Digest Fields
draft-ietf-httpbis-digest-headers-07
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.
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|---|---|---|---|
| Authors | Roberto Polli , Lucas Pardue | ||
| Last updated | 2021-11-16 | ||
| Replaces | draft-polli-resource-digests-http | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Stream | WG state | In WG Last Call | |
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draft-ietf-httpbis-digest-headers-07
HTTP R. Polli
Internet-Draft Team Digitale, Italian Government
Obsoletes: 3230 (if approved) L. Pardue
Intended status: Standards Track Cloudflare
Expires: 20 May 2022 16 November 2021
Digest Fields
draft-ietf-httpbis-digest-headers-07
Abstract
This document defines HTTP fields that support integrity checksums.
The Digest field can be used for the integrity of HTTP
representations. The Content-Digest field can be used for the
integrity of HTTP message content. Want-Digest and Want-Content-
Digest can be used to indicate a sender's desire to receive integrity
fields respectively.
This document obsoletes RFC 3230.
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."
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This Internet-Draft will expire on 20 May 2022.
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
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document Structure . . . . . . . . . . . . . . . . . . . 4
1.2. Concept Overview . . . . . . . . . . . . . . . . . . . . 4
1.3. Replacing RFC 3230 . . . . . . . . . . . . . . . . . . . 5
1.4. Notational Conventions . . . . . . . . . . . . . . . . . 6
2. Representation Digest . . . . . . . . . . . . . . . . . . . . 6
3. The Digest Field . . . . . . . . . . . . . . . . . . . . . . 7
4. The Content-Digest Field . . . . . . . . . . . . . . . . . . 8
5. Want-Digest and Want-Content-Digest Fields . . . . . . . . . 9
6. Digest Algorithm Values . . . . . . . . . . . . . . . . . . . 9
7. Using Digest in State-Changing Requests . . . . . . . . . . . 12
7.1. Digest and Content-Location in Responses . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8.1. Digest Does Not Protect the Full HTTP Message . . . . . . 13
8.2. Digest for End-to-End Integrity . . . . . . . . . . . . . 14
8.3. Usage in Signatures . . . . . . . . . . . . . . . . . . . 14
8.4. Usage in Trailer Fields . . . . . . . . . . . . . . . . . 15
8.5. Usage with Encryption . . . . . . . . . . . . . . . . . . 15
8.6. Algorithm Agility . . . . . . . . . . . . . . . . . . . . 15
8.7. Duplicate digest-algorithm in field value . . . . . . . . 16
8.8. Resource exhaustion . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9.1. Establish the HTTP Digest Algorithm Values Registry . . . 16
9.2. Obsolete "contentMD5" token in Digest Algorithm . . . . . 16
9.3. Changes Compared to RFC3230 . . . . . . . . . . . . . . . 17
9.4. Changes Compared to RFC5843 . . . . . . . . . . . . . . . 17
9.5. Want-Digest Field Registration . . . . . . . . . . . . . 17
9.6. Digest Field Registration . . . . . . . . . . . . . . . . 17
9.7. Want-Content-Digest Field Registration . . . . . . . . . 17
9.8. Content-Digest Field Registration . . . . . . . . . . . . 18
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10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Resource Representation and Representation-Data . . 21
Appendix B. Examples of Unsolicited Digest . . . . . . . . . . . 23
B.1. Server Returns Full Representation Data . . . . . . . . . 23
B.2. Server Returns No Representation Data . . . . . . . . . . 24
B.3. Server Returns Partial Representation Data . . . . . . . 24
B.4. Client and Server Provide Full Representation Data . . . 25
B.5. Client Provides Full Representation Data, Server Provides
No Representation Data . . . . . . . . . . . . . . . . . 26
B.6. Client and Server Provide Full Representation Data . . . 26
B.7. POST Response does not Reference the Request URI . . . . 27
B.8. POST Response Describes the Request Status . . . . . . . 28
B.9. Digest with PATCH . . . . . . . . . . . . . . . . . . . . 28
B.10. Error responses . . . . . . . . . . . . . . . . . . . . . 29
B.11. Use with Trailer Fields and Transfer Coding . . . . . . . 30
Appendix C. Examples of Want-Digest Solicited Digest . . . . . . 30
C.1. Server Selects Client's Least Preferred Algorithm . . . . 31
C.2. Server Selects Algorithm Unsupported by Client . . . . . 31
C.3. Server Does Not Support Client Algorithm and Returns an
Error . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 32
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Code Samples . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Since draft-ietf-httpbis-digest-headers-06 . . . . . . . . . . 35
Since draft-ietf-httpbis-digest-headers-05 . . . . . . . . . . 36
Since draft-ietf-httpbis-digest-headers-04 . . . . . . . . . . 36
Since draft-ietf-httpbis-digest-headers-03 . . . . . . . . . . 36
Since draft-ietf-httpbis-digest-headers-02 . . . . . . . . . . 36
Since draft-ietf-httpbis-digest-headers-01 . . . . . . . . . . 37
Since draft-ietf-httpbis-digest-headers-00 . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
HTTP does not define a means to protect the integrity of
representations. When HTTP messages are transferred between
endpoints, the protocol might choose to make use of features of the
lower layer in order to provide some integrity protection; for
instance, TCP checksums or TLS records [RFC2818].
