This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
EID 8158
Internet Engineering Task Force (IETF) R. Polli
Request for Comments: 9530 Team Digitale, Italian Government
Obsoletes: 3230 L. Pardue
Category: Standards Track Cloudflare
ISSN: 2070-1721 February 2024
Digest Fields
Abstract
This document defines HTTP fields that support integrity digests.
The Content-Digest field can be used for the integrity of HTTP
message content. The Repr-Digest field can be used for the integrity
of HTTP representations. Want-Content-Digest and Want-Repr-Digest
can be used to indicate a sender's interest and preferences for
receiving the respective Integrity fields.
This document obsoletes RFC 3230 and the Digest and Want-Digest HTTP
fields.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9530.
Copyright Notice
Copyright (c) 2024 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Document Structure
1.2. Concept Overview
1.3. Obsoleting RFC 3230
1.4. Notational Conventions
2. The Content-Digest Field
3. The Repr-Digest Field
3.1. Using Repr-Digest in State-Changing Requests
3.2. Repr-Digest and Content-Location in Responses
4. Integrity Preference Fields
5. Hash Algorithm Considerations and Registration
6. Security Considerations
6.1. HTTP Messages Are Not Protected in Full
6.2. End-to-End Integrity
6.3. Usage in Signatures
6.4. Usage in Trailer Fields
6.5. Variations within Content-Encoding
6.6. Algorithm Agility
6.7. Resource Exhaustion
7. IANA Considerations
7.1. HTTP Field Name Registration
7.2. Creation of the Hash Algorithms for HTTP Digest Fields
Registry
7.3. Deprecate the Hypertext Transfer Protocol (HTTP) Digest
Algorithm Values Registry
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Resource Representation and Representation Data
Appendix B. Examples of Unsolicited Digest
B.1. Server Returns Full Representation Data
B.2. Server Returns No Representation Data
B.3. Server Returns Partial Representation Data
B.4. Client and Server Provide Full Representation Data
B.5. Client Provides Full Representation Data and Server
Provides No Representation Data
B.6. Client and Server Provide Full Representation Data
B.7. POST Response Does Not Reference the Request URI
B.8. POST Response Describes the Request Status
B.9. Digest with PATCH
B.10. Error Responses
B.11. Use with Trailer Fields and Transfer Coding
Appendix C. Examples of Want-Repr-Digest Solicited Digest
C.1. Server Selects Client's Least Preferred Algorithm
C.2. Server Selects Algorithm Unsupported by Client
C.3. Server Does Not Support Client Algorithm and Returns an
Error
Appendix D. Sample Digest Values
Appendix E. Migrating from RFC 3230
Acknowledgements
Authors' Addresses
1. Introduction
HTTP does not define the means to protect the data integrity of
content or representations. When HTTP messages are transferred
between endpoints, lower-layer features or properties such as TCP
checksums or TLS records [TLS] can provide some integrity protection.
However, transport-oriented integrity provides a limited utility
because it is opaque to the application layer and only covers the
extent of a single connection. HTTP messages often travel over a
chain of separate connections. In between connections, there is a
possibility for data corruption. An HTTP integrity mechanism can
provide the means for endpoints, or applications using HTTP, to
detect data corruption and make a choice about how to act on it. An
example use case is to aid fault detection and diagnosis across
system boundaries.
This document defines two digest integrity mechanisms for HTTP.
First, content integrity, which acts on conveyed content (Section 6.4
of [HTTP]). Second, representation data integrity, which acts on
representation data (Section 8.1 of [HTTP]). This supports advanced
use cases, such as validating the integrity of a resource that was
reconstructed from parts retrieved using multiple requests or
connections.
This document obsoletes [RFC3230] and therefore the Digest and Want-
Digest HTTP fields; see Section 1.3.
1.1. Document Structure
This document is structured as follows:
* New request and response header and trailer field definitions.
- Section 2 (Content-Digest),
- Section 3 (Repr-Digest), and
- Section 4 (Want-Content-Digest and Want-Repr-Digest).
* Considerations specific to representation data integrity.
- Section 3.1 (State-changing requests),
- Section 3.2 (Content-Location),
- Appendix A contains worked examples of representation data in
message exchanges, and
- Appendixes B and C contain worked examples of Repr-Digest and
Want-Repr-Digest fields in message exchanges.
* Section 5 presents hash algorithm considerations and defines
registration procedures for future entries.
1.2. Concept Overview
The HTTP fields defined in this document can be used for HTTP
integrity. Senders choose a hashing algorithm and calculate a digest
from an input related to the HTTP message. The algorithm identifier
and digest are transmitted in an HTTP field. Receivers can validate
the digest for integrity purposes. Hashing algorithms are registered
in the "Hash Algorithms for HTTP Digest Fields" registry (see
Section 7.2).
Selecting the data on which digests are calculated depends on the use
case of the HTTP messages. This document provides different fields
for HTTP representation data and HTTP content.
There are use cases where a simple digest of the HTTP content bytes
is required. The Content-Digest request and response header and
trailer field is defined to support digests of content (Section 6.4
of [HTTP]); see Section 2.
For more advanced use cases, the Repr-Digest request and response
header and trailer field (Section 3) is defined. It contains a
digest value computed by applying a hashing algorithm to selected
representation data (Section 8.1 of [HTTP]). Basing Repr-Digest on
the selected representation makes it straightforward to apply it to
use cases where the message content requires some sort of
manipulation to be considered as representation of the resource or
the content conveys a partial representation of a resource, such as
range requests (see Section 14 of [HTTP]).
