Digest Headers
draft-ietf-httpbis-digest-headers-05
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9530.
|
|
|---|---|---|---|
| Authors | Roberto Polli , Lucas Pardue | ||
| Last updated | 2021-04-13 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Associated WG milestone |
|
||
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9530 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-httpbis-digest-headers-05
HTTP R. Polli
Internet-Draft Team Digitale, Italian Government
Obsoletes: 3230 (if approved) L. Pardue
Intended status: Standards Track Cloudflare
Expires: 15 October 2021 13 April 2021
Digest Headers
draft-ietf-httpbis-digest-headers-05
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 to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/
(https://lists.w3.org/Archives/Public/ietf-http-wg/).
The source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions (https://github.com/httpwg/
http-extensions).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Polli & Pardue Expires 15 October 2021 [Page 1]
Internet-Draft Digest Headers April 2021
This Internet-Draft will expire on 15 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. A Brief History of HTTP Integrity Fields . . . . . . . . 4
1.2. This Proposal . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Notational Conventions . . . . . . . . . . . . . . . . . 6
2. Representation Digest . . . . . . . . . . . . . . . . . . . . 6
3. The Digest Field . . . . . . . . . . . . . . . . . . . . . . 7
4. The Want-Digest Field . . . . . . . . . . . . . . . . . . . . 8
5. Digest Algorithm Values . . . . . . . . . . . . . . . . . . . 8
6. Use of Digest when acting on resources . . . . . . . . . . . 11
6.1. Digest and PATCH . . . . . . . . . . . . . . . . . . . . 11
7. Deprecate Negotiation of Content-MD5 . . . . . . . . . . . . 11
8. Obsolete Digest Field Parameters . . . . . . . . . . . . . . 12
9. Relationship to Subresource Integrity (SRI) . . . . . . . . . 12
9.1. Supporting Both SRI and Representation Digest . . . . . . 13
10. Examples of Unsolicited Digest . . . . . . . . . . . . . . . 13
10.1. Server Returns Full Representation Data . . . . . . . . 13
10.2. Server Returns No Representation Data . . . . . . . . . 14
10.3. Server Returns Partial Representation Data . . . . . . . 14
10.4. Client and Server Provide Full Representation Data . . . 15
10.5. Client Provides Full Representation Data, Server Provides
No Representation Data . . . . . . . . . . . . . . . . 15
10.6. Client and Server Provide Full Representation Data, Client
Uses id-sha-256. . . . . . . . . . . . . . . . . . . . 16
10.7. POST Response does not Reference the Request URI . . . . 17
10.8. POST Response Describes the Request Status . . . . . . . 18
10.9. Digest with PATCH . . . . . . . . . . . . . . . . . . . 18
10.10. Error responses . . . . . . . . . . . . . . . . . . . . 19
10.11. Use with Trailer Fields and Transfer Coding . . . . . . 20
11. Examples of Want-Digest Solicited Digest . . . . . . . . . . 21
Polli & Pardue Expires 15 October 2021 [Page 2]
Internet-Draft Digest Headers April 2021
11.1. Server Selects Client's Least Preferred Algorithm . . . 21
11.2. Server Selects Algorithm Unsupported by Client . . . . . 22
11.3. Server Does Not Support Client Algorithm and Returns an
Error . . . . . . . . . . . . . . . . . . . . . . . . . 22
12. Security Considerations . . . . . . . . . . . . . . . . . . . 22
12.1. Digest Does Not Protect the Full HTTP Message . . . . . 22
12.2. Broken Cryptographic Algorithms . . . . . . . . . . . . 23
12.3. Other Deprecated Algorithms . . . . . . . . . . . . . . 23
12.4. Digest for End-to-End Integrity . . . . . . . . . . . . 23
12.5. Digest and Content-Location in Responses . . . . . . . . 23
12.6. Usage in Signatures . . . . . . . . . . . . . . . . . . 24
12.7. Usage in Trailer Fields . . . . . . . . . . . . . . . . 24
12.8. Usage with Encryption . . . . . . . . . . . . . . . . . 25
12.9. Algorithm Agility . . . . . . . . . . . . . . . . . . . 25
12.9.1. Duplicate digest-algorithm in field value . . . . . 25
12.10. Resource exhaustion . . . . . . . . . . . . . . . . . . 26
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
13.1. Establish the HTTP Digest Algorithm Values Registry . . 26
13.2. The "status" Field in the HTTP Digest Algorithm Values
Registry . . . . . . . . . . . . . . . . . . . . . . . 26
13.3. Deprecate "MD5" Digest Algorithm . . . . . . . . . . . . 26
13.4. Update "UNIXsum" Digest Algorithm . . . . . . . . . . . 26
13.5. Update "UNIXcksum" Digest Algorithm . . . . . . . . . . 27
13.6. Update "CRC32c" Digest Algorithm . . . . . . . . . . . . 27
13.7. Deprecate "SHA" Digest Algorithm . . . . . . . . . . . . 27
13.8. Obsolete "ADLER32" Digest Algorithm . . . . . . . . . . 28
13.9. Obsolete "contentMD5" token in Digest Algorithm . . . . 28
13.10. The "id-sha-256" Digest Algorithm . . . . . . . . . . . 28
13.11. The "id-sha-512" Digest Algorithm . . . . . . . . . . . 29
13.12. Changes Compared to RFC5843 . . . . . . . . . . . . . . 29
13.13. Want-Digest Field Registration . . . . . . . . . . . . . 29
13.14. Digest Field Registration . . . . . . . . . . . . . . . 29
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
14.1. Normative References . . . . . . . . . . . . . . . . . . 30
14.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Resource Representation and Representation-Data . . 33
Appendix B. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 35
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 36
Code Samples . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Since draft-ietf-httpbis-digest-headers-04 . . . . . . . . . . 38
Since draft-ietf-httpbis-digest-headers-03 . . . . . . . . . . 38
Since draft-ietf-httpbis-digest-headers-02 . . . . . . . . . . 38
Since draft-ietf-httpbis-digest-headers-01 . . . . . . . . . . 38
Since draft-ietf-httpbis-digest-headers-00 . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
Polli & Pardue Expires 15 October 2021 [Page 3]
Internet-Draft Digest Headers April 2021
1. 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.
