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

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

stand_alone: yes 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

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 at rest, be used across multiple hops in order to provide end-to-end integrity guarantees, 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 7 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 13.2 of {{SEMANTICS}}).

Changes are semantically compatible with existing implementations and better cover both the request and response cases.

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 7.5.1 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: : The digest mechanism described here does not cover the full HTTP message nor its semantic, as representation metadata are not included in the checksum.

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

Authentication: : The digest mechanisms described here are not meant to support authentication of the source of a digest or of a 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: : The digest mechanisms described here are not meant to support authorization or other kinds of access controls.

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] and [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 5.7.1 of {{SEMANTICS}}.

The definitions "representation", "selected representation", "representation data", "representation metadata", and "payload body" 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 payload body (see Section 5.5.4 of {{SEMANTICS}}). It uses the representation data (see Section 7.2 of {{SEMANTICS}}), that can be fully or partially contained in the payload body, 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 payload body can be affected by Range Requests or methods such as HEAD, while the way the payload body is transferred "on the wire" is dependent on other transformations (eg. transfer codings for HTTP/1.1 see 6.1 of {{?HTTP11=I-D.ietf-httpbis-messaging}}): {{resource-representation}} contains several examples to help illustrate those effects.

A representation digest consists of the value of a checksum computed on the entire selected representation data (see Section 7 of {{SEMANTICS}}) of a resource identified according to Section 5.5.2 of {{SEMANTICS}} together with an indication of the algorithm used (and any parameters)

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

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 which uses base64 encoding.

   sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

The Digest Field {#digest}

The Digest field contains a list of one or more representation digest values as defined in {{representation-digest}}. It can be used in both request and response.

   Digest = "Digest" ":" OWS 1#representation-data-digest

The relationship between Content-Location (see Section 7.8 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. This could be useful for responses expected to reside in caches shared by users with different browsers, for example.

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 using incremental digest-algorithms this allows the sender and the receiver to dynamically compute the digest value while streaming the content.

Two examples of its use are

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

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

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.

   Want-Digest = "Want-Digest" ":" OWS 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 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. For some digest-algorithms, one or more parameters can be supplied.

   digest-algorithm = token

The BNF for "parameter" is defined in Section 5.7.6 of {{SEMANTICS}}. All digest-algorithm values are case-insensitive but the lower case is preferred.

The Internet Assigned Numbers Authority (IANA) acts as a registry for digest-algorithm values. The registry contains the tokens listed below.

Some digest-algorithms, although registered, rely on vulnerable algorithms: the "md5" digest-algorithm MUST NOT be used due to collision attacks [CMU-836068] and the "sha" digest-algorithm MUST NOT be used due to collision attacks [IACR-2020-014].

{: 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 MUST NOT be used as it's now vulnerable to collision attacks [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 MUST NOT be used as it's now vulnerable to collision attacks [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 {{?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 the POST one, 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.

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.

The SRI digest value is calculated over the identity encoding of the resource, not the selected representation (as specified for representation-data-digest in this document). Section 3.4.5 of [SRI] describes the benefit of the identity approach - the SRI integrity attribute can contain multiple algorithm-value pairs where each applies to a different identity encoded payload. This allows for protection of distinct resources sharing a URL. However, this is a contrast to the design of representation digests, where multiple Digest field-values all protect the same representation.

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.

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.

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.

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

Request:

GET /items/123

Response:

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

{"hello": "world"}

Server Returns No Representation Data

Requests without a payload body can still send a Digest field applying the digest-algorithm to an empty representation.

As there is no content coding applied, the "sha-256" and the "id-sha-256" digest-values in the response are the same.

Request:

HEAD /items/123 HTTP/1.1
Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

Response:

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

Server Returns Partial Representation Data

Request:

GET /items/123
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 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.

The response body is displayed as a base64-encoded string because it contains non-printable characters.

Request:

PUT /items/123
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

{"hello": "world"}

Response:

Content-Type: application/json
Content-Encoding: br
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

Client Provides Full Representation Data, Server Provides No Representation Data

Request Digest value is calculated on the enclosed payload. Response Digest value depends on the representation metadata header fields, including Content-Encoding: br even when the response does not contain a payload body.

Request:

PUT /items/123
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
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
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
        id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

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

Request Digest 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 5.5.2).

Digest is thus computed on the enclosed representation.

