draft-ietf-httpbis-digest-headers.md

---

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

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

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

venue: group: HTTP type: Working Group home: https://httpwg.org/ mail: ietf-http-wg@w3.org arch: https://lists.w3.org/Archives/Public/ietf-http-wg/ repo: digest-headers github-issue-label: digest-headers

author:

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

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

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

informative: RFC2818: HTTP11: I-D.ietf-httpbis-messaging PATCH: RFC5789 NO-MD5: RFC6151 NO-SHA: RFC6194 SIGNATURES: I-D.ietf-httpbis-message-signatures

--- 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.

--- middle

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 [RFC2818] 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 unintended or malicious 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 SEMANTICS}}). Second, representation data integrity, which acts on representation data ({{Section 3.2 of SEMANTICS}}). 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 RFC 3230 and therefore the Digest and Want-Digest HTTP fields; see {{obsolete-3230}}.

Document Structure

This document is structured as follows:

• {{content-digest}} defines the Content-Digest request and response header and trailer field,
• {{representation-digest}} defines the Repr-Digest request and response header and trailer field,
• {{want-fields}} defines the Want-Repr-Digest and Want-Content-Digest request and response header and trailer field,
• {{algorithms}} describes algorithms and their relation to the fields defined in this document,
• {{state-changing-requests}} details computing representation digests,
• {{examples-unsolicited}} and {{examples-solicited}} provide examples of using Repr-Digest and Want-Repr-Digest.

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" (see {{algorithms}}).

Selecting the data on which digests are calculated depends on the use case of HTTP messages. This document provides different headers 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 3.2 of SEMANTICS}}); see {{content-digest}}.

For more advanced use-cases, the Repr-Digest request and response header and trailer field ({{representation-digest}}) is defined. It contains a digest value computed by applying a hashing algorithm to selected representation data ({{Section 3.2 of SEMANTICS}}). Basing Repr-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}}).

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 preference of algorithms 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 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. 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.

Obsoleting RFC 3230 {#obsolete-3230}

[RFC3230] defined the Digest and Want-Digest HTTP fields for HTTP integrity. It also coined the term "instance" and "instance manipulation" in order to explain concepts that are now more universally defined, and implemented, as HTTP semantics such as selected representation data ({{Section 3.2 of SEMANTICS}}).

Experience has shown that implementations of [RFC3230] have interpreted the meaning of "instance" inconsistently, leading to interoperability issues. The most common mistake being the calculation of 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 ({{representation-digest}} and {{content-digest}}) and Integrity preference fields ({{want-fields}}) defined in this document are better aligned with current HTTP semantics and have names that more clearly articulate the intended usages.

Notational Conventions

{::boilerplate bcp14-tagged}

This document uses the Augmented BNF defined in [RFC5234] and updated by [RFC7405].

This document uses the following terminology from {{Section 3 of !STRUCTURED-FIELDS=RFC8941}} 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 {{SEMANTICS}}.

Hashing algorithm names respect the casing used in their definition document (e.g. SHA-1, CRC32c) whereas hashing algorithm keys are quoted (e.g. "sha", "crc32c").

The term "checksum" describes the output of the application of an algorithm to a sequence of bytes, whereas "digest" is only used in relation to the value contained in the fields.

Integrity fields: collective term for Content-Digest and Repr-Digest

Integrity preference fields: collective term for Want-Repr-Digest and Want-Content-Digest

The Content-Digest Field {#content-digest}

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 SEMANTICS}}). It is a Dictionary (see {{Section 3.2 of STRUCTURED-FIELDS}}) where each:

• key conveys the hashing algorithm (see {{algorithms}}) used to compute the digest;
• value is a Byte Sequence ({{Section 3.3.5 of STRUCTURED-FIELDS}}), that contains the output of the digest calculation.

For example:

NOTE: '\' line wrapping per RFC 8792

Content-Digest: \
  sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
  iYllu7BNNyealdVLvRwEmTHWXvJwew==:

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 {{sec-agility}}).

NOTE: '\' line wrapping per RFC 8792

Repr-Digest: \
  sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:,\
  sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
  iYllu7BNNyealdVLvRwEmTHWXvJwew==:

A recipient MAY ignore any or all digests. This allows the recipient to choose which hashing algorithm(s) to use for validation instead of verifying every digest.