1.1. A Brief History of HTTP Integrity Fields
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".
1.2. This Proposal
The concept of "selected representation" defined in Section 3.2 of
[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]).
Polli & Pardue Expires 15 October 2021 [Page 4]
Internet-Draft Digest Headers April 2021
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 Section 7, "Digest" field parameters are obsoleted
Section 8, "md5" and "sha" digest-algorithms are obsoleted
Section 12.2 and the "adler32" algorithm is deprecated Section 12.3.
The value of "Digest" is calculated on selected representation, which
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").
1.3. Goals
The goals of this proposal are:
1. Digest coverage for either the resource's "representation data"
or "selected representation data" communicated via HTTP.
2. Support for multiple digest-algorithms.
3. Negotiation of the use of digests.
The goals 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.
Polli & Pardue Expires 15 October 2021 [Page 5]
Internet-Draft Digest Headers April 2021
1.4. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405] along with the "#rule" extension defined in
Section 5.6.1 of [SEMANTICS].
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").
2. Representation Digest
The representation digest is an integrity mechanism for HTTP
resources which uses a checksum that is calculated independently of
the content (see Section 6.4 of [SEMANTICS]). It uses the
representation data (see Section 8.1 of [SEMANTICS]), that can be
fully or partially contained in the content, or not contained at all:
representation-data := Content-Encoding( Content-Type( bits ) )
This takes into account the effect of the HTTP semantics on the
messages; for example, the content can be affected by Range Requests
or methods such as HEAD, while the way the content is transferred "on
the wire" is dependent on other transformations (e.g. transfer
codings for HTTP/1.1 - see Section 6.1 of [HTTP11]). To help
illustrate how such things affect "Digest", several examples are
provided in Appendix A.
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>
Polli & Pardue Expires 15 October 2021 [Page 6]
Internet-Draft Digest Headers April 2021
When a message has no representation data it is still possible to
assert that no representation data was sent computing the
representation digest on an empty string (see Section 12.6).
The checksum is computed using one of the digest-algorithms listed in
Section 5 and then encoded in the associated format.
The example below shows the "sha-256" digest-algorithm that uses
base64 encoding.
sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
3. The Digest Field
The "Digest" field contains a list of one or more representation
digest values as defined in Section 2. It can be used in both
requests and responses.
Digest = 1#representation-data-digest
For example:
Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
The relationship between "Content-Location" (see Section 8.7 of
[SEMANTICS]) and "Digest" is demonstrated in Section 10.7. A
comprehensive set of examples showing the impacts of representation
metadata, payload transformations and HTTP methods on Digest is
provided in Section 10 and Section 11.
A "Digest" field MAY contain multiple representation-data-digest
values. For example, a server may provide representation-data-digest
values using different algorithms, allowing it to support a
population of clients with different evolving capabilities; this is
particularly useful in support of transitioning away from weaker
algorithms should the need arise (see Section 12.9).
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.
Polli & Pardue Expires 15 October 2021 [Page 7]
Internet-Draft Digest Headers April 2021
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-algorithms is used, the sender and the receiver can
dynamically compute the digest value while streaming the content.
4. The Want-Digest Field
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 = 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
5. Digest Algorithm Values
Digest-algorithm values are used to indicate a specific digest
computation.
digest-algorithm = token
All digest-algorithm values are case-insensitive but 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.
Polli & Pardue Expires 15 October 2021 [Page 8]
Internet-Draft Digest Headers April 2021
Some digest-algorithms, although registered, rely on vulnerable
algorithms and MUST not be used:
* "md5", see [CMU-836068] and [NO-MD5];
* "sha", see [IACR-2020-014] and [NO-SHA1].
See the references above for further information.
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. 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 MUST NOT be used as it's now vulnerable to
collision attacks. See [NO-SHA1] and [IACR-2020-014].
* Reference: [RFC3174], [RFC6234], [RFC4648], this document.
* Status: deprecated
Polli & Pardue Expires 15 October 2021 [Page 9]
Internet-Draft Digest Headers April 2021
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:
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.