Content-Digest and Repr-Digest support hashing algorithm agility.
The Want-Content-Digest and Want-Repr-Digest fields allow endpoints
to express interest in Content-Digest and Repr-Digest, respectively,
and to express algorithm preferences in either.
Content-Digest and Repr-Digest are collectively termed "Integrity
fields". Want-Content-Digest and Want-Repr-Digest are collectively
termed "Integrity preference fields".
Integrity fields are tied to the Content-Encoding and Content-Type
header fields. Therefore, a given resource may have multiple
different digest values when transferred with HTTP.
Integrity fields apply to HTTP message content or HTTP
representations. They do not apply to 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. For example, HTTP Message
Signatures [SIGNATURES] could be used to sign Integrity fields, thus
providing coverage for HTTP content or representation data.
This specification does not define means for authentication,
authorization, or privacy.
1.3. Obsoleting RFC 3230
[RFC3230] defined the Digest and Want-Digest HTTP fields for HTTP
integrity. It also coined the terms "instance" and "instance
manipulation" in order to explain concepts, such as selected
representation data (Section 8.1 of [HTTP]), that are now more
universally defined and implemented as HTTP semantics.
Experience has shown that implementations of [RFC3230] have
interpreted the meaning of "instance" inconsistently, leading to
interoperability issues. The most common issue relates to the
mistake of calculating the digest using (what we now call) message
content, rather than using (what we now call) representation data as
was originally intended. Interestingly, time has also shown that a
digest of message content can be beneficial for some use cases, so it
is difficult to detect if non-conformance to [RFC3230] is intentional
or unintentional.
In order to address potential inconsistencies and ambiguity across
implementations of Digest and Want-Digest, this document obsoletes
[RFC3230]. The Integrity fields (Sections 2 and 3) and Integrity
preference fields (Section 4) defined in this document are better
aligned with current HTTP semantics and have names that more clearly
articulate the intended usages.
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]. This includes the rules CR (carriage return), LF (line
feed), and CRLF (CR LF).
This document uses the following terminology from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing: Boolean, Byte
Sequence, Dictionary, Integer, and List.
The definitions "representation", "selected representation",
"representation data", "representation metadata", "user agent", and
"content" in this document are to be interpreted as described in
[HTTP].
This document uses the line folding strategies described in
[FOLDING].
Hashing algorithm names respect the casing used in their definition
document (e.g., SHA-1, CRC32c).
HTTP messages indicate hashing algorithms using an Algorithm Key
(algorithms). Where the document refers to an Algorithm Key in
prose, it is quoted (e.g., "sha", "crc32c").
The term "checksum" describes the output of applying an algorithm to
a sequence of bytes, whereas "digest" is only used in relation to the
value contained in the fields.
"Integrity fields" is the collective term for Content-Digest and
Repr-Digest.
"Integrity preference fields" is the collective term for Want-Repr-
Digest and Want-Content-Digest.
2. The Content-Digest Field
The Content-Digest HTTP field can be used in requests and responses
to communicate digests that are calculated using a hashing algorithm
applied to the actual message content (see Section 6.4 of [HTTP]).
It is a Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]), where
each:
* key conveys the hashing algorithm (see Section 5) used to compute
the digest;
* value is a Byte Sequence (Section 3.3.5 of [STRUCTURED-FIELDS])
that conveys an encoded version of the byte output produced by the
digest calculation.
For example:
NOTE: '\' line wrapping per RFC 8792
Content-Digest: \
sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:
The Dictionary type can be used, for example, to attach multiple
digests calculated using different hashing algorithms in order to
support a population of endpoints with different or evolving
capabilities. Such an approach could support transitions away from
weaker algorithms (see Section 6.6).
NOTE: '\' line wrapping per RFC 8792
Content-Digest: \
sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\
sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:
A recipient MAY ignore any or all digests. Application-specific
behavior or local policy MAY set additional constraints on the
processing and validation practices of the conveyed digests. The
security considerations cover some of the issues related to ignoring
digests (see Section 6.6) and validating multiple digests (see
Section 6.7).
A sender MAY send a digest without knowing whether the recipient
supports a given hashing algorithm. A sender MAY send a digest if it
knows 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 [HTTP].
3. The Repr-Digest Field
The Repr-Digest HTTP field can be used in requests and responses to
communicate digests that are calculated using a hashing algorithm
applied to the entire selected representation data (see Section 8.1
of [HTTP]).
Representations take into account the effect of the HTTP semantics on
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 [HTTP/1.1]). To help
illustrate HTTP representation concepts, several examples are
provided in Appendix A.
When a message has no representation data, it is still possible to
assert that no representation data was sent by computing the digest
on an empty string (see Section 6.3).
Repr-Digest is a Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]),
where each:
* key conveys the hashing algorithm (see Section 5) used to compute
the digest;
* value is a Byte Sequence that conveys an encoded version of the
byte output produced by the digest calculation.
For example:
NOTE: '\' line wrapping per RFC 8792
Repr-Digest: \
sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:
The Dictionary type can be used to attach multiple digests calculated
using different hashing algorithms in order to support a population
of endpoints with different or evolving capabilities. Such an
approach could support transitions away from weaker algorithms (see
Section 6.6).
NOTE: '\' line wrapping per RFC 8792
Repr-Digest: \
sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\
sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:
A recipient MAY ignore any or all digests. Application-specific
behavior or local policy MAY set additional constraints on the
processing and validation practices of the conveyed digests. The
security considerations cover some of the issues related to ignoring
digests (see Section 6.6) and validating multiple digests (see
Section 6.7).