This document defines two digest integrity mechanisms for HTTP.
First, representation data integrity, which acts on representation
data (Section 3.2 of [SEMANTICS]). Second, content digest integrity,
which acts on conveyed content (Section 6.4 of [SEMANTICS]). Both
mechanisms operate independent of transport integrity, offering the
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potential to detect programming errors and corruption of data in
flight or at rest. They can be used across multiple hops in order to
provide end-to-end integrity guarantees, which can aid fault
diagnosis when resources are transferred across hops and system
boundaries. Finally, they can be used to validate integrity when
reconstructing a resource fetched using different HTTP connections.
This document obsoletes [RFC3230].
1.1. Document Structure
This document is structured as follows:
* Section 2 describes concepts related to representation digests,
* Section 3 defines the Digest request and response header and
trailer field,
* Section 4 defines the Content-Digest request and response header
and trailer field,
* Section 5 defines the Want-Digest and Want-Content-Digest request
and response header and trailer field,
* Section 6 describes algorithms and their relation to Digest,
* Section 7 details computing representation digests,
* Appendix B and Appendix C provide examples of using Digest and
Want-Digest.
1.2. Concept Overview
This document defines the Digest request and response header and
trailer field; see Section 3. At a high level, the value contains a
checksum, computed over selected representation data (Section 3.2 of
[SEMANTICS]), that the recipient can use to validate integrity.
Basing Digest on the selected representation makes it straightforward
to apply it to use-cases where the transferred data requires some
sort of manipulation to be considered a representation or conveys a
partial representation of a resource, such as Range Requests (see
Section 14.2 of [SEMANTICS]).
To support use-cases where a simple checksum of the content bytes is
required, this document introduces the Content-Digest request and
response header and trailer field; see Section 4.
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Digest and Content-Digest support algorithm agility. The Want-Digest
and Want-Content-Digest fields allows endpoints to express interest
in Digest and Content-Digest respectively, and preference of
algorithms in either.
Digest field calculations are tied to the Content-Encoding and
Content-Type header fields. Therefore, a given resource may have
multiple different checksum values when transferred with HTTP.
Digest fields do not provide integrity for HTTP messages or fields.
However, they can be combined with other mechanisms that protect
metadata, such as digital signatures, in order to protect the phases
of an HTTP exchange in whole or in part.
This specification does not define means for authentication,
authorization or privacy.
1.3. Replacing RFC 3230
Historically, the Content-MD5 header field provided an HTTP integrity
mechanism but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it due to
inconsistent handling of partial responses. [RFC3230] defined the
concept of "instance" digests and a more flexible integrity scheme to
help address issues with Content-MD5. It first introduced the Digest
and Want-Digest fields. HTTP terminology has evolved since [RFC3230]
was published. The concept of "instance" has been superseded by
selected representation.
This document replaces [RFC3230]. The changes described in the
following paragraphs are intended to be semantically compatible with
existing implementations where possible.
The Digest and Want-Digest field definitions are updated to align
with the terms and notational conventions in [SEMANTICS].
Negotiation of Content-MD5 is deprecated and has been replaced by
Content-Digest negotiation via Want-Content-Digest.
Sections 4.1.1 and 4.2 of [RFC3230] defined field parameters. This
document obsoletes the usage of parameters with Digest because this
feature has not been widely deployed and complicates field-value
processing. [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.
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The algorithm table has been updated to reflect the current state of
the art, (see Section 6).
1.4. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405] along with the "#rule" extension defined in
Section 5.6.1 of [SEMANTICS] and the "qvalue" rule defined in
Section 12.4.2 of [SEMANTICS].
The definitions "representation", "selected representation",
"representation data", "representation metadata", and "content" in
this document are to be interpreted as described in [SEMANTICS].
Algorithm names respect the casing used in their definition document
(e.g. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted
(e.g. "sha", "crc32c").
2. Representation Digest
The representation digest is an integrity mechanism for HTTP
resources which uses a checksum that is calculated independently of
the content (see Section 6.4 of [SEMANTICS]). It uses the
representation data (see Section 8.1 of [SEMANTICS]), that can be
fully or partially contained in the content, or not contained at all.
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>
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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 8.3).
The checksum is computed using one of the digest-algorithms listed in
the HTTP Digest Algorithm Values Registry (see Section 6) and then
encoded in the associated format.
3. The Digest Field
The Digest field contains a comma-separated list of one or more
representation digest values as defined in Section 2. It can be used
in both requests and responses.