Polli & Pardue Expires 15 October 2021 [Page 10]
Internet-Draft Digest Headers April 2021
6. Use of Digest when acting on resources
POST and PATCH requests can appear to convey partial representations
but are semantically acting on resources. The enclosed
representation, including its metadata, refers to that action.
In these requests the representation digest MUST be computed on the
representation-data of that action. This is the only possible choice
because representation digest requires complete representation
metadata (see Section 2).
In responses,
* if the representation describes the status of the request,
"Digest" MUST be computed on the enclosed representation (see
Section 10.8 );
* 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.
6.1. 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]).
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.
7. Deprecate Negotiation of Content-MD5
This RFC deprecates the negotiation of Content-MD5 as it has been
obsoleted by [RFC7231]. The "contentMD5" token defined in Section 5
of [RFC3230] MUST NOT be used as a digest-algorithm.
Polli & Pardue Expires 15 October 2021 [Page 11]
Internet-Draft Digest Headers April 2021
8. Obsolete Digest Field Parameters
Section 4.1.1 and 4.2 of [RFC3230] defined field parameters. This
document obsoletes the usage of parameters with "Digest" because this
feature has not been widely deployed and complicates field-value
processing.
[RFC3230] intended field parameters to provide a common way to attach
additional information to a representation-data-digest. However, if
parameters are used as an input to validate the checksum, an attacker
could alter them to steer the validation behavior.
A digest-algorithm can still be parameterized by defining its own way
to encode parameters into the representation-data-digest, in such a
way as to mitigate security risks related to its computation.
9. Relationship to Subresource Integrity (SRI)
Subresource Integrity [SRI] is an integrity mechanism that shares
some similarities to the present document's mechanism. However,
there are differences in motivating factors, threat model and
specification of integrity digest generation, signalling and
validation.
SRI allows a first-party authority to declare an integrity assertion
on a resource served by a first or third party authority. This is
done via the "integrity" attribute that can be added to "script" or
"link" HTML elements. Therefore, the integrity assertion is always
made out-of-band to the resource fetch. In contrast, the "Digest"
field is supplied in-band alongside the selected representation,
meaning that an authority can only declare an integrity assertion for
itself. Methods to improve the security properties of representation
digests are presented in Section 12. This contrast is interesting
because on one hand self-assertion is less likely to be affected by
coordination problems such as the first-party holding stale
information about the third party, but on the other hand the self-
assertion is only as trustworthy as the authority that provided it.
The SRI "integrity" attribute contains a cryptographic hash algorithm
and digest value which is similar to "representation-data-digest"
(see Section 2). The major differences are in serialization format.
SRI does not specify handling of partial representation data (e.g.
Range requests). In contrast, this document specifies handling in
terms that are fully compatible with core HTTP concepts (an example
is provided in Section 10.3).
Polli & Pardue Expires 15 October 2021 [Page 12]
Internet-Draft Digest Headers April 2021
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.
9.1. Supporting Both SRI and Representation Digest
The SRI and Representation Digest mechanisms are different and
complementary but one is not capable of replacing the other because
they have different threat, security and implementation properties.
A user agent that supports both mechanisms is expected to apply the
rules specified for each but since the two mechanisms are
independent, the ordering is not important. However, a user agent
supporting both could benefit from performing representation digest
validation first because it does not always require a conversion into
identity encoding.
There is a chance that a user agent supporting both mechanisms may
find one validates successfully while the other fails. This document
specifies no requirements or guidance for user agents that experience
such cases.
10. Examples of Unsolicited Digest
The following examples demonstrate interactions where a server
responds with a "Digest" field even though the client did not solicit
one using "Want-Digest".
Some examples include JSON objects in the content. For presentation
purposes, objects that fit completely within the line-length limits
are presented on a single line using compact notation with no leading
space. Objects that would exceed line-length limits are presented
across multiple lines (one line per key-value pair) with 2 spaced of
leading indentation.
"Digest" is media-type agnostic and does not provide canonicalization
algorithms for specific formats. Examples of "Digest" are calculated
inclusive of any space.
10.1. Server Returns Full Representation Data
Request:
Polli & Pardue Expires 15 October 2021 [Page 13]
Internet-Draft Digest Headers April 2021
GET /items/123 HTTP/1.1
Host: foo.example
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
10.2. Server Returns No Representation Data
In this example, a HEAD request is used to retrieve the checksum of a
resource.
The response "Digest" field-value is calculated over the JSON object
"{"hello": "world"}", which is not shown because there is no payload
data.
Request:
HEAD /items/123 HTTP/1.1
Host: foo.example
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
10.3. Server Returns Partial Representation Data
In this example, the client makes a range request and the server
responds with partial content. The "Digest" field-value represents
the entire JSON object "{"hello": "world"}".
Request:
GET /items/123 HTTP/1.1
Host: foo.example
Range: bytes=1-7
Response:
Polli & Pardue Expires 15 October 2021 [Page 14]
Internet-Draft Digest Headers April 2021
HTTP/1.1 206 Partial Content
Content-Type: application/json
Content-Range: bytes 1-7/18
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
"hello"
10.4. Client and Server Provide Full Representation Data
The request contains a "Digest" field-value calculated on the
enclosed representation. It also includes an "Accept-Encoding: br"
header field that advertises the client supports brotli encoding.