A sender MAY send a digest without knowing whether the recipient
supports a given hashing algorithm. A sender MAY send a digest if it
knows the recipient will ignore it.
Repr-Digest can be sent in a trailer section. In this case, Repr-
Digest MAY be merged into the header section; see Section 6.5.1 of
[HTTP].
3.1. Using Repr-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 3).
In responses,
* if the representation describes the status of the request, Repr-
Digest MUST be computed on the enclosed representation (see
Appendix B.8);
* if there is a referenced resource, Repr-Digest MUST be computed on
the selected representation of the referenced resource even if
that is different from the target resource. This might or might
not result in computing Repr-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 [HTTP]). In contrast, the Location header field does
not affect Repr-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 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.
3.2. Repr-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 Repr-Digest is computed accordingly. An example is
given in Appendix B.7.
4. Integrity Preference Fields
Senders can indicate their interest in Integrity fields and hashing
algorithm preferences using the Want-Content-Digest or Want-Repr-
Digest HTTP fields. These can be used in both requests and
responses.
Want-Content-Digest indicates that the sender would like to receive
(via the Content-Digest field) a content digest on messages
associated with the request URI and representation metadata. Want-
Repr-Digest indicates that the sender would like to receive (via the
Repr-Digest field) a representation digest on messages associated
with the request URI and representation metadata.
If Want-Content-Digest or Want-Repr-Digest are used in a response, it
indicates that the server would like the client to provide the
respective Integrity field on future requests.
Integrity preference fields are only a hint. The receiver of the
field can ignore it and send an Integrity field using any algorithm
or omit the field entirely; for example, see Appendix C.2. It is not
a protocol error if preferences are ignored. Applications that use
Integrity fields and Integrity preferences can define expectations or
constraints that operate in addition to this specification. Ignored
preferences are an application-specific concern.
Want-Content-Digest and Want-Repr-Digest are of type Dictionary where
each:
* key conveys the hashing algorithm (see Section 5);
* value is an Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) that
conveys an ascending, relative, weighted preference. It must be
in the range 0 to 10 inclusive. 1 is the least preferred, 10 is
the most preferred, and a value of 0 means "not acceptable".
Examples:
Want-Repr-Digest: sha-256=1
Want-Repr-Digest: sha-512=3, sha-256=10, unixsum=0
Want-Content-Digest: sha-256=1
Want-Content-Digest: sha-512=3, sha-256=10, unixsum=0
5. Hash Algorithm Considerations and Registration
There are a wide variety of hashing algorithms that can be used for
the purposes of integrity. The choice of algorithm depends on
several factors such as the integrity use case, implementation needs
or constraints, or application design and workflows.
An initial set of algorithms will be registered with IANA in the
"Hash Algorithms for HTTP Digest Fields" registry; see Section 7.2.
Additional algorithms can be registered in accordance with the
policies set out in this section.
Each algorithm has a status field that is intended to provide an aid
to implementation selection.
Algorithms with a status value of "Active" are suitable for many
purposes and it is RECOMMENDED that applications use these
algorithms. These can be used in adversarial situations where hash
functions might need to provide resistance to collision, first-
preimage, and second-preimage attacks. For adversarial situations,
selection of the acceptable "Active" algorithms will depend on the
level of protection the circumstances demand. More considerations
are presented in Section 6.6.
Algorithms with a status value of "Deprecated" either provide none of
these properties or are known to be weak (see [NO-MD5] and [NO-SHA]).
These algorithms MAY be used to preserve integrity against
corruption, but MUST NOT be used in a potentially adversarial
setting, for example, when signing Integrity fields' values for
authenticity. Permitting the use of these algorithms can help some
applications (such as those that previously used [RFC3230], are
migrating to this specification (Appendix E), and have existing
stored collections of computed digest values) avoid undue operational
overhead caused by recomputation using other more-secure algorithms.
Such applications are not exempt from the requirements in this
section. Furthermore, applications without such legacy or history
ought to follow the guidance for using algorithms with the status
value "Active".
Discussion of algorithm agility is presented in Section 6.6.
Registration requests for the "Hash Algorithms for HTTP Digest
Fields" registry use the Specification Required policy (Section 4.6
of [RFC8126]). Requests should use the following template:
Algorithm Key: The Structured Fields key value used in Content-
Digest, Repr-Digest, Want-Content-Digest, or Want-Repr-Digest
field Dictionary member keys.
Status: The status of the algorithm. The options are:
"Active": Algorithms without known problems
"Provisional": Unproven algorithms
"Deprecated": Deprecated or insecure algorithms
Description: A short description of the algorithm.
Reference(s): Pointer(s) to the primary document(s) defining the
Algorithm Key and technical details of the algorithm.
When reviewing registration requests, the designated expert(s) should
pay attention to the requested status. The status value should
reflect standardization status and the broad opinion of relevant
interest groups such as the IETF or security-related Standards
Development Organizations (SDOs). The "Active" status is not
suitable for an algorithm that is known to be weak, broken, or
experimental. If a registration request attempts to register such an
algorithm as "Active", the designated expert(s) should suggest an
alternative status of "Deprecated" or "Provisional".
When reviewing registration requests, the designated expert(s) cannot
use a status of "Deprecated" or "Provisional" as grounds for
rejection.
Requests to update or change the fields in an existing registration
are permitted. For example, this could allow for the transition of
an algorithm status from "Active" to "Deprecated" as the security
environment evolves.