Digest = 1#representation-data-digest
For example:
Digest: sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
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 8.6).
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
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. In this case, Digest MAY be
merged into the header section; see Section 6.5.1 of [SEMANTICS].
When an incremental digest-algorithm is used, the sender and the
receiver can dynamically compute the digest value while streaming the
content.
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A non-comprehensive set of examples showing the impacts of
representation metadata, payload transformations and HTTP methods on
Digest is provided in Appendix B and Appendix C.
4. The Content-Digest Field
The Content-Digest field contains a comma-separated list of one or
more content digest values. A content digest value is computed by
applying a digest-algorithm to the actual message content (see
Section 6.4 of [SEMANTICS]). It can be used in both requests and
responses.
Content-Digest = 1#content-digest
content-digest = digest-algorithm "="
<encoded digest output>
For example:
Content-Digest: sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
A Content-Digest field MAY contain multiple content-digest values,
similarly to Digest (see Section 3)
Content-Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
A recipient MAY ignore any or all of the content-digests in a
Content-Digest field. This allows the recipient to choose which
digest-algorithm(s) to use for validation instead of verifying every
received content-digest.
A sender MAY send a content-digest using a digest-algorithm without
knowing whether the recipient supports the digest-algorithm, or even
knowing that the recipient will ignore it.
Content-Digest can be sent in a trailer section. In this case,
Content-Digest MAY be merged into the header section; see
Section 6.5.1 of [SEMANTICS].
When an incremental digest-algorithm is used, the sender and the
receiver can dynamically compute the digest value while streaming the
content.
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5. Want-Digest and Want-Content-Digest Fields
Senders can indicate their integrity checksum preferences using the
Want-Digest or Want-Content-Digest fields. These can be used in both
requests and responses.
Want-Digest indicates the sender's desire to receive a representation
digest on messages associated with the request URI and representation
metadata, using the Digest field.
Want-Content-Digest indicates the sender's desire to receive a
content digest on messages associated with the request URI and
representation metadata, using the Content-Digest field.
Want-Digest = 1#want-digest-value
Want-Content-Digest = 1#want-digest-value
want-digest-value = digest-algorithm [ ";" "q" "=" qvalue]
qvalue indicates the sender's digest-algorithm preferences.
Section 12.4.2 of [SEMANTICS]) describes qvalue usage and semantics.
Senders can provide multiple digest-algorithm items with the same
qvalue.
Examples:
Want-Digest: sha-256
Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0
Want-Content-Digest: sha-256
Want-Content-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0
6. Digest Algorithm Values
Digest-algorithm values are used to indicate a specific digest
computation.
digest-algorithm = token
All digest-algorithm token values are case-insensitive but lower case
is preferred; digest-algorithm token values MUST be compared in a
case-insensitive fashion.
Every digest-algorithm defines its computation procedure and encoding
output. Unless specified otherwise, comparison of encoded output is
case-sensitive.
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The "HTTP Digest Algorithm Values Registry", maintained by IANA at
https://www.iana.org/assignments/http-dig-alg/
(https://www.iana.org/assignments/http-dig-alg/) registers digest-
algorithm values. Registrations MUST include the following fields:
* Digest algorithm: the token value. The registry can be used to
reserve token values
* Status: the status of the algorithm. Use "standard" for
standardized algorithms without known problems; "experimental" or
some other appropriate value
- e.g. according to the type and status of the primary document
in which the algorithm is defined; "insecure" when the
algorithm is insecure; "reserved" when Digest algorithm
references a reserved token value
* Description: the description of the digest-algorithm and its
encoding
* Reference: a set of pointers to the primary documents defining the
digest-algorithm
The associated encoding for new digest-algorithms MUST either be
represented as a quoted string or MUST NOT include ";" or "," in the
character sets used for the encoding.
Insecure digest algorithms MAY be used to preserve integrity against
accidental change, but MUST NOT be used in a potentially adversarial
setting; for example, when signing the digest of content for
authenticity.
The registry is initialized with the tokens listed below.
sha-512
* Digest Algorithm: sha-512
* Description: The SHA-512 algorithm [RFC6234]. The output of
this algorithm is encoded using the base64 encoding [RFC4648].
* Reference: [RFC6234], [RFC4648], this document.
* Status: standard
sha-256
* Digest Algorithm: sha-256
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* 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
md5
* Digest Algorithm: md5
* Description: The MD5 algorithm, as specified in [RFC1321]. The
output of this algorithm is encoded using the base64 encoding
[RFC4648]. This digest-algorithm is now vulnerable to
collision attacks. See [NO-MD5] and [CMU-836068].
* Reference: [RFC1321], [RFC4648], this document.
* Status: insecure
sha
* Digest Algorithm: sha
* Description: The SHA-1 algorithm [RFC3174]. The output of this
algorithm is encoded using the base64 encoding [RFC4648]. This
digest-algorithm is now vulnerable to collision attacks. See
[NO-SHA1] and [IACR-2020-014].