A sender MAY send a digest without knowing whether the recipient supports a given hashing 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}}.

The Repr-Digest Field {#representation-digest}

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 SEMANTICS}}).

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 HTTP11}}). To help illustrate HTTP representation concepts, several examples are provided in {{resource-representation}}.

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 {{usage-in-signatures}}).

Repr-Digest is a Dictionary (see {{Section 3.2 of STRUCTURED-FIELDS}}) where each:

• key conveys the hashing algorithm (see {{algorithms}}) used to compute the digest;
• value is a Byte Sequence that contains the output of the digest calculation.

For example:

NOTE: '\' line wrapping per RFC 8792

Repr-Digest: \
  sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
  iYllu7BNNyealdVLvRwEmTHWXvJwew==:

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 {{sec-agility}}).

NOTE: '\' line wrapping per RFC 8792

Repr-Digest: \
  sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:,\
  sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
  iYllu7BNNyealdVLvRwEmTHWXvJwew==:

A recipient MAY ignore any or all digests. This allows the recipient to choose which hashing algorithm(s) to use for validation instead of verifying every digest.

A sender MAY send a digest without knowing whether the recipient supports a given hashing algorithm, or even knowing that 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 SEMANTICS}}.

Using Repr-Digest in State-Changing Requests {#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 {{representation-digest}}).

In responses,

• if the representation describes the status of the request, Repr-Digest MUST be computed on the enclosed representation (see {{post-referencing-status}} );

• 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. That 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 SEMANTICS}}). 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 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.

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

When a state-changing method returns the Content-Location header field, the enclosed representation refers to the resource identified by its value and Repr-Digest is computed accordingly. An example is given in {{post-not-request-uri}}.

Integrity preference fields {#want-fields}

Senders can indicate their interest in Integrity fields and hashing algorithm preferences using the Want-Content-Digest or Want-Repr-Digest fields. These can be used in both requests and responses.

Want-Content-Digest indicates that the sender would like to receive a content digest on messages associated with the request URI and representation metadata, using the Content-Digest field.

Want-Repr-Digest indicates that the sender would like to receive a representation digest on messages associated with the request URI and representation metadata, using the Repr-Digest field.

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.

Want-Content-Digest and Want-Repr-Digest are of type Dictionary where each:

• key conveys the hashing algorithm (see {{algorithms}});
• 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

Hash Algorithms for HTTP Digest Fields Registry {#algorithms}

The "Hash Algorithms for HTTP Digest Fields", maintained by IANA at https://www.iana.org/assignments/http-dig-alg/, registers algorithms for use with the Integrity and Integrity preference fields defined in this document.

This registry uses the Specification Required policy ({{Section 4.6 of !RFC8126}}).

Registrations MUST include the following fields:

• 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. 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 the algorithm references a reserved token value
• Description: a short description of the algorithm
• Reference(s): a set of pointers to the primary documents defining the algorithm and key

Insecure hashing 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.

The entries in {{iana-hash-algorithm-table}} are registered by this document.

| -------------- | -------- | ----------------------------------- | -------------- |

Algorithm Key	Status	Description	Reference(s)
sha-512	standard	The SHA-512 algorithm.	[RFC6234], [RFC4648], this document.
sha-256	standard	The SHA-256 algorithm.	[RFC6234], [RFC4648], this document.
md5	insecure	The MD5 algorithm. It is vulnerable to collision attacks; see {{NO-MD5}} and [CMU-836068]	[RFC1321], [RFC4648], this document.
sha	insecure	The SHA-1 algorithm. It is vulnerable to collision attacks; see {{NO-SHA}} and [IACR-2020-014]	[RFC3174], [RFC4648], [RFC6234] this document.
unixsum	insecure	The algorithm used by the UNIX "sum" command.	[RFC4648], [RFC6234], [UNIX], this document.
unixcksum	insecure	The algorithm used by the UNIX "cksum" command.	[RFC4648], [RFC6234], [UNIX], this document.
adler	insecure	The ADLER32 algorithm.	[RFC1950], this document.
crc32c	insecure	The CRC32c algorithm.	{{!RFC4960}} appendix B, this document.
--------------	--------	-----------------------------------	--------------
{: #iana-hash-algorithm-table title="Initial Hash Algorithms"}

Security Considerations {#security}

HTTP Messages Are Not Protected In Full {#sec-limitations}

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. This can be achieved by combining it with other approaches such as transport-layer security or digital signatures (for example, HTTP Message Signatures {{SIGNATURES}}).