6. Security Considerations
6.1. HTTP Messages Are Not Protected in Full
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 corruption.
Integrity fields are not intended to be a general protection against
malicious tampering with HTTP messages. In the absence of additional
security mechanisms, an on-path malicious actor can either remove a
digest value entirely or substitute it with a new digest value
computed over manipulated representation data or content. This
attack can be mitigated by combining mechanisms described in this
document with other approaches such as Transport Layer Security (TLS)
or digital signatures (for example, HTTP Message Signatures
[SIGNATURES]).
6.2. End-to-End Integrity
Integrity fields can help detect representation data or content
modification due to implementation errors, undesired "transforming
proxies" (see Section 7.7 of [HTTP]), 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 a user agent can validate that resource retrieval succeeded
before handing off to an HTML parser, video player, etc., for
parsing.
Note that using these mechanisms 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 6.3.
6.3. Usage in Signatures
Digital signatures are widely used together with checksums to provide
the certain identification of the origin of a message [FIPS186-5].
Such signatures can protect one or more HTTP fields and there are
additional considerations when Integrity fields are included in this
set.
There are no restrictions placed on the type or format of digital
signature that Integrity fields can be used with. One possible
approach is to combine them with HTTP Message Signatures
[SIGNATURES].
Digests explicitly depend on the "representation metadata" (e.g., the
values of Content-Type, Content-Encoding, etc.). A signature that
protects Integrity fields 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 3.2.
Signatures are likely to be deemed an adversarial setting when
applying Integrity fields; see Section 5. Repr-Digest offers an
interesting possibility when combined with signatures. In the
scenario where there is no content to send, the digest of an empty
string can be included in the message and, if signed, can help the
recipient detect if content was added either as a result of accident
or purposeful manipulation. The opposite scenario is also supported;
including an Integrity field for content and signing it can help a
recipient detect where the content was removed.
Any mangling of Integrity fields might affect signature validation.
Examples of such mangling include de-duplicating digests or combining
different field values (see Section 5.2 of [HTTP]).
6.4. Usage in Trailer Fields
Before sending Integrity fields in a trailer section, the sender
should consider that intermediaries are explicitly allowed to drop
any trailer (see Section 6.5.2 of [HTTP]).
When Integrity 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.
One of the benefits of using Integrity fields in a trailer section is
that it allows hashing of bytes as they are sent. However, it is
possible to design a hashing algorithm that requires processing of
content in such a way that would negate these benefits. For example,
Merkle Integrity Content Encoding [MICE] requires content to be
processed in reverse order. This means the complete data needs to be
available, which means there is negligible processing difference in
sending an Integrity field in a header versus a trailer section.
6.5. Variations within Content-Encoding
Content coding mechanisms can support different encoding parameters,
meaning that the same input content can produce different outputs.
For example, GZIP supports multiple compression levels. Such
encoding parameters are generally not communicated as representation
metadata. For instance, different compression levels would all use
the same "Content-Encoding: gzip" field. Other examples include
where encoding relies on nonces or timestamps, such as the aes128gcm
content coding defined in [RFC8188].
Since it is possible for there to be variation within content coding,
the checksum conveyed by the Integrity fields cannot be used to
provide a proof of integrity "at rest" unless the whole content is
persisted.
6.6. Algorithm Agility
The security properties of hashing algorithms are not fixed.
Algorithm agility (see [RFC7696]) is achieved by providing
implementations with flexibility to choose hashing algorithms from
the IANA Hash Algorithms for HTTP Digest Fields registry; see
Section 7.2.
Transition from weak algorithms is supported by negotiation of
hashing algorithm using Want-Content-Digest or Want-Repr-Digest (see
Section 4) or by sending multiple digests from which the receiver
chooses. A receiver that depends on a digest for security will be
vulnerable to attacks on the weakest algorithm it is willing to
accept. Endpoints are advised that sending multiple values consumes
resources that may be wasted if the receiver ignores them (see
Section 3).
While algorithm agility allows the migration to stronger algorithms,
it does not prevent the use of weaker algorithms. Integrity fields
do not provide any mitigations for downgrade or substitution attacks
(see Section 1 of [RFC6211]) of the hashing algorithm. To protect
against such attacks, endpoints could restrict their set of supported
algorithms to stronger ones and protect the fields' values by using
TLS and/or digital signatures.
6.7. Resource Exhaustion
Integrity field validation consumes computational resources. In
order to avoid resource exhaustion, implementations can restrict
validation of the algorithm types, the number of validations, or the
size of content. In these cases, skipping validation entirely or
ignoring validation failure of a more-preferred algorithm leaves the
possibility of a downgrade attack (see Section 6.6).
7. IANA Considerations
7.1. HTTP Field Name Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name
Registry" [HTTP] as shown in the table below:
+=====================+===========+========================+
| Field Name | Status | Reference |
+=====================+===========+========================+
| Content-Digest | permanent | Section 2 of RFC 9530 |
+---------------------+-----------+------------------------+
| Repr-Digest | permanent | Section 3 of RFC 9530 |
+---------------------+-----------+------------------------+
| Want-Content-Digest | permanent | Section 4 of RFC 9530 |
+---------------------+-----------+------------------------+
| Want-Repr-Digest | permanent | Section 4 of RFC 9530 |
+---------------------+-----------+------------------------+
| Digest | obsoleted | [RFC3230], Section 1.3 |
| | | of RFC 9530 |
+---------------------+-----------+------------------------+
| Want-Digest | obsoleted | [RFC3230], Section 1.3 |
| | | of RFC 9530 |
+---------------------+-----------+------------------------+
Table 1: Hypertext Transfer Protocol (HTTP) Field Name
Registry Update
7.2. Creation of the Hash Algorithms for HTTP Digest Fields Registry
IANA has created the new "Hash Algorithms for HTTP Digest Fields"
registry at <https://www.iana.org/assignments/http-digest-hash-alg/>
and populated it with the entries in Table 2. The procedure for new
registrations is provided in Section 5.