* Reference: [RFC3174], [RFC6234], [RFC4648], this document.
* Status: insecure
unixsum
* Digest Algorithm: unixsum
* Description: The algorithm computed by the UNIX "sum" command,
as defined by the Single UNIX Specification, Version 2 [UNIX].
The output of this algorithm is an ASCII decimal-digit string
representing the 16-bit checksum, which is the first word of
the output of the UNIX "sum" command.
* Reference: [UNIX], this document.
* Status: insecure
unixcksum
* Digest Algorithm: unixcksum
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* 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: insecure
adler32
* Digest Algorithm: adler32
* Description: The ADLER32 algorithm is a checksum specified in
[RFC1950] "ZLIB Compressed Data Format". The 32-bit output is
encoded in hexadecimal (using between 1 and 8 ASCII characters
from 0-9, A-F, and a-f; leading 0's are allowed). For example,
adler32=03da0195 and adler32=3DA0195 are both valid checksums
for the 4-byte message "Wiki". This algorithm is obsoleted and
SHOULD NOT be used.
* Reference: [RFC1950], this document.
* Status: insecure
crc32c
* Digest Algorithm: crc32c
* Description: The CRC32c algorithm is a 32-bit cyclic redundancy
check. It achieves a better hamming distance (for better
error-detection performance) than many other 32-bit CRC
functions. Other places it is used include iSCSI and SCTP.
The 32-bit output is encoded in hexadecimal (using between 1
and 8 ASCII characters from 0-9, A-F, and a-f; leading 0's are
allowed). For example, crc32c=0a72a4df and crc32c=A72A4DF are
both valid checksums for the 3-byte message "dog".
* Reference: [RFC4960] appendix B, this document.
* Status: insecure
7. Using Digest in State-Changing Requests
When the representation enclosed in a state-changing request does not
describe the target resource, the representation digest MUST be
computed on the representation-data. This is the only possible
choice because representation digest requires complete representation
metadata (see Section 2).
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In responses,
* if the representation describes the status of the request, Digest
MUST be computed on the enclosed representation (see Appendix B.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 is done according to the HTTP semantics of the given
method, for example using the Content-Location header field (see
Section 8.7 of [SEMANTICS]). In contrast, the Location header field
does not affect Digest because it is not representation metadata.
For example, in PATCH requests, the representation digest will be
computed on the patch document because the representation metadata
refers to the patch document and not to the target resource (see
Section 2 of [PATCH]). In responses, instead, the representation
digest will be computed on the selected representation of the patched
resource.
7.1. 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. An example is given
in Appendix B.7.
8. Security Considerations
8.1. Digest Does Not Protect the Full HTTP Message
This document specifies a data integrity mechanism that protects HTTP
representation data or content, but not HTTP header and trailer
fields, from certain kinds of accidental corruption.
Digest fields are not intended to be a general protection against
malicious tampering with HTTP messages. This can be achieved by
combining it with other approaches such as transport-layer security
or digital signatures.
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8.2. Digest for End-to-End Integrity
Digest fields can help detect representation data or content
modification due to implementation errors, undesired "transforming
proxies" (see Section 7.7 of [SEMANTICS]) or other actions as the
data passes across multiple hops or system boundaries. Even a simple
mechanism for end-to-end representation data integrity is valuable
because user-agent can validate that resource retrieval succeeded
before handing off to a HTML parser, video player etc. for parsing.
Note that using digest fields alone does not provide end-to-end
integrity of HTTP messages over multiple hops, since metadata could
be manipulated at any stage. Methods to protect metadata are
discussed in Section 8.3.
8.3. Usage in Signatures
Digital signatures are widely used together with checksums to provide
the certain identification of the origin of a message [NIST800-32].
Such signatures can protect one or more HTTP fields and there are
additional considerations when Digest is included in this set.
Since digest fields are hashes of resource representations, they
explicitly depend on the representation metadata (e.g. 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 7.1.
Digest fields 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 checksum of an empty representation
allows receiving endpoints to detect if an eventual payload has been
stripped or added.
Any mangling of digest fields, including de-duplication of
representation-data-digest values or combining different field values
(see Section 5.2 of [SEMANTICS]) might affect signature validation.
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8.4. Usage in Trailer Fields
Before sending digest fields in a trailer section, the sender should
consider that intermediaries are explicitly allowed to drop any
trailer (see Section 6.5.2 of [SEMANTICS]).
When digest fields are used in a trailer section, the field-values
are received after the content. Eager processing of content before
the trailer section prevents digest validation, possibly leading to
processing of invalid data.
Not every digest-algorithm is suitable for use in the trailer
section, some may require to pre-process the whole payload before
sending a message (e.g. see [I-D.thomson-http-mice]).
8.5. Usage with Encryption
Digest fields may expose details of encrypted payload when the
checksum is computed on the unencrypted data.