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 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 a user agent can validate that resource retrieval succeeded before handing off to a 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 {{usage-in-signatures}}.

Usage in Signatures {#usage-in-signatures}

Digital signatures are widely used together with checksums to provide the certain identification of the origin of a message [NIST800-32]. Such signatures can protect one or more HTTP fields and there are additional considerations when Integrity fields are included in this set.

There are no restrictions placed on the type or format of digitial 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 {{digest-and-content-location}}.

Signatures are likely to be deemed an adversarial setting when applying Integrity fields; see {{algorithms}}. 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 Integrity fields, including digests' de-duplication or combining different field values (see {{Section 5.2 of SEMANTICS}}) might affect signature validation.

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 SEMANTICS}}).

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.

Not every hashing 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}}).

Usage with Encryption

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.

Algorithm Agility {#sec-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 {{establish-hash-algorithm-registry}}.

The "standard" algorithms listed in this document are suitable for many purposes, including adversarial situations where hash functions might need to provide resistance to collision, first-preimage and second-preimage attacks. Algorithms listed as "insecure" either provide none of these properties, or are known to be weak (see {{NO-MD5}} and {{NO-SHA}}).

For adversarial situations, which of the "standard" algorithms are acceptable will depend on the level of protection the circumstances demand. As there is no negotiation, endpoints that depend on a digest for security will be vulnerable to attacks on the weakest algorithm they are willing to accept.

Transition from weak algorithms is supported by negotiation of hashing algorithm using Want-Content-Digest or Want-Repr-Digest (see {{want-fields}}) or by sending multiple digests from which the receiver chooses. Endpoints are advised that sending multiple values consumes resources, which may be wasted if the receiver ignores them (see {{representation-digest}}).

While algorithm agility allows the migration to stronger algorithms it does not prevent the use of weaker algorithms. Integrity fields do not provide any mitigiations 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 value by using TLS and/or digital signatures.

Resource exhaustion

Integrity 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.

IANA Considerations

HTTP Field Name Registration

IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry ({{SEMANTICS}}) according to the table below:

|---------------------|-----------|-----------------------------------------------|

Field Name	Status	Reference
Content-Digest	permanent	{{content-digest}} of this document
Repr-Digest	permanent	{{representation-digest}} of this document
Want-Content-Digest	permanent	{{want-fields}} of this document
Want-Repr-Digest	permanent	{{want-fields}} of this document
Digest	obsoleted	[RFC3230], {{obsolete-3230}} of this document
Want-Digest	obsoleted	[RFC3230], {{obsolete-3230}} of this document
---------------------	-----------	-----------------------------------------------

Establish the Hash Algorithms for HTTP Digest Fields Registry {#establish-hash-algorithm-registry}

This memo sets this specification to be the establishing document for the Hash Algorithms for HTTP Digest Fields registry defined in {{algorithms}}.

IANA is asked to initialize the registry with the entries in {{iana-hash-algorithm-table}}.

--- back

Resource Representation and Representation Data {#resource-representation}

The following examples show how representation metadata, payload transformations and method impacts on the message and content. When the content contains non-printable characters (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"}

{: title="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=

{: title="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.

PUT /entries/1234 HTTP/1.1
Host: foo.example
Content-Type: application/json

H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

{: title="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

{: title="Request for partial content"}

HTTP/1.1 206 Partial Content
Content-Encoding: gzip
Content-Type: application/json
Content-Range: bytes 1-7/18

iwgAla3RXA==

{: title="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

{: title="HEAD request"}

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

{: title="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/.

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

{"author": "Camilleri"}

{: title="POST request"}

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

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

{: title="Response with Content-Location header"}

Examples of Unsolicited Digest {#examples-unsolicited}

The following examples demonstrate interactions where a server responds with a Content-Digest or Repr-Digest fields 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 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, Content-Digest and Repr-Digest can be returned independently.

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

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"}, and thus have the same value.