+===========+============+============================+============+
| Algorithm | Status | Description | Reference |
| Key | | | |
+===========+============+============================+============+
| sha-512 | Active | The SHA-512 algorithm. | [RFC6234], |
| | | | [RFC4648], |
| | | | RFC 9530 |
+-----------+------------+----------------------------+------------+
| sha-256 | Active | The SHA-256 algorithm. | [RFC6234], |
| | | | [RFC4648], |
| | | | RFC 9530 |
+-----------+------------+----------------------------+------------+
| md5 | Deprecated | The MD5 algorithm. It is | [RFC1321], |
| | | vulnerable to collision | [RFC4648], |
| | | attacks; see [NO-MD5] and | RFC 9530 |
| | | [CMU-836068] | |
+-----------+------------+----------------------------+------------+
| sha | Deprecated | The SHA-1 algorithm. It | [RFC3174], |
| | | is vulnerable to collision | [RFC4648], |
| | | attacks; see [NO-SHA] and | [RFC6234], |
| | | [IACR-2020-014] | RFC 9530 |
+-----------+------------+----------------------------+------------+
| unixsum | Deprecated | The algorithm used by the | [RFC4648], |
| | | UNIX "sum" command. | [RFC6234], |
| | | | [UNIX], |
| | | | RFC 9530 |
+-----------+------------+----------------------------+------------+
| unixcksum | Deprecated | The algorithm used by the | [RFC4648], |
| | | UNIX "cksum" command. | [RFC6234], |
| | | | [UNIX], |
| | | | RFC 9530 |
+-----------+------------+----------------------------+------------+
| adler | Deprecated | The ADLER32 algorithm. | [RFC1950], |
| | | | RFC 9530 |
+-----------+------------+----------------------------+------------+
| crc32c | Deprecated | The CRC32c algorithm. | Appendix A |
| | | | of |
| | | | [RFC9260], |
| | | | RFC 9530 |
+-----------+------------+----------------------------+------------+
Table 2: Initial Hash Algorithms
7.3. Deprecate the Hypertext Transfer Protocol (HTTP) Digest Algorithm
Values Registry
IANA has deprecated the "Hypertext Transfer Protocol (HTTP) Digest
Algorithm Values" registry at <https://www.iana.org/assignments/http-
dig-alg/> and replaced the note on that registry with the following
text:
| This registry is deprecated since it lists the algorithms that can
| be used with the Digest and Want-Digest fields defined in
| [RFC3230], which has been obsoleted by RFC 9530. While
| registration is not closed, new registrations are encouraged to
| use the Hash Algorithms for HTTP Digest Fields
| (https://www.iana.org/assignments/http-digest-hash-alg/) registry
| instead.
8. References
8.1. Normative References
[FOLDING] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
"Handling Long Lines in Content of Internet-Drafts and
RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
<https://www.rfc-editor.org/info/rfc8792>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC1950] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/info/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/info/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/info/rfc3174>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[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/info/rfc5234>.
[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/info/rfc6234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/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/info/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/info/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
8.2. Informative References
[CMU-836068]
Carnegie Mellon University, Software Engineering
Institute, "MD5 vulnerable to collision attacks", December
2008, <https://www.kb.cert.org/vuls/id/836068/>.
[FIPS186-5]
National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", FIPS PUB 186-5,
DOI 10.6028/NIST.FIPS.186-5, February 2023,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-5.pdf>.
[HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.
[IACR-2020-014]
Leurent, G. and T. Peyrin, "SHA-1 is a Shambles", January
2020, <https://eprint.iacr.org/2020/014.pdf>.
[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/info/rfc6151>.
[NO-SHA] 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/info/rfc6194>.
[PATCH] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
<https://www.rfc-editor.org/info/rfc5789>.
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
RFC 3230, DOI 10.17487/RFC3230, January 2002,
<https://www.rfc-editor.org/info/rfc3230>.
[RFC6211] Schaad, J., "Cryptographic Message Syntax (CMS) Algorithm
Identifier Protection Attribute", RFC 6211,
DOI 10.17487/RFC6211, April 2011,
<https://www.rfc-editor.org/info/rfc6211>.
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
<https://www.rfc-editor.org/info/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/info/rfc7696>.
[RFC8188] Thomson, M., "Encrypted Content-Encoding for HTTP",
RFC 8188, DOI 10.17487/RFC8188, June 2017,
<https://www.rfc-editor.org/info/rfc8188>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/info/rfc9260>.
[RFC9457] Nottingham, M., Wilde, E., and S. Dalal, "Problem Details
for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023,
<https://www.rfc-editor.org/info/rfc9457>.
[SIGNATURES]
Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
Message Signatures", RFC 9421, DOI 10.17487/RFC9421,
February 2024, <https://www.rfc-editor.org/info/rfc9421>.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[UNIX] The Open Group, "The Single UNIX Specification, Version 2
- 6 Vol Set for UNIX 98", January 1998.
Appendix A. Resource Representation and Representation Data
The following examples show how representation metadata, content
transformations, and methods impact the message and content. These
examples a not exhaustive.
