Digest mechanisms do not cover the full HTTP message nor its semantic, as representation metadata is not included in the checksum.

HTTP field integrity: : Digest mechanisms cover only representation and selected representation data, and do not protect the integrity of associated representation metadata or other message fields.

Authentication: : Digest mechanisms do not support authentication of 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.

Authorization: : Digest mechanisms do not support authorization or other kinds of access controls.

Notational Conventions

{::boilerplate bcp14}

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}}.

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. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg. "sha", "crc32c").

Representation Digest {#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 ) )

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=I-D.ietf-httpbis-messaging}}). To help illustrate how such things affect Digest, several examples are provided in {{resource-representation}}.

A representation digest consists of the value of a checksum computed on the entire selected 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 {{usage-in-signatures}}).

The checksum is computed using one of the digest-algorithms listed in {{algorithms}} and then encoded in the associated format.

The example below shows the "sha-256" digest-algorithm that uses base64 encoding.

   sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

The Digest Field {#digest}

The Digest field contains a comma-separated list of one or more representation digest values as defined in {{representation-digest}}. 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 {{post-not-request-uri}}. A comprehensive set of examples showing the impacts of representation metadata, payload transformations and HTTP methods on Digest is provided in {{examples-unsolicited}} and {{examples-solicited}}.

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 {{algorithm-agility}}).

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 can be sent in a trailer section. 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 {#want-digest}

The Want-Digest field indicates the sender's desire to receive a representation digest on messages associated with the request URI and representation metadata. It can be used in both requests and responses.

   Want-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" (see Section 12.4.2 of{{SEMANTICS}}), it is treated as if its associated "qvalue" were 1.0.

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 sender's preferred digest-algorithm is the one (or ones) with the highest "qvalue".

Two examples of its use are:

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

Digest Algorithm Values {#algorithms}

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; digest-algorithm token values MUST be compared in a case-insensitive fashion.

The Internet Assigned Numbers Authority (IANA) maintains a registry for digest-algorithm values. The registry is initialized with the tokens listed below.

Deprecated digest algorithms MUST NOT be used:

"md5", see [CMU-836068] and {{?NO-MD5=RFC6151}};
"sha", see [IACR-2020-014] and {{?NO-SHA1=RFC6194}}.

See the references above for further information.

{: vspace="0"} sha-256 : * Description: The SHA-256 algorithm [RFC6234]. The output of this algorithm is encoded using the base64 encoding [RFC4648]. * Reference: [RFC6234], [RFC4648], this document. * Status: standard

sha-512 : * Description: The SHA-512 algorithm [RFC6234]. The output of this algorithm is encoded using the base64 encoding [RFC4648]. * Reference: [RFC6234], [RFC4648], this document. * Status: standard

md5 : * Description: The MD5 algorithm, as specified in [RFC1321]. The output of this algorithm is encoded using the base64 encoding [RFC4648]. This digest-algorithm is now vulnerable to collision attacks. See {{NO-MD5}} and [CMU-836068]. * Reference: [RFC1321], [RFC4648], this document. * Status: deprecated

sha : * Description: The SHA-1 algorithm [RFC3174]. The output of this algorithm is encoded using the base64 encoding [RFC4648]. This digest-algorithm is now vulnerable to collision attacks. See {{NO-SHA1}} and [IACR-2020-014]. * Reference: [RFC3174], [RFC6234], [RFC4648], this document. * Status: deprecated

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

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

To allow sender and recipient to provide a checksum which is independent from Content-Encoding, the following additional digest-algorithms are defined:

{: vspace="0"} id-sha-512 : * Description: The sha-512 digest of the 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 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 "," in the character sets used for the encoding.

Use of Digest when acting on resources {#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 representation-data of that action. This is the only possible choice because representation digest requires complete representation metadata (see {{representation-digest}}).

In responses,

if the representation describes the status of the request, Digest MUST be computed on the enclosed representation (see {{post-referencing-action}} );
if there is a referenced resource 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 on the enclosed representation.

The latter case might be done according to the HTTP semantics of the given method, for example using the Content-Location header field. In contrast, the Location header field does not affect Digest because it is not representation metadata.

Digest and PATCH

In PATCH requests, the representation digest MUST 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=RFC5789}}).

In PATCH responses, the representation digest MUST 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.

Deprecate Negotiation of Content-MD5 {#deprecate-contentMD5}

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.

Obsolete Digest Field Parameters {#obsolete-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.

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 in {{security-considerations}}. 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 and digest value which is similar to representation-data-digest (see {{representation-digest}}). 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 {{server-returns-partial-representation-data}}).

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 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.

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.