The response includes a "Content-Encoding: br" that indicates the
selected representation is brotli encoded. The "Digest" field-value
is therefore different compared to the request.
For presentation purposes, the response body is displayed as a
base64-encoded string because it contains non-printable characters.
Request:
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
Content-Location: /items/123
Content-Encoding: br
Content-Length: 22
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
10.5. Client Provides Full Representation Data, Server Provides No
Representation Data
The request "Digest" field-value is calculated on the enclosed
payload.
Polli & Pardue Expires 15 October 2021 [Page 15]
Internet-Draft Digest Headers April 2021
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=
10.6. Client and Server Provide Full Representation Data, Client Uses
id-sha-256.
The response contains two digest values:
* one with no content coding applied, which in this case
accidentally matches the unencoded digest-value sent in the
request;
* one taking into account the "Content-Encoding".
As the response body contains non-printable characters, it is
displayed as a base64-encoded string.
Request:
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Response:
Polli & Pardue Expires 15 October 2021 [Page 16]
Internet-Draft Digest Headers April 2021
HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: br
Content-Location: /items/123
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
10.7. POST Response does not Reference the Request URI
The request "Digest" field-value is computed on the enclosed
representation (see Section 6).
The representation enclosed in the response refers to the resource
identified by "Content-Location" (see [SEMANTICS], Section 6.4.2).
"Digest" is thus computed on the enclosed representation.
Request:
POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Response:
HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=
{
"id": "123",
"title": "New Title"
}
Note that a "204 No Content" response without content but with the
same "Digest" field-value would have been legitimate too.
Polli & Pardue Expires 15 October 2021 [Page 17]
Internet-Draft Digest Headers April 2021
10.8. POST Response Describes the Request Status
The request "Digest" field-value is computed on the enclosed
representation (see Section 6).
The representation enclosed in the response describes the status of
the request, so "Digest" is computed on that enclosed representation.
Response "Digest" has no explicit relation with the resource
referenced by "Location".
Request:
POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
Location: /books/123
{"title": "New Title"}
Response:
HTTP/1.1 201 Created
Content-Type: application/json
Digest: id-sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
Location: /books/123
{
"status": "created",
"id": "123",
"ts": 1569327729,
"instance": "/books/123"
}
10.9. Digest with PATCH
This case is analogous to a POST request where the target resource
reflects the effective request URI.
The PATCH request uses the "application/merge-patch+json" media type
defined in [RFC7396].
"Digest" is calculated on the enclosed payload, which corresponds to
the patch document.
Polli & Pardue Expires 15 October 2021 [Page 18]
Internet-Draft Digest Headers April 2021
The response "Digest" field-value is computed on the complete
representation of the patched resource.
Request:
PATCH /books/123 HTTP/1.1
Host: foo.example
Content-Type: application/merge-patch+json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=
{
"id": "123",
"title": "New Title"
}
Note that a "204 No Content" response without content but with the
same "Digest" field-value would have been legitimate too.
10.10. Error responses
In error responses, the representation-data does not necessarily
refer to the target resource. Instead, it refers to the
representation of the error.
In the following example a client attempts to patch the resource
located at /books/123. However, the resource does not exist 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:
Polli & Pardue Expires 15 October 2021 [Page 19]
Internet-Draft Digest Headers April 2021
PATCH /books/123 HTTP/1.1
Host: foo.example
Content-Type: application/merge-patch+json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Response:
HTTP/1.1 404 Not Found
Content-Type: application/problem+json
Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=
{
"title": "Not Found",
"detail": "Cannot PATCH a non-existent resource",
"status": 404
}
10.11. Use with Trailer Fields and Transfer Coding
An origin server sends "Digest" as trailer field, so it can calculate
digest-value while streaming content and thus mitigate resource
consumption. The "Digest" field-value is the same as in Section 10.1
because "Digest" is designed to be independent from the use of one or
more transfer codings (see Section 2).
Request:
GET /items/123 HTTP/1.1
Host: foo.example
Response:
Polli & Pardue Expires 15 October 2021 [Page 20]
Internet-Draft Digest Headers April 2021
HTTP/1.1 200 OK
Content-Type: application/json
Transfer-Encoding: chunked
Trailer: Digest
8\r\n
{"hello"\r\n
8
: "world\r\n
2\r\n
"}\r\n
0\r\n
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
11. Examples of Want-Digest Solicited Digest
The following examples demonstrate interactions where a client
solicits a "Digest" using "Want-Digest".
Some examples include JSON objects in the content. For presentation
purposes, objects that fit completely within the line-length limits
are presented on a single line using compact notation with no leading
space. Objects that would exceed line-length limits are presented
across multiple lines (one line per key-value pair) with 2 spaced of
leading indentation.
"Digest" is media-type agnostic and does not provide canonicalization
algorithms for specific formats. Examples of "Digest" are calculated
inclusive of any space.
11.1. Server Selects Client's Least Preferred Algorithm
The client requests a digest, preferring "sha". The server is free
to reply with "sha-256" anyway.