Unless otherwise indicated, the examples are based on the JSON object
{"hello": "world"} followed by an LF. When the content contains non-
printable characters (e.g., when it is encoded), it is shown as a
sequence of hex-encoded bytes.
Consider a client that wishes to upload a JSON object using the PUT
method. It could do this using the application/json Content-Type
without any content coding.
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Length: 19
{"hello": "world"}
Figure 1: Request Containing a JSON Object without Any Content Coding
However, the use of content coding is quite common. The client could
also upload the same data with a GZIP coding (Section 8.4.1.3 of
[HTTP]). Note that in this case, the Content-Length contains a
larger value due to the coding overheads.
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Encoding: gzip
Content-Length: 39
1F 8B 08 00 88 41 37 64 00 FF
AB 56 CA 48 CD C9 C9 57 B2 52
50 2A CF 2F CA 49 51 AA E5 02
00 D9 E4 31 E7 13 00 00 00
Figure 2: Request Containing a GZIP-Encoded JSON Object
Sending the GZIP-coded data without indicating it via Content-
Encoding means that the content is malformed. In this case, the
server can reply with an error.
PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Length: 39
1F 8B 08 00 88 41 37 64 00 FF
AB 56 CA 48 CD C9 C9 57 B2 52
50 2A CF 2F CA 49 51 AA E5 02
00 D9 E4 31 E7 13 00 00 00
Figure 3: Request Containing Malformed JSON
HTTP/1.1 400 Bad Request
Figure 4: An Error Response for Malformed Content
A Range-Request affects the transferred message content. In this
example, the client is accessing the resource at /entries/1234, which
is the JSON object {"hello": "world"} followed by an LF. However,
the client has indicated a preferred content coding and a specific
byte range.
GET /entries/1234 HTTP/1.1
Host: foo.example
Accept-Encoding: gzip
Range: bytes=1-7
Figure 5: Request for Partial Content
The server satisfies the client request by responding with a partial
representation (equivalent to the first 10 bytes of the JSON object
displayed in whole in Figure 2).
HTTP/1.1 206 Partial Content
Content-Encoding: gzip
Content-Type: application/json
Content-Range: bytes 0-9/39
1F 8B 08 00 A5 B4 BD 62 02 FF
Figure 6: Partial Response from a GZIP-Encoded Representation
Aside from content coding or range requests, the method can also
affect the transferred message content. For example, the response to
a HEAD request does not carry content, but this example case includes
Content-Length; see Section 8.6 of [HTTP].
HEAD /entries/1234 HTTP/1.1
Host: foo.example
Accept: application/json
Accept-Encoding: gzip
Figure 7: HEAD Request
HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: gzip
Content-Length: 39
Figure 8: Response to HEAD Request (Empty Content)
Finally, the semantics of a response might decouple the target URI
from the enclosed representation. In the example below, the client
issues a POST request directed to /authors/, but the response
includes a Content-Location header field indicating that the enclosed
representation refers to the resource available at /authors/123.
Note that Content-Length is not sent in this example.
POST /authors/ HTTP/1.1
Host: foo.example
Accept: application/json
Content-Type: application/json
{"author": "Camilleri"}
Figure 9: POST Request
HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /authors/123
Location: /authors/123
{"id": "123", "author": "Camilleri"}
Figure 10: Response with Content-Location Header
Appendix B. Examples of Unsolicited Digest
The following examples demonstrate interactions where a server
responds with a Content-Digest or Repr-Digest field, even though the
client did not solicit one using Want-Content-Digest or Want-Repr-
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 two spaces
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, Content-Digest and Repr-Digest can be returned
independently.
B.1. Server Returns Full Representation Data
In this example, the message content conveys complete representation
data. This means that in the response, Content-Digest and Repr-
Digest are both computed over the JSON object {"hello": "world"}
followed by an LF; thus, they have the same value.
GET /items/123 HTTP/1.1
Host: foo.example
Figure 11: GET Request for an Item
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 19
Content-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
{"hello": "world"}
Figure 12: Response with Identical Repr-Digest and 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 Content-Digest field-value is computed on empty content.
Repr-Digest is calculated over the JSON object {"hello": "world"}
followed by an LF, which is not shown because there is no content.
HEAD /items/123 HTTP/1.1
Host: foo.example
Figure 13: HEAD Request for an Item
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Content-Digest: \
sha-256=:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=:
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
Figure 14: Response with Both Content-Digest
and Repr-Digest (Empty Content)
EID 8158 (Verified) is as follows:Section: B.2
Original Text:
Figure 14: Response with Both Content-Digest
and Digest (Empty Content)
Corrected Text:
Figure 14: Response with Both Content-Digest
and Repr-Digest (Empty Content)
Notes:
Minor Editorial correction
B.3. Server Returns Partial Representation Data
In this example, the client makes a range request and the server
responds with partial content.
GET /items/123 HTTP/1.1
Host: foo.example
Range: bytes=10-18
Figure 15: Request for Partial Content
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 206 Partial Content
Content-Type: application/json
Content-Range: bytes 10-18/19
Content-Digest: \
sha-256=:jjcgBDWNAtbYUXI37CVG3gRuGOAjaaDRGpIUFsdyepQ=:
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
"world"}
Figure 16: Partial Response with Both Content-Digest and Repr-Digest
In the response message above, note that the Repr-Digest and Content-
Digests are different. The Repr-Digest field-value is calculated
across the entire JSON object {"hello": "world"} followed by an LF,
and the field appears as follows:
NOTE: '\' line wrapping per RFC 8792
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
However, since the message content is constrained to bytes 10-18, the
Content-Digest field-value is calculated over the sequence "world"}
followed by an LF, thus resulting in the following:
NOTE: '\' line wrapping per RFC 8792
Content-Digest: \
sha-256=:jjcgBDWNAtbYUXI37CVG3gRuGOAjaaDRGpIUFsdyepQ=:
B.4. Client and Server Provide Full Representation Data
The request contains a Repr-Digest field-value calculated on the
enclosed representation. It also includes an Accept-Encoding: br
header field that advertises that the client supports Brotli
encoding.