A user agent supporting both mechanisms:

can legitimately ignore Digest when using SRI, because {{digest}} specifies that "a recipient MAY ignore any or all of the representation-data-digests";
enforce both Digest and SRI: in this case it can be useful to provide enough information to identify whether the mismatch happened at the Digest or the SRI level.

Examples of Unsolicited Digest {#examples-unsolicited}

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.

Server Returns Full Representation Data {#example-full-representation}

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"}

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=

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"

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==

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.

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=

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==

POST Response does not Reference the Request URI {#post-not-request-uri}

The request Digest field-value is computed on the enclosed representation (see {{acting-on-resources}}).

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.

POST Response Describes the Request Status {#post-referencing-action}

The request Digest field-value is computed on the enclosed representation (see {{acting-on-resources}}).

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"
}

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.

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 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.

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
}

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 {{example-full-representation}} because Digest is designed to be independent from the use of one or more transfer codings (see {{representation-digest}}).

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=

Examples of Want-Digest Solicited Digest {#examples-solicited}

The following examples demonstrate interactions where a client solicits a Digest 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.

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"}

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"}

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

Security Considerations

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.

Broken Cryptographic Algorithms {#broken-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.

Other Deprecated Algorithms {#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.

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 {{usage-in-signatures}}.

Digest and Content-Location in Responses {#digest-and-content-location}

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.

Usage in Signatures {#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 {{digest-and-content-location}}.

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.

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}}).

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.

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 {{iana-digest-algorithm-registry}}.

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 {{iana-digest-algorithm-status}}.

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 {{want-digest}}) 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 {{digest}}).

Duplicate digest-algorithm in field value

An endpoint might receive multiple representation-data-digest values (see {{digest}}) 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.

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.

IANA Considerations

Establish the HTTP Digest Algorithm Values Registry {#iana-digest-algorithm-registry}

This memo sets this specification to be the establishing document for the HTTP Digest Algorithm Values registry.

The "status" Field in the HTTP Digest Algorithm Values Registry {#iana-digest-algorithm-status}

This memo adds the field "Status" to the HTTP Digest Algorithm Values registry. The allowed values for the "Status" fields are described below.

{: vspace="0"} 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.

Deprecate "MD5" Digest Algorithm {#iana-md5}

This memo updates the "MD5" digest-algorithm in the HTTP Digest Algorithm Values registry:

Digest Algorithm: md5
Description: As specified in {{algorithms}}.
Status: As specified in {{algorithms}}.

Update "UNIXsum" Digest Algorithm {#iana-unixsum}

This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest Algorithm Values registry:

Digest Algorithm: As specified in {{algorithms}}.
Description: As specified in {{algorithms}}.
Status: As specified in {{algorithms}}.

Update "UNIXcksum" Digest Algorithm {#iana-unixcksum}

This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest Algorithm Values registry:

Digest Algorithm: As specified in {{algorithms}}.
Description: As specified in {{algorithms}}.
Status: As specified in {{algorithms}}.

Update "CRC32c" Digest Algorithm {#iana-crc32c}

This memo updates the "CRC32c" digest-algorithm in the HTTP Digest Algorithm Values 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.

Deprecate "SHA" Digest Algorithm {#iana-sha}

This memo updates the "SHA" digest-algorithm in the HTTP Digest Algorithm Values registry:

Digest Algorithm: sha
Description: As specified in {{algorithms}}.
Status: As specified in {{algorithms}}.

Obsolete "ADLER32" Digest Algorithm {#iana-adler-32}

This memo updates the "ADLER32" digest-algorithm in the HTTP Digest Algorithm Values 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

Obsolete "contentMD5" token in Digest Algorithm {#iana-contentMD5}

This memo adds the "contentMD5" token in the HTTP Digest Algorithm Values 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: {{iana-contentMD5}} of this document, Section 5 of [RFC3230].
Status: obsoleted

The "id-sha-256" Digest Algorithm {#iana-id-sha-256}

This memo registers the "id-sha-256" digest-algorithm in the HTTP Digest Algorithm Values registry:

Digest Algorithm: id-sha-256
Description: As specified in {{algorithms}}.
Status: As specified in {{algorithms}}.

The "id-sha-512" Digest Algorithm {#iana-id-sha-512}

This memo registers the "id-sha-512" digest-algorithm in the HTTP Digest Algorithm Values registry:

Digest Algorithm: id-sha-512
Description: As specified in {{algorithms}}.
Status: As specified in {{algorithms}}.

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.

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): {{want-digest}} of this document

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): {{digest}} of this document

--- back

Resource Representation and Representation-Data {#resource-representation}

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:

HEAD /entries/1234 HTTP/1.1
Host: foo.example
Accept: application/json
Accept-Encoding: gzip

Response:

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

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.