The checksum of an encrypted payload can change between different
messages depending on the encryption algorithm used; in those cases
its value could not be used to provide a proof of integrity "at rest"
unless the whole (e.g. encoded) content is persisted.
8.6. Algorithm Agility
The security properties of digest-algorithms are not fixed.
Algorithm Agility (see [RFC7696]) is achieved by providing
implementations with flexibility choose digest-algorithms from the
IANA Digest Algorithm Values registry in Section 9.1.
To help endpoints distinguish weaker algorithms from stronger ones,
this document adds to the IANA Digest Algorithm Values registry a new
"Status" field containing the most recent appraisal of the digest-
algorithm.
An endpoint might have a preference for algorithms, such as
preferring "standard" algorithms over "insecure" ones. Transition
from weak algorithms is supported by negotiation of digest-algorithm
using Want-Digest or Want-Content-Digest (see Section 5) or by
sending multiple representation-data-digest values from which the
receiver chooses. Endpoints are advised that sending multiple values
consumes resources, which may be wasted if the receiver ignores them
(see Section 3).
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8.7. Duplicate digest-algorithm in field value
An endpoint might receive multiple representation-data-digest values
(see Section 3) that use the same digest-algorithm with different or
identical digest-values. For example:
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=,
sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
A receiver is permitted to ignore any representation-data-digest
value, so validation of duplicates is left as an implementation
decision. Endpoints might select all, some, or none of the values
for checksum comparison and, based on the intersection of those
results, conditionally pass or fail digest validation.
8.8. Resource exhaustion
Digest fields validation consumes computational resources. In order
to avoid resource exhaustion, implementations can restrict validation
of the algorithm types, number of validations, or the size of
content.
9. IANA Considerations
9.1. Establish the HTTP Digest Algorithm Values Registry
This memo sets this specification to be the establishing document for
the HTTP Digest Algorithm Values (https://www.iana.org/assignments/
http-dig-alg/) registry.
IANA is asked to update the "Reference" for this registry to refer
this document and to inizialize the registry with the tokens defined
in Section 6.
This registry uses the Specification Required policy (Section 4.6 of
[RFC8126]).
9.2. Obsolete "contentMD5" token in Digest Algorithm
This memo adds the "contentMD5" token in the HTTP Digest Algorithm
Values (https://www.iana.org/assignments/http-dig-alg/) registry:
* Digest Algorithm: contentMD5
* Description: Section 5 of [RFC3230] defined the "contentMD5" token
to be used only in Want-Digest. This token is obsoleted and MUST
NOT be used.
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* Reference: Section 9.2 of this document, Section 5 of [RFC3230].
* Status: obsoleted
9.3. Changes Compared to RFC3230
The contentMD5 digest-algorithm token defined in Section 5 of
[RFC3230] has been added to the HTTP Digest Algorithm Values Registry
with the "obsoleted" status.
All digest-algorithms defined in [RFC3230] are now "insecure".
9.4. Changes Compared to RFC5843
The digest-algorithm tokens for "MD5", "SHA", "SHA-256", "SHA-512"
have been updated to lowercase.
The status of "MD5" and "SHA" has been updated to "insecure", and
their description has been modified accordingly.
9.5. Want-Digest Field Registration
This section registers the Want-Digest field in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
Field name: Want-Digest
Status: permanent
Specification document(s): Section 5 of this document
9.6. Digest Field Registration
This section registers the Digest field in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" [SEMANTICS].
Field name: Digest
Status: permanent
Specification document(s): Section 3 of this document
9.7. Want-Content-Digest Field Registration
This section registers the Want-Content-Digest field in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
Field name: Want-Content-Digest
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Status: permanent
Specification document(s): Section 5 of this document
9.8. Content-Digest Field Registration
This section registers the Content-Digest field in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].
Field name: Content-Digest
Status: permanent
Specification document(s): Section 4 of this document
10. References
10.1. Normative References
[CMU-836068]
Carnagie Mellon University, Software Engineering
Institute, "MD5 Vulnerable to collision attacks", 31
December 2008, <https://www.kb.cert.org/vuls/id/836068/>.
[IACR-2020-014]
Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", 5
January 2020, <https://eprint.iacr.org/2020/014.pdf>.
[NIST800-32]
National Institute of Standards and Technology, U.S.
Department of Commerce, "Introduction to Public Key
Technology and the Federal PKI Infrastructure", February
2001, <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
nistspecialpublication800-32.pdf>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/rfc/rfc1321>.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/rfc/rfc1950>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
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[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
<https://www.rfc-editor.org/rfc/rfc3174>.
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
RFC 3230, DOI 10.17487/RFC3230, January 2002,
<https://www.rfc-editor.org/rfc/rfc3230>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/rfc/rfc4960>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC5843] Bryan, A., "Additional Hash Algorithms for HTTP Instance
Digests", RFC 5843, DOI 10.17487/RFC5843, April 2010,
<https://www.rfc-editor.org/rfc/rfc5843>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/rfc/rfc6234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
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[SEMANTICS]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.