The response includes a Content-Encoding: br that indicates the
selected representation is Brotli-encoded. The Repr-Digest field-
value is therefore different compared to the request.
For presentation purposes, the response body is displayed as a
sequence of hex-encoded bytes because it contains non-printable
characters.
NOTE: '\' line wrapping per RFC 8792
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
{"hello": "world"}
Figure 17: PUT Request with Digest
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Content-Location: /items/123
Content-Encoding: br
Content-Length: 23
Repr-Digest: \
sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:
8B 08 80 7B 22 68 65 6C 6C 6F
22 3A 20 22 77 6F 72 6C 64 22
7D 0A 03
Figure 18: Response with Digest of Encoded Response
B.5. Client Provides Full Representation Data and Server Provides No
Representation Data
The request Repr-Digest field-value is calculated on the enclosed
content, which is the JSON object {"hello": "world"} followed by an
LF.
The response Repr-Digest field-value depends on the representation
metadata header fields, including Content-Encoding: br, even when the
response does not contain content.
NOTE: '\' line wrapping per RFC 8792
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Length: 19
Accept-Encoding: br
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:
{"hello": "world"}
HTTP/1.1 204 No Content
Content-Type: application/json
Content-Encoding: br
Repr-Digest: sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:
Figure 19: Empty Response with Digest
B.6. Client and Server Provide Full Representation Data
The response contains two digest values using different algorithms.
For presentation purposes, the response body is displayed as a
sequence of hex-encoded bytes because it contains non-printable
characters.
NOTE: '\' line wrapping per RFC 8792
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:
{"hello": "world"}
Figure 20: PUT Request with Digest
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: br
Content-Location: /items/123
Repr-Digest: \
sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\
sha-512=:db7fdBbgZMgX1Wb2MjA8zZj+rSNgfmDCEEXM8qLWfpfoNY0sCpHAzZbj\
09X1/7HAb7Od5Qfto4QpuBsFbUO3dQ==:
8B 08 80 7B 22 68 65 6C 6C 6F
22 3A 20 22 77 6F 72 6C 64 22
7D 0A 03
Figure 21: Response with Digest of Encoded Content
B.7. POST Response Does Not Reference the Request URI
The request Repr-Digest field-value is computed on the enclosed
representation (see Section 3.1), which is the JSON object {"title":
"New Title"} followed by an LF.
The representation enclosed in the response is a multiline JSON
object followed by an LF. It refers to the resource identified by
Content-Location (see Section 6.4.2 of [HTTP]); thus, an application
can use Repr-Digest in association with the resource referenced by
Content-Location.
POST /books HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:
{"title": "New Title"}
Figure 22: POST Request with Digest
HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Repr-Digest: sha-256=:uVSlinTTdQUwm2On4k8TJUikGN1bf/Ds8WPX4oe0h9I=:
{
"id": "123",
"title": "New Title"
}
Figure 23: Response with Digest of Resource
B.8. POST Response Describes the Request Status
The request Repr-Digest field-value is computed on the enclosed
representation (see Section 3.1), which is the JSON object {"title":
"New Title"} followed by an LF.
The representation enclosed in the response describes the status of
the request, so Repr-Digest is computed on that enclosed
representation. It is a multiline JSON object followed by an LF.
Response Repr-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
Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:
{"title": "New Title"}
Figure 24: POST Request with Digest
HTTP/1.1 201 Created
Content-Type: application/json
Repr-Digest: sha-256=:yXIGDTN5VrfoyisKlXgRKUHHMs35SNtyC3szSz1dbO8=:
Location: /books/123
{
"status": "created",
"id": "123",
"ts": 1569327729,
"instance": "/books/123"
}
Figure 25: Response with Digest of Representation
B.9. Digest with PATCH
This case is analogous to a POST request where the target resource
reflects the target URI.
The PATCH request uses the application/merge-patch+json media type
defined in [RFC7396]. Repr-Digest is calculated on the content that
corresponds to the patch document and is the JSON object {"title":
"New Title"} followed by an LF.
The response Repr-Digest field-value is computed on the complete
representation of the patched resource. It is a multiline JSON
object followed by an LF.
PATCH /books/123 HTTP/1.1
Host: foo.example
Content-Type: application/merge-patch+json
Accept: application/json
Accept-Encoding: identity
Repr-Digest: sha-256=:mEkdbO7Srd9LIOegftO0aBX+VPTVz7/CSHes2Z27gc4=:
{"title": "New Title"}
Figure 26: PATCH Request with Digest
HTTP/1.1 200 OK
Content-Type: application/json
Repr-Digest: sha-256=:uVSlinTTdQUwm2On4k8TJUikGN1bf/Ds8WPX4oe0h9I=:
{
"id": "123",
"title": "New Title"
}
Figure 27: Response with Digest of Representation
Note that a 204 No Content response without content, but with the
same Repr-Digest field-value, would have been legitimate too. In
that case, Content-Digest would have been computed on an empty
content.