Request:
GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha-256;q=0.3, sha;q=1
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Polli & Pardue Expires 15 October 2021 [Page 21]
Internet-Draft Digest Headers April 2021
11.2. Server Selects Algorithm Unsupported by Client
The client requests a "sha" digest only. The server is currently
free to reply with a Digest containing an unsupported algorithm.
Request:
GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
{"hello": "world"}
11.3. Server Does Not Support Client Algorithm and Returns an Error
The client requests a "sha" Digest, the server advises "sha-256" and
"sha-512".
Request:
GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1
Response:
HTTP/1.1 400 Bad Request
Want-Digest: sha-256, sha-512
12. Security Considerations
12.1. Digest Does Not Protect the Full HTTP Message
This document specifies a data integrity mechanism that protects HTTP
"representation data", but not HTTP "representation metadata" fields,
from certain kinds of accidental corruption.
"Digest" is not intended to be a general protection against malicious
tampering with HTTP messages. This can be achieved by combining it
with other approaches such as transport-layer security or digital
signatures.
Polli & Pardue Expires 15 October 2021 [Page 22]
Internet-Draft Digest Headers April 2021
12.2. Broken Cryptographic Algorithms
Cryptographic algorithms are intended to provide a proof of integrity
suited towards cryptographic constructions such as signatures.
However, these rely on collision-resistance for their security proofs
[CMU-836068]. The "md5" and "sha" digest-algorithms are vulnerable
to collisions attacks, so they MUST NOT be used with "Digest".
12.3. Other Deprecated Algorithms
The ADLER32 algorithm defined in [RFC1950] has been deprecated by
[RFC3309] because, under certain conditions, it provides weak
detection of errors. It is now NOT RECOMMENDED for use with
"Digest".
12.4. Digest for End-to-End Integrity
"Digest" only covers the "representation data" and not the
"representation metadata". "Digest" could help protect the
"representation data" from buggy manipulation, undesired
"transforming proxies" (see Section 7.7 of [SEMANTICS]) or other
actions as the data passes across multiple hops or system boundaries.
Even a simple mechanism for end-to-end "representation data"
integrity is valuable because user-agent can validate that resource
retrieval succeeded before handing off to a HTML parser, video player
etc. for parsing.
Identity digest-algorithms (e.g. "id-sha-256" and "id-sha-512") are
particularly useful for end-to-end integrity because they allow
piecing together a resource from different sources with different
HTTP messaging characteristics. For example, different servers that
apply different content codings.
Note that using "Digest" alone does not provide end-to-end integrity
of HTTP messages over multiple hops, since metadata could be
manipulated at any stage. Methods to protect metadata are discussed
in Section 12.6.
12.5. Digest and Content-Location in Responses
When a state-changing method returns the "Content-Location" header
field, the enclosed representation refers to the resource identified
by its value and "Digest" is computed accordingly.
Polli & Pardue Expires 15 October 2021 [Page 23]
Internet-Draft Digest Headers April 2021
12.6. Usage in Signatures
Digital signatures are widely used together with checksums to provide
the certain identification of the origin of a message [NIST800-32].
Such signatures can protect one or more HTTP fields and there are
additional considerations when "Digest" is included in this set.
Since the "Digest" field is a hash of a resource representation, it
explicitly depends on the "representation metadata" (eg. the values
of "Content-Type", "Content-Encoding" etc). A signature that
protects "Digest" but not other "representation metadata" can expose
the communication to tampering. For example, an actor could
manipulate the "Content-Type" field-value and cause a digest
validation failure at the recipient, preventing the application from
accessing the representation. Such an attack consumes the resources
of both endpoints. See also Section 12.5.
"Digest" SHOULD always be used over a connection that provides
integrity at the transport layer that protects HTTP fields.
A "Digest" field using NOT RECOMMENDED digest-algorithms SHOULD NOT
be used in signatures.
Using signatures to protect the "Digest" of an empty representation
allows receiving endpoints to detect if an eventual payload has been
stripped or added.
Any mangling of "Digest", including de-duplication of representation-
data-digest values or combining different field values (see
Section 5.2 of [SEMANTICS]) might affect signature validation.
12.7. Usage in Trailer Fields
When "Digest" is used in trailer fields, the receiver gets the digest
value after the content and may thus be tempted to process the data
before validating the digest value. It is prefereable that data is
only be processed after validating the Digest.
If received in trailers, "Digest" MUST NOT be discarded; instead, it
MAY be merged in the header section (See Section 6.5.1 of
[SEMANTICS]).
Not every digest-algorithm is suitable for use in the trailer
section, some may require to pre-process the whole payload before
sending a message (eg. see [I-D.thomson-http-mice]).
Polli & Pardue Expires 15 October 2021 [Page 24]
Internet-Draft Digest Headers April 2021
12.8. Usage with Encryption
"Digest" may expose details of encrypted payload when the checksum is
computed on the unencrypted data. For example, the use of the "id-
sha-256" digest-algorithm in conjunction with the encrypted content-
coding [RFC8188].