[UNIX] The Open Group, "The Single UNIX Specification, Version 2
- 6 Vol Set for UNIX 98", February 1997.
10.2. Informative References
[HTTP11] Fielding, R. T., Nottingham, M., and J. Reschke,
"HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
httpbis-messaging-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
messaging-19>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-header-structure-19, 3 June 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
header-structure-19>.
[I-D.thomson-http-mice]
Thomson, M. and J. Yasskin, "Merkle Integrity Content
Encoding", Work in Progress, Internet-Draft, draft-
thomson-http-mice-03, 13 August 2018,
<https://datatracker.ietf.org/doc/html/draft-thomson-http-
mice-03>.
[NO-MD5] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/rfc/rfc6151>.
[NO-SHA1] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/rfc/rfc6194>.
[PATCH] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
<https://www.rfc-editor.org/rfc/rfc5789>.
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[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/rfc/rfc2818>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/rfc/rfc7231>.
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
<https://www.rfc-editor.org/rfc/rfc7396>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/rfc/rfc7696>.
[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://www.rfc-editor.org/rfc/rfc7807>.
Appendix A. Resource Representation and Representation-Data
The following examples show how representation metadata, payload
transformations and method impacts on the message and content. When
the content contains non-printable characters (e.g. when it is
compressed) it is shown as a Base64-encoded string.
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
{"hello": "world"}
Figure 1: Request containing a JSON object without any content coding
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Encoding: gzip
H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
Figure 2: Request containing a gzip-encoded JSON object
Now the same content conveys a malformed JSON object, because the
request does not indicate a content coding.
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PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
Figure 3: Request containing malformed JSON
A Range-Request alters the content, conveying a partial
representation.
GET /entries/1234 HTTP/1.1
Host: foo.example
Range: bytes=1-7
Figure 4: Request for partial content
HTTP/1.1 206 Partial Content
Content-Encoding: gzip
Content-Type: application/json
Content-Range: bytes 1-7/18
iwgAla3RXA==
Figure 5: Partial response from a gzip-encoded representation
The method can also alter the content. For example, the response to
a HEAD request does not carry content.
HEAD /entries/1234 HTTP/1.1
Host: foo.example
Accept: application/json
Accept-Encoding: gzip
Figure 6: HEAD request
HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: gzip
Figure 7: Response to HEAD request (empty content)
Finally, the semantics of an HTTP response might decouple the
effective request URI from the enclosed representation. In the
example response below, the Content-Location header field indicates
that the enclosed representation refers to the resource available at
/authors/123, even though the request is directed to /authors/.
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POST /authors/ HTTP/1.1
Host: foo.example
Accept: application/json
Content-Type: application/json
{"author": "Camilleri"}
Figure 8: POST request
HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /authors/123
Location: /authors/123
{"id": "123", "author": "Camilleri"}
Figure 9: Response with Content-Location header
Appendix B. Examples of Unsolicited Digest
The following examples demonstrate interactions where a server
responds with a Digest or Content-Digest fields even though the
client did not solicit one using Want-Digest or Want-Content-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.
Checksum mechanisms defined in this document are media-type agnostic
and do not provide canonicalization algorithms for specific formats.
Examples are calculated inclusive of any space. While examples can
include both fields, Digest and Content-Digest can be returned
independently.
B.1. Server Returns Full Representation Data
In this example, the message content conveys complete representation
data, so Digest and Content-Digest have the same value.
GET /items/123 HTTP/1.1
Host: foo.example
Figure 10: GET request for an item
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HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Figure 11: Response with Content-Digest
B.2. Server Returns No Representation Data
In this example, a HEAD request is used to retrieve the checksum of a
resource.
The response Digest field-value is calculated over the JSON object
{"hello": "world"}, which is not shown because there is no payload
data. Content-Digest is computed on empty content.
HEAD /items/123 HTTP/1.1
Host: foo.example
Figure 12: HEAD request for an item
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
Figure 13: Response with both Content-Digest and Digest; empty
content
B.3. Server Returns Partial Representation Data
In this example, the client makes a range request and the server
responds with partial content. The Digest field-value represents the
entire JSON object {"hello": "world"}, while the Content-Digest
field-value is computed on the message content "hello".
GET /items/123 HTTP/1.1
Host: foo.example
Range: bytes=1-7
Figure 14: Request for partial content
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HTTP/1.1 206 Partial Content
Content-Type: application/json
Content-Range: bytes 1-7/18
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Digest: sha-256=Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=
"hello"
Figure 15: Partial response with both Content-Digest and Digest
B.4. Client and Server Provide Full Representation Data
The request contains a Digest field-value calculated on the enclosed
representation. It also includes an Accept-Encoding: br header field
that advertises the client supports Brotli encoding.