Request:

POST /authors/ HTTP/1.1
Host: foo.example
Accept: application/json
Content-Type: application/json

{"author": "Camilleri"}

Response:

HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /authors/123
Location: /authors/123

{"id": "123", "author": "Camilleri"}

FAQ

Why remove all references to content-md5?

Those were unnecessary to understanding and using this specification.
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.
How to use Digest with PATCH method?

See {{acting-on-resources}}.
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.
Why remove references to Digest Authentication?

This specification seems to me completely unrelated to Digest Authentication but for the word "Digest".
What changes in Want-Digest?

The contentMD5 token defined in Section 5 of [RFC3230] is deprecated by {{deprecate-contentMD5}}.

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

While mid-stream trailer fields 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

{:numbered="false"} 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

{:numbered="false"}

RFC Editor: Please remove this section before publication.

How can I generate and validate the 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"}

print("Encoding | digest-algorithm | digest-value")
print("Identity | sha256 |", digest(item))
# Encoding | digest-algorithm | digest-value
# Identity | sha256 | X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

print("Encoding | digest-algorithm | digest-value")
print("Brotli | sha256 |", digest(item, encoding=brotli.compress))
# Encoding | digest-algorithm | digest-value
# Brotli | sha256 | 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

print("Encoding | digest-algorithm | digest-value")
print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512))
# Encoding | digest-algorithm | digest-value
# Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
#                      '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='

Changes

{:numbered="false"}

RFC Editor: Please remove this section before publication.

Since draft-ietf-httpbis-digest-headers-04

{:numbered="false"}

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

{:numbered="false"}

Reference semantics-12
Detail encryption quirks
Details on Algorithm agility #1250
Obsolete parameters #850

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

{:numbered="false"}

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

{:numbered="false"}

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

{:numbered="false"}

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

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Introduction

A Brief History of HTTP Integrity Fields {#history}

This Proposal

Goals

Notational Conventions

Representation Digest {#representation-digest}

The Digest Field {#digest}

The Want-Digest Field {#want-digest}

Digest Algorithm Values {#algorithms}

Use of Digest when acting on resources {#acting-on-resources}

Digest and PATCH

Deprecate Negotiation of Content-MD5 {#deprecate-contentMD5}

Obsolete Digest Field Parameters {#obsolete-parameters}

Relationship to Subresource Integrity (SRI)

Supporting Both SRI and Representation Digest

Examples of Unsolicited Digest {#examples-unsolicited}

Server Returns Full Representation Data {#example-full-representation}

Server Returns No Representation Data

Server Returns Partial Representation Data

Client and Server Provide Full Representation Data

Client Provides Full Representation Data, Server Provides No Representation Data

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

POST Response does not Reference the Request URI {#post-not-request-uri}

POST Response Describes the Request Status {#post-referencing-action}

Digest with PATCH

Error responses

Use with Trailer Fields and Transfer Coding

Examples of Want-Digest Solicited Digest {#examples-solicited}

Server Selects Client's Least Preferred Algorithm

Server Selects Algorithm Unsupported by Client

Server Does Not Support Client Algorithm and Returns an Error

Security Considerations

Digest Does Not Protect the Full HTTP Message

Broken Cryptographic Algorithms {#broken-algorithms}

Other Deprecated Algorithms {#deprecated-algorithms}

Digest for End-to-End Integrity

Digest and Content-Location in Responses {#digest-and-content-location}

Usage in Signatures {#usage-in-signatures}

Usage in Trailer Fields

Usage with Encryption

Algorithm Agility

Duplicate digest-algorithm in field value

Resource exhaustion

IANA Considerations

Establish the HTTP Digest Algorithm Values Registry {#iana-digest-algorithm-registry}

The "status" Field in the HTTP Digest Algorithm Values Registry {#iana-digest-algorithm-status}

Deprecate "MD5" Digest Algorithm {#iana-md5}

Update "UNIXsum" Digest Algorithm {#iana-unixsum}

Update "UNIXcksum" Digest Algorithm {#iana-unixcksum}

Update "CRC32c" Digest Algorithm {#iana-crc32c}

Deprecate "SHA" Digest Algorithm {#iana-sha}

Obsolete "ADLER32" Digest Algorithm {#iana-adler-32}

Obsolete "contentMD5" token in Digest Algorithm {#iana-contentMD5}

The "id-sha-256" Digest Algorithm {#iana-id-sha-256}

The "id-sha-512" Digest Algorithm {#iana-id-sha-512}

Changes Compared to RFC5843

Want-Digest Field Registration

Digest Field Registration

Resource Representation and Representation-Data {#resource-representation}

FAQ

Acknowledgements

Code Samples

Changes

Since draft-ietf-httpbis-digest-headers-04

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

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

Since draft-ietf-httpbis-digest-headers-01

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