The representation-data-digest of an encrypted payload can change
between different messages depending on the encryption algorithm
used; in those cases its value could not be used to provide a proof
of integrity "at rest" unless the whole (e.g. encoded) content is
persisted.
12.9. Algorithm Agility
The security properties of digest-algorithms are not fixed.
Algorithm Agility (see [RFC7696]) is achieved by providing
implementations with flexibility choose digest-algorithms from the
IANA Digest Algorithm Values registry in Section 13.1.
To help endpoints understand weaker algorithms from stronger ones,
this document adds to the IANA Digest Algorithm Values registry a new
"Status" field containing the most-recent appraisal of the digest-
algorithm; the allowed values are specified in Section 13.2.
An endpoint might have a preference for algorithms, such as
preferring "standard" algorithms over "deprecated" ones. Transition
from weak algorithms is supported by negotiation of digest-algorithm
using "Want-Digest" (see Section 4) or by sending multiple
representation-data-digest values from which the receiver chooses.
Endpoints are advised that sending multiple values consumes
resources, which may be wasted if the receiver ignores them (see
Section 3).
12.9.1. Duplicate digest-algorithm in field value
An endpoint might receive multiple representation-data-digest values
(see Section 3) that use the same digest-algorithm with different or
identical digest-values. For example:
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=,
sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
A receiver is permitted to ignore any representation-data-digest
value, so validation of duplicates is left as an implementation
decision. Endpoints might select all, some or none of the values for
checksum comparison and, based on the intersection of those results,
conditionally pass or fail digest validation.
Polli & Pardue Expires 15 October 2021 [Page 25]
Internet-Draft Digest Headers April 2021
12.10. Resource exhaustion
"Digest" validation consumes computational resources. In order to
avoid resource exhaustion, implementations can restrict validation of
the algorithm types, number of validations, or the size of content.
13. IANA Considerations
13.1. Establish the HTTP Digest Algorithm Values Registry
This memo sets this specification to be the establishing document for
the HTTP Digest Algorithm Values (https://www.iana.org/assignments/
http-dig-alg/http-dig-alg.xhtml) registry.
13.2. The "status" Field in the HTTP Digest Algorithm Values Registry
This memo adds the field "Status" to the HTTP Digest Algorithm Values
(https://www.iana.org/assignments/http-dig-alg/http-dig-alg.xhtml)
registry. The allowed values for the "Status" fields are described
below.
Status
* "standard" for standardized algorithms without known problems;
* "experimental", "obsoleted" or some other appropriate value -
e.g. according to the type and status of the primary document
in which the algorithm is defined;
* "deprecated" when the algorithm is insecure or otherwise
undesirable.
13.3. Deprecate "MD5" Digest Algorithm
This memo updates the "MD5" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry:
* Digest Algorithm: md5
* Description: As specified in Section 5.
* Status: As specified in Section 5.
13.4. Update "UNIXsum" Digest Algorithm
This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry:
Polli & Pardue Expires 15 October 2021 [Page 26]
Internet-Draft Digest Headers April 2021
* Digest Algorithm: As specified in Section 5.
* Description: As specified in Section 5.
* Status: As specified in Section 5.
13.5. Update "UNIXcksum" Digest Algorithm
This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry:
* Digest Algorithm: As specified in Section 5.
* Description: As specified in Section 5.
* Status: As specified in Section 5.
13.6. Update "CRC32c" Digest Algorithm
This memo updates the "CRC32c" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry:
* Digest Algorithm: crc32c
* Description: The CRC32c algorithm is a 32-bit cyclic redundancy
check. It achieves a better hamming distance (for better error-
detection performance) than many other 32-bit CRC functions.
Other places it is used include iSCSI and SCTP. The 32-bit output
is encoded in hexadecimal (using between 1 and 8 ASCII characters
from 0-9, A-F, and a-f; leading 0's are allowed). For example,
crc32c=0a72a4df and crc32c=A72A4DF are both valid checksums for
the 3-byte message "dog".
* Reference: [RFC4960] appendix B, this document.
* Status: standard.
13.7. Deprecate "SHA" Digest Algorithm
This memo updates the "SHA" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry:
* Digest Algorithm: sha
* Description: As specified in Section 5.
Polli & Pardue Expires 15 October 2021 [Page 27]
Internet-Draft Digest Headers April 2021
* Status: As specified in Section 5.
13.8. Obsolete "ADLER32" Digest Algorithm
This memo updates the "ADLER32" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry:
* Digest Algorithm: adler32
* Description: The ADLER32 algorithm is a checksum specified in
[RFC1950] "ZLIB Compressed Data Format". The 32-bit output is
encoded in hexadecimal (using between 1 and 8 ASCII characters
from 0-9, A-F, and a-f; leading 0's are allowed). For example,
adler32=03da0195 and adler32=3DA0195 are both valid checksums for
the 4-byte message "Wiki". This algorithm is obsoleted and SHOULD
NOT be used.
* Status: obsoleted
13.9. Obsolete "contentMD5" token in Digest Algorithm
This memo adds the "contentMD5" token in the HTTP Digest Algorithm
Values (https://www.iana.org/assignments/http-dig-alg/http-dig-
alg.xhtml) registry:
* Digest Algorithm: contentMD5
* Description: Section 5 of [RFC3230] defined the "contentMD5" token
to be used only in Want-Digest. This token is obsoleted and MUST
NOT be used.