The response includes a Content-Encoding: br that indicates the
selected representation is Brotli-encoded. The Digest field-value is
therefore different compared to the request.
For presentation purposes, the response body is displayed as a
Base64-encoded string because it contains non-printable characters.
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Figure 16: PUT Request with Digest
HTTP/1.1 200 OK
Content-Type: application/json
Content-Location: /items/123
Content-Encoding: br
Content-Length: 22
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
Figure 17: Response with Digest of encoded response
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B.5. Client Provides Full Representation Data, Server Provides No
Representation Data
The request Digest field-value is calculated on the enclosed payload.
The response Digest field-value depends on the representation
metadata header fields, including Content-Encoding: br even when the
response does not contain content.
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Length: 18
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
HTTP/1.1 204 No Content
Content-Type: application/json
Content-Encoding: br
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
Figure 18: Empty response with Digest
B.6. Client and Server Provide Full Representation Data
The response contains two digest values using different algorithms.
As the response body contains non-printable characters, it is
displayed as a base64-encoded string.
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Figure 19: PUT Request with Digest
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HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: br
Content-Location: /items/123
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
sha-512=pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE6
0jBCwnMPyA/s3NF3ZO5oIWA7lf8ukk+\n5KJzm3p5og==
iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
Figure 20: Response with Digest of Encoded Content
B.7. POST Response does not Reference the Request URI
The request Digest field-value is computed on the enclosed
representation (see Section 7).
The representation enclosed in the response refers to the resource
identified by Content-Location (see Section 6.4.2 of [SEMANTICS]).
Digest is thus computed on the enclosed representation.
POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Figure 21: POST Request with Digest
HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Digest: sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=
{
"id": "123",
"title": "New Title"
}
Figure 22: Response with Digest of Resource
Note that a 204 No Content response without content but with the same
Digest field-value would have been legitimate too. In that case,
Content-Digest would have been computed on an empty content.
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B.8. POST Response Describes the Request Status
The request Digest field-value is computed on the enclosed
representation (see Section 7).
The representation enclosed in the response describes the status of
the request, so Digest is computed on that enclosed representation.
Response Digest has no explicit relation with the resource referenced
by Location.
POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Figure 23: POST Request with Digest
HTTP/1.1 201 Created
Content-Type: application/json
Digest: sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
Location: /books/123
{
"status": "created",
"id": "123",
"ts": 1569327729,
"instance": "/books/123"
}
Figure 24: Response with Digest of Representation
B.9. Digest with PATCH
This case is analogous to a POST request where the target resource
reflects the effective request URI.
The PATCH request uses the application/merge-patch+json media type
defined in [RFC7396].
Digest is calculated on the enclosed payload, which corresponds to
the patch document.
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The response Digest field-value is computed on the complete
representation of the patched resource.
PATCH /books/123 HTTP/1.1
Host: foo.example
Content-Type: application/merge-patch+json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Figure 25: PATCH Request with Digest
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=
{
"id": "123",
"title": "New Title"
}
Figure 26: Response with Digest of Representation
Note that a 204 No Content response without content but with the same
Digest field-value would have been legitimate too.
B.10. Error responses
In error responses, the representation-data does not necessarily
refer to the target resource. Instead, it refers to the
representation of the error.
In the following example, a client sends the same request from
Figure 25 to patch the resource located at /books/123. However, the
resource does not exist and the server generates a 404 response with
a body that describes the error in accordance with [RFC7807].
The response Digest field-value is computed on this enclosed
representation.
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HTTP/1.1 404 Not Found
Content-Type: application/problem+json
Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=
{
"title": "Not Found",
"detail": "Cannot PATCH a non-existent resource",
"status": 404
}
Figure 27: Response with Digest of Error Representation
B.11. Use with Trailer Fields and Transfer Coding
An origin server sends Digest as trailer field, so it can calculate
digest-value while streaming content and thus mitigate resource
consumption. The Digest field-value is the same as in Appendix B.1
because Digest is designed to be independent from the use of one or
more transfer codings (see Section 2).
GET /items/123 HTTP/1.1
Host: foo.example
Figure 28: GET Request
HTTP/1.1 200 OK
Content-Type: application/json
Transfer-Encoding: chunked
Trailer: Digest
8\r\n
{"hello"\r\n
8
: "world\r\n
2\r\n
"}\r\n
0\r\n
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Figure 29: Chunked Response with Digest
Appendix C. Examples of Want-Digest Solicited Digest
The following examples demonstrate interactions where a client
solicits a Digest using Want-Digest. The behavior of Content-Digest
and Want-Content-Digest is identical.
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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.
Checksum mechanisms described in this document are media-type
agnostic and do not provide canonicalization algorithms for specific
formats. Examples are calculated inclusive of any space.
C.1. Server Selects Client's Least Preferred Algorithm
The client requests a digest, preferring "sha". The server is free
to reply with "sha-256" anyway.
GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha-256;q=0.3, sha;q=1
Figure 30: GET Request with Want-Digest
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Figure 31: Response with Different Algorithm
C.2. Server Selects Algorithm Unsupported by Client
The client requests a only "sha" digest because that is the only
algorithm it supports. The server is not obliged to produce a
response containing a "sha" digest, it instead uses a different
algorithm.
GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1
Figure 32: GET Request with Want-Digest
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HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
{"hello": "world"}
Figure 33: Response with Unsupported Algorithm
C.3. Server Does Not Support Client Algorithm and Returns an Error
Appendix C.2 is an example where a server ignores the client's
preferred digest algorithm. Alternatively a server can also reject
the request and return an error.
In this example, the client requests a "sha" Digest, and the server
returns an error with problem details [RFC7807] contained in the
content. The problem details contain a list of the digest algorithms
that the server supports. This is purely an example, this
specification does not define any format or requirements for such
content.
GET /items/123 HTTP/1.1
Host: foo.example
Want-Digest: sha;q=1
Figure 34: GET Request with Want-Digest
HTTP/1.1 400 Bad Request
Content-Type: application/problem+json
{
"title": "Bad Request",
"detail": "Supported digest-algorithms: sha-256, sha-512",
"status": 400
}
Figure 35: Response advertising the supported algorithms
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.
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Thanks to Julian Reschke for his valuable contributions to this
document, and to the following contributors that have helped improve
this specification by reporting bugs, asking smart questions,
drafting or reviewing text, and evaluating open issues: Mike Bishop,
Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean
Turner, and Erik Wilde.
FAQ
_RFC Editor: Please remove this section before publication._
1. Why remove all references to content-md5?
Those were unnecessary to understanding and using this
specification.
2. Why remove references to instance manipulation?
Those were unnecessary for correctly using and applying the
specification. An example with Range Request is more than
enough. This document uses the term "partial representation"
which should group all those cases.
3. How to use Digest with PATCH method?
See Section 7.
4. Why remove references to delta-encoding?
Unnecessary for a correct implementation of this specification.
The revised specification can be nicely adapted to "delta
encoding", but all the references here to delta encoding don't
add anything to this RFC. Another job would be to refresh delta
encoding.
5. Why remove references to Digest Authentication?
This specification seems to me completely unrelated to Digest
Authentication but for the word "Digest".
6. What changes in Want-Digest?
The contentMD5 token defined in Section 5 of [RFC3230] is
deprecated by this document.
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-
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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. 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.
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?
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.
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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))
print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512, encoding=brotli.compress))
# Encoding | digest-algorithm | digest-value
# Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
# '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='
# Brotli | sha512 | b'pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE6'
# '0jBCwnMPyA/s3NF3ZO5oIWA7lf8ukk+\n5KJzm3p5og=='
Changes
_RFC Editor: Please remove this section before publication._
Since draft-ietf-httpbis-digest-headers-06
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* Remove id-sha-256 and id-sha-512 from the list of supported
algorithms #855
Since draft-ietf-httpbis-digest-headers-05
* Reboot digest-algorithm values registry #1567
* Add Content-Digest #1542
* Remove SRI section #1478
Since draft-ietf-httpbis-digest-headers-04
* Improve SRI section #1354
* About duplicate digest-algorithms #1221
* Improve security considerations #852
* md5 and sha deprecation references #1392
* Obsolete 3230 #1395
* Editorial #1362
Since draft-ietf-httpbis-digest-headers-03
* Reference semantics-12
* Detail encryption quirks
* Details on Algorithm agility #1250
* Obsolete parameters #850
Since draft-ietf-httpbis-digest-headers-02
* Deprecate SHA-1 #1154
* Avoid id-* with encrypted content
* Digest is independent from MESSAGING and HTTP/1.1 is not normative
#1215
* Identity is not a valid field value for content-encoding #1223
* Mention trailers #1157
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* Reference httpbis-semantics #1156
* Add contentMD5 as an obsoleted digest-algorithm #1249
* Use lowercase digest-algorithms names in the doc and in the
digest-algorithm IANA table.
Since draft-ietf-httpbis-digest-headers-01
* Digest of error responses is computed on the error representation-
data #1004
* Effect of HTTP semantics on payload and message body moved to
appendix #1122
* Editorial refactoring, moving headers sections up. #1109-#1112,
#1116, #1117, #1122-#1124
Since draft-ietf-httpbis-digest-headers-00
* Align title with document name
* Add id-sha-* algorithm examples #880
* Reference [RFC6234] and [RFC3174] instead of FIPS-1
* Deprecate MD5
* Obsolete ADLER-32 but don't forbid it #828
* Update CRC32C value in IANA table #828
* Use when acting on resources (POST, PATCH) #853
* Added Relationship with SRI, draft Use Cases #868, #971
* Warn about the implications of Content-Location
Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
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Lucas Pardue
Cloudflare
Email: lucaspardue.24.7@gmail.com
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