Request:

POST /books HTTP/1.1
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
Digest: id-sha-256=BZlF2v0IzjuxN01RQ97EUXriaNNLhtI8Chx8Eq+XYSc=
Content-Location: /books/123

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

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

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

Request Digest 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
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=0o/WKwSfnmIoSlop2LV/ISaBDth05IeW27zzNMUh5l8=
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 is computed on the complete representation of the patched resource.

Request:

PATCH /books/123 HTTP/1.1
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=BZlF2v0IzjuxN01RQ97EUXriaNNLhtI8Chx8Eq+XYSc=

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

Note that a 204 No Content response without a payload body 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 digest of the response is computed on this enclosed representation.

Request:

PATCH /books/123 HTTP/1.1
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=UJSojgEzqUe4UoHzmNl5d2xkmrW3BOdmvsvWu1uFeu0=

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

Use with trailers and transfer coding

An origin server sends Digest in the HTTP trailer, so it can calculate digest-value while streaming content and thus mitigate resource consumption. The 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

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.

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
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
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 for sha-256 and sha-512

Request:

GET /items/123
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 as 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

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

The ADLER32 algorithm defined in [RFC1950] has been deprecated by [RFC3309] because under certain conditions it provides weak detection of errors and is now NOT RECOMMENDED for use with Digest.

Digest for End-to-End Integrity

Digest alone does not provide end-to-end integrity of HTTP messages over multiple hops, as it just covers the representation data and not the representation metadata.

Besides, it allows to protect representation data from buggy manipulation, buggy compression, etc.

Moreover identity digest-algorithms (eg. "id-sha-256" and "id-sha-512") allow piecing together a resource from different sources (e.g. different servers that perhaps apply different content codings) enabling the user-agent to detect that the application-layer tasks completed properly, before handing off to say the HTML parser, video player etc.

Even a simple mechanism for end-to-end validation is thus valuable.

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 which 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.

Usage in trailers

When used in trailers, the receiver gets the digest value after the payload body and may thus be tempted to process the data before validating the digest value. Instead, data should 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 5.6.2 of {{SEMANTICS}}).

Not every digest-algorithm is suitable for trailers, as they 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 information details of encrypted payload when the checksum is computed on the unencrypted data. An example of that is the use of the "id-sha-256" digest-algorithm in conjuction with the encrypted content-coding {{?RFC8188}}.

Algorithm Agility

...

IANA Considerations

Establish the HTTP Digest Algorithm Values

This memo sets this spec to be the establishing document for the HTTP Digest Algorithm Values

The "status" Field in the HTTP Digest Algorithm Values

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

Status : Specify "standard", "experimental", "historic", "obsoleted", or "deprecated" according to the type and status of the primary document in which the algorithm is defined.

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.

Obsolete "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 Header 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 payload body. When the payload body 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
Content-Type: application/json

{"hello": "world"}

Here is a gzip-compressed json object using a content coding.

Request:

PUT /entries/1234 HTTP/1.1
Content-Type: application/json
Content-Encoding: gzip

H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

Now the same payload body conveys a malformed json object.

Request:

PUT /entries/1234 HTTP/1.1
Content-Type: application/json

H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

A Range-Request alters the payload body, conveying a partial representation.

Request:

GET /entries/1234 HTTP/1.1
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 payload body.

Request:

HEAD /entries/1234 HTTP/1.1
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
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 spec.
Why remove references to instance manipulation?

Those were unnecessary for correctly using and applying the spec. An example with Range Request is more than enough. This doc 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 spec. The revised spec 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 RFC 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 spec changes supported algorithms?

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 trailers?

While mid-stream trailers 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


def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256):
    json_bytes = json.dumps(item).encode()
    content_encoded = encoding(json_bytes)
    checksum_bytes = algorithm(content_encoded).digest()
    return base64.encodebytes(checksum_bytes).strip()


item = {"hello": "world"}

print("Encoding | digest-algorithm | digest-value")
print("Identity | sha256 |", digest(item))
# Encoding | digest-algorithm | digest-value
# Identity | sha256 | 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=

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/7XgHaAy8pmojAkGWoRx2UFChF41A2s
vX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==\n'

Changes

{:numbered="false"}

RFC Editor: Please remove this section before publication.

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

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-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.

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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}

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 trailers 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

Other 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 trailers

Usage with encryption

Algorithm Agility

IANA Considerations

Establish the HTTP Digest Algorithm Values

The "status" Field in the HTTP Digest Algorithm Values

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

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

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

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

Obsolete "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 Header Field Registration

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

FAQ

Acknowledgements

Code Samples

Changes

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

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

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