* Reference: Section 13.9 of this document, Section 5 of [RFC3230].
* Status: obsoleted
13.10. The "id-sha-256" Digest Algorithm
This memo registers the "id-sha-256" digest-algorithm in the HTTP
Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
alg/http-dig-alg.xhtml) registry:
* Digest Algorithm: id-sha-256
* Description: As specified in Section 5.
* Status: As specified in Section 5.
Polli & Pardue Expires 15 October 2021 [Page 28]
Internet-Draft Digest Headers April 2021
13.11. The "id-sha-512" Digest Algorithm
This memo registers the "id-sha-512" digest-algorithm in the HTTP
Digest Algorithm Values (https://www.iana.org/assignments/http-dig-
alg/http-dig-alg.xhtml) registry:
* Digest Algorithm: id-sha-512
* Description: As specified in Section 5.
* Status: As specified in Section 5.
13.12. Changes Compared to RFC5843
The digest-algorithm values for "MD5", "SHA", "SHA-256", "SHA-512",
"UNIXcksum", "UNIXsum", "ADLER32" and "CRC32c" have been updated to
lowercase.
The status of "MD5" has been updated to "deprecated", and its
description states that this algorithm MUST NOT be used.
The status of "SHA" has been updated to "deprecated", and its
description states that this algorithm MUST NOT be used.
The status for "CRC2c", "UNIXsum" and "UNIXcksum" has been updated to
"standard".
The "id-sha-256" and "id-sha-512" algorithms have been added to the
registry.
13.13. Want-Digest Field Registration
This section registers the "Want-Digest" field in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
Field name: "Want-Digest"
Status: permanent
Specification document(s): Section 4 of this document
13.14. Digest Field Registration
This section registers the "Digest" field in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" [SEMANTICS].
Field name: "Digest"
Polli & Pardue Expires 15 October 2021 [Page 29]
Internet-Draft Digest Headers April 2021
Status: permanent
Specification document(s): Section 3 of this document
14. References
14.1. Normative References
[CMU-836068]
Carnagie Mellon University, Software Engineering
Institute, "MD5 Vulnerable to collision attacks", 31
December 2008, <https://www.kb.cert.org/vuls/id/836068/>.
[IACR-2020-014]
Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", 5
January 2020, <https://eprint.iacr.org/2020/014.pdf>.
[NIST800-32]
National Institute of Standards and Technology, U.S.
Department of Commerce, "Introduction to Public Key
Technology and the Federal PKI Infrastructure", February
2001, <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
nistspecialpublication800-32.pdf>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/rfc/rfc1321>.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/rfc/rfc1950>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
<https://www.rfc-editor.org/rfc/rfc3174>.
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
RFC 3230, DOI 10.17487/RFC3230, January 2002,
<https://www.rfc-editor.org/rfc/rfc3230>.
Polli & Pardue Expires 15 October 2021 [Page 30]
Internet-Draft Digest Headers April 2021
[RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
DOI 10.17487/RFC3309, September 2002,
<https://www.rfc-editor.org/rfc/rfc3309>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/rfc/rfc4960>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC5843] Bryan, A., "Additional Hash Algorithms for HTTP Instance
Digests", RFC 5843, DOI 10.17487/RFC5843, April 2010,
<https://www.rfc-editor.org/rfc/rfc5843>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/rfc/rfc6234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[SEMANTICS]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-15, 30 March 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
15>.
[UNIX] The Open Group, "The Single UNIX Specification, Version 2
- 6 Vol Set for UNIX 98", February 1997.
14.2. Informative References
Polli & Pardue Expires 15 October 2021 [Page 31]
Internet-Draft Digest Headers April 2021
[HTTP11] Fielding, R. T., Nottingham, M., and J. Reschke,
"HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
httpbis-messaging-15, 30 March 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
15>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-header-structure-19, 3 June 2020,
<https://tools.ietf.org/html/draft-ietf-httpbis-header-
structure-19>.
[I-D.thomson-http-mice]
Thomson, M. and J. Yasskin, "Merkle Integrity Content
Encoding", Work in Progress, Internet-Draft, draft-
thomson-http-mice-03, 13 August 2018,
<https://tools.ietf.org/html/draft-thomson-http-mice-03>.
[NO-MD5] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/rfc/rfc6151>.
[NO-SHA1] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/rfc/rfc6194>.
[PATCH] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
<https://www.rfc-editor.org/rfc/rfc5789>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/rfc/rfc2818>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/rfc/rfc7231>.
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
<https://www.rfc-editor.org/rfc/rfc7396>.
Polli & Pardue Expires 15 October 2021 [Page 32]
Internet-Draft Digest Headers April 2021
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/rfc/rfc7696>.
[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://www.rfc-editor.org/rfc/rfc7807>.
[RFC8188] Thomson, M., "Encrypted Content-Encoding for HTTP",
RFC 8188, DOI 10.17487/RFC8188, June 2017,
<https://www.rfc-editor.org/rfc/rfc8188>.