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 26 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 [RFC9457].
The response Repr-Digest field-value is computed on this enclosed
representation. It is a multiline JSON object followed by an LF.
HTTP/1.1 404 Not Found
Content-Type: application/problem+json
Repr-Digest: sha-256=:EXB0S2VF2H7ijkAVJkH1Sm0pBho0iDZcvVUHHXTTZSA=:
{
"title": "Not Found",
"detail": "Cannot PATCH a non-existent resource",
"status": 404
}
Figure 28: Response with Digest of Error Representation
B.11. Use with Trailer Fields and Transfer Coding
An origin server sends Repr-Digest as trailer field, so it can
calculate digest-value while streaming content and thus mitigate
resource consumption. The Repr-Digest field-value is the same as in
Appendix B.1 because Repr-Digest is designed to be independent of the
use of one or more transfer codings (see Section 3).
In the response content below, the string "\r\n" represents the CRLF
bytes.
GET /items/123 HTTP/1.1
Host: foo.example
Figure 29: GET Request
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Transfer-Encoding: chunked
Trailer: Repr-Digest
8\r\n
{"hello"\r\n
8\r\n
: "world\r\n
3\r\n
"}\n\r\n
0\r\n
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:\r\n
Figure 30: Chunked Response with Digest
Appendix C. Examples of Want-Repr-Digest Solicited Digest
The following examples demonstrate interactions where a client
solicits a Repr-Digest using Want-Repr-Digest. The behavior of
Content-Digest and Want-Content-Digest is identical.
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 two spaces
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 and prefers "sha". The server is free
to reply with "sha-256" anyway.
GET /items/123 HTTP/1.1
Host: foo.example
Want-Repr-Digest: sha-256=3, sha=10
Figure 31: GET Request with Want-Repr-Digest
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Repr-Digest: \
sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg==:
{"hello": "world"}
Figure 32: Response with Different Algorithm
C.2. Server Selects Algorithm Unsupported by Client
The client requests a "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-Repr-Digest: sha=10
Figure 33: GET Request with Want-Repr-Digest
NOTE: '\' line wrapping per RFC 8792
HTTP/1.1 200 OK
Content-Type: application/json
Repr-Digest: \
sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\
yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:
{"hello": "world"}
Figure 34: 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 a response with an error status code such as
4xx or 5xx. This specification does not prescribe any requirement on
status code selection; the following example illustrates one possible
option.
In this example, the client requests a "sha" Repr-Digest, and the
server returns an error with problem details [RFC9457] contained in
the content. The problem details contain a list of the hashing
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-Repr-Digest: sha=10
Figure 35: GET Request with Want-Repr-Digest
HTTP/1.1 400 Bad Request
Content-Type: application/problem+json
{
"title": "Bad Request",
"detail": "Supported hashing algorithms: sha-256, sha-512",
"status": 400
}
Figure 36: Response Advertising the Supported Algorithms
Appendix D. Sample Digest Values
This section shows examples of digest values for different hashing
algorithms. The input value is the JSON object {"hello": "world"}.
The digest values are each produced by running the relevant hashing
algorithm over the input and running the output bytes through Byte
Sequence serialization; see Section 4.1.8 of [STRUCTURED-FIELDS].
NOTE: '\' line wrapping per RFC 8792
sha-512 - :WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+\
AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
sha-256 - :X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
md5 - :Sd/dVLAcvNLSq16eXua5uQ==:
sha - :07CavjDP4u3/TungoUHJO/Wzr4c=:
unixsum - :GQU=:
unixcksum - :7zsHAA==:
adler - :OZkGFw==:
crc32c - :Q3lHIA==:
Appendix E. Migrating from RFC 3230
HTTP digests are computed by applying a hashing algorithm to input
data. [RFC3230] defined the input data as an "instance", a term it
also defined. The concept of an instance has since been superseded
by the HTTP semantic term "representation". It is understood that
some implementations of [RFC3230] mistook "instance" to mean HTTP
content. Using content for the Digest field is an error that leads
to interoperability problems between peers that implement [RFC3230].
[RFC3230] was only ever intended to use what HTTP now defines as
selected representation data. The semantic concept of digest and
representation are explained alongside the definition of the Repr-
Digest field (Section 3).
While the syntax of Digest and Repr-Digest are different, the
considerations and examples this document gives for Repr-Digest apply
equally to Digest because they operate on the same input data; see
Sections 3.1, 6 and 6.3.
[RFC3230] could never communicate the digest of HTTP message content
in the Digest field; Content-Digest now provides that capability.
[RFC3230] allowed algorithms to define their output encoding format
for use with the Digest field. This resulted in a mix of formats
such as base64, hex, or decimal. By virtue of using Structured
Fields, Content-Digest, and Repr-Digest use only a single encoding
format. Further explanation and examples are provided in Appendix D.
Acknowledgements
This document is based on ideas from [RFC3230], so thanks to Jeff
Mogul and Arthur Van Hoff for their great work. The original idea of
refreshing [RFC3230] arose from an interesting discussion with Mark
Nottingham, Jeffrey Yasskin, and Martin Thomson when reviewing the
MICE content coding.
Thanks to Julian Reschke for his valuable contributions to this
document, and to the following contributors that have helped improve
this specification by reporting bugs, asking smart questions,
drafting or reviewing text, and evaluating open issues: Mike Bishop,
Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean
Turner, Justin Richer, and Erik Wilde.
Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
Lucas Pardue
Cloudflare
Email: lucas@lucaspardue.com