[SRI] Akhawe, D., Braun, F., Marier, F., and J. Weinberger,
"Subresource Integrity", W3C Recommendation REC-SRI-
20160623, 23 June 2016,
<https://www.w3.org/TR/2016/REC-SRI-20160623/>.
Appendix A. Resource Representation and Representation-Data
The following examples show how representation metadata, payload
transformations and method impacts on the message and content. When
the content contains non-printable characters (eg. when it is
compressed) it is shown as base64-encoded string.
A request with a JSON object without any content coding.
Request:
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
{"hello": "world"}
Here is a gzip-compressed JSON object using a content coding.
Request:
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Encoding: gzip
H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
Now the same content conveys a malformed JSON object.
Polli & Pardue Expires 15 October 2021 [Page 33]
Internet-Draft Digest Headers April 2021
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:
Polli & Pardue Expires 15 October 2021 [Page 34]
Internet-Draft Digest Headers April 2021
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"}
Appendix B. FAQ
1. Why remove all references to content-md5?
Those were unnecessary to understanding and using this
specification.
2. Why remove references to instance manipulation?
Those were unnecessary for correctly using and applying the
specification. An example with Range Request is more than
enough. This document uses the term "partial representation"
which should group all those cases.
3. How to use "Digest" with "PATCH" method?
See Section 6.
4. Why remove references to delta-encoding?
Unnecessary for a correct implementation of this specification.
The revised specification can be nicely adapted to "delta
encoding", but all the references here to delta encoding don't
add anything to this RFC. Another job would be to refresh delta
encoding.
5. Why remove references to Digest Authentication?
This specification seems to me completely unrelated to Digest
Authentication but for the word "Digest".
6. What changes in "Want-Digest"?
Polli & Pardue Expires 15 October 2021 [Page 35]
Internet-Draft Digest Headers April 2021
The contentMD5 token defined in Section 5 of [RFC3230] is
deprecated by Section 7.
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.
7. Does this specification change supported algorithms?
Yes. This RFC updates [RFC5843] which is still delegated for all
algorithms updates, and adds two more algorithms: "id-sha-256"
and "id-sha-512" which allows to send a checksum of a resource
representation with no content codings applied. To simplify a
future transition to Structured Fields
[I-D.ietf-httpbis-header-structure] we suggest to use lowercase
for digest-algorithms.
8. What about mid-stream trailer fields?
While mid-stream trailer fields (https://github.com/httpwg/http-
core/issues/313#issuecomment-584389706) are interesting, since
this specification is a rewrite of [RFC3230] we do not think we
should face that. As a first thought, nothing in this document
precludes future work that would find a use for mid-stream
trailers, for example an incremental digest-algorithm. A
document defining such a digest-algorithm is best positioned to
describe how it is used.
Acknowledgements
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
_RFC Editor: Please remove this section before publication._
How can I generate and validate the "Digest" values shown in the
examples throughout this document?
Polli & Pardue Expires 15 October 2021 [Page 36]
Internet-Draft Digest Headers April 2021
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
_RFC Editor: Please remove this section before publication._
Polli & Pardue Expires 15 October 2021 [Page 37]
Internet-Draft Digest Headers April 2021
Since draft-ietf-httpbis-digest-headers-04
* Improve SRI section #1354
* About duplicate digest-algorithms #1221
* Improve security considerations #852
* md5 and sha deprecation references #1392
* Obsolete 3230 #1395
* Editorial #1362
Since draft-ietf-httpbis-digest-headers-03
* Reference semantics-12
* Detail encryption quirks
* Details on Algorithm agility #1250
* Obsolete parameters #850
Since draft-ietf-httpbis-digest-headers-02
* Deprecate SHA-1 #1154
* Avoid id-* with encrypted content
* Digest is independent from MESSAGING and HTTP/1.1 is not normative
#1215
* Identity is not a valid field value for content-encoding #1223
* Mention trailers #1157
* Reference httpbis-semantics #1156
* Add contentMD5 as an obsoleted digest-algorithm #1249
* Use lowercase digest-algorithms names in the doc and in the
digest-algorithm IANA table.
Since draft-ietf-httpbis-digest-headers-01
* Digest of error responses is computed on the error representation-
data #1004
Polli & Pardue Expires 15 October 2021 [Page 38]
Internet-Draft Digest Headers April 2021
* Effect of HTTP semantics on payload and message body moved to
appendix #1122
* Editorial refactoring, moving headers sections up. #1109-#1112,
#1116, #1117, #1122-#1124
Since draft-ietf-httpbis-digest-headers-00
* Align title with document name
* Add id-sha-* algorithm examples #880
* Reference [RFC6234] and [RFC3174] instead of FIPS-1
* Deprecate MD5
* Obsolete ADLER-32 but don't forbid it #828
* Update CRC32C value in IANA table #828
* Use when acting on resources (POST, PATCH) #853
* Added Relationship with SRI, draft Use Cases #868, #971
* Warn about the implications of "Content-Location"
Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
Lucas Pardue
Cloudflare
Email: lucaspardue.24.7@gmail.com
Polli & Pardue Expires 15 October 2021 [Page 39]