GET /items/123 HTTP/1.1
Host: foo.example

{: title="GET request for an item"}

NOTE: '\' line wrapping per RFC 8792

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

{"hello": "world"}

{: title="Response with identical Repr-Digest and Content-Digest"}

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"}, which is not shown because there is no payload data.

HEAD /items/123 HTTP/1.1
Host: foo.example

{: title="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=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

{: title="Response with both Content-Digest and Digest; empty content"}

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=1-7

{: title="Request for partial content"}

NOTE: '\' line wrapping per RFC 8792

HTTP/1.1 206 Partial Content
Content-Type: application/json
Content-Range: bytes 1-7/18
Content-Digest: \
  sha-256=:Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=:
Repr-Digest: \
  sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

"hello"

{: title="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"}, and the field is

Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

However, since the message content is constrained to bytes 1-7, the Content-Digest field-value is calculated over the byte sequence "hello", thus resulting in

NOTE: '\' line wrapping per RFC 8792

Content-Digest: \
  sha-256=:Wqdirjg/u3J688ejbUlApbjECpiUUtIwT8lY/z81Tno=:

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 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 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
Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

{"hello": "world"}

{: title="PUT Request with Digest"}

HTTP/1.1 200 OK
Content-Type: application/json
Content-Location: /items/123
Content-Encoding: br
Content-Length: 22
Repr-Digest: sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

{: title="Response with Digest of encoded response"}

Client Provides Full Representation Data, Server Provides No Representation Data

The request Repr-Digest field-value is calculated on the enclosed payload.

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.

PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Content-Length: 18
Accept-Encoding: br
Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

{"hello": "world"}

{: title="PUT Request with Digest}

HTTP/1.1 204 No Content
Content-Type: application/json
Content-Encoding: br
Repr-Digest: sha-256=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:

{: title="Empty response with Digest"}

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
Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

{"hello": "world"}

{: title="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=:4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=:,\
  sha-512=:pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE60jBCwnMPyA/\
  s3NF3ZO5oIWA7lf8ukk+5KJzm3p5og==:

iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

{: title="Response with Digest of Encoded Content"}

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

The request Repr-Digest field-value is computed on the enclosed representation (see {{state-changing-requests}}).

The representation enclosed in the response refers to the resource identified by Content-Location (see {{Section 6.4.2 of SEMANTICS}}). Repr-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
Repr-Digest: sha-256=:bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=:

{"title": "New Title"}

{: title="POST Request with Digest"}

HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Repr-Digest: sha-256=:yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=:

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

{: title="Response with Digest of Resource"}

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.

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

The request Repr-Digest field-value is computed on the enclosed representation (see {{state-changing-requests}}).

The representation enclosed in the response describes the status of the request, so Repr-Digest is computed on that enclosed representation.

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=:bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=:

{"title": "New Title"}

{: title="POST Request with Digest"}

HTTP/1.1 201 Created
Content-Type: application/json
Repr-Digest: sha-256=:2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=:
Location: /books/123

{
  "status": "created",
  "id": "123",
  "ts": 1569327729,
  "instance": "/books/123"
}

{: title="Response with Digest of Representation"}

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}}.

Repr-Digest is calculated on the enclosed payload, which corresponds to the patch document.

The response Repr-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
Repr-Digest: sha-256=:bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=:

{"title": "New Title"}

{: #fig-patch title="PATCH Request with Digest"}

HTTP/1.1 200 OK
Content-Type: application/json
Repr-Digest: sha-256=:yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE=:

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

{: title="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.

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 {{fig-patch}} 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 Repr-Digest field-value is computed on this enclosed representation.

HTTP/1.1 404 Not Found
Content-Type: application/problem+json
Repr-Digest: sha-256=:KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk=:

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

{: title="Response with Digest of Error Representation"}

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

GET /items/123 HTTP/1.1
Host: foo.example

{: title="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
Repr-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

{: title="Chunked Response with Digest"}

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

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 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.

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-Repr-Digest: sha-256=3, sha=10

{: title="GET Request with Want-Repr-Digest"}

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

{"hello": "world"}

{: title="Response with Different Algorithm"}

Server Selects Algorithm Unsupported by Client {#ex-server-selects-unsupported-algorithm}

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

{: title="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=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrI\
  iYllu7BNNyealdVLvRwEmTHWXvJwew==:

{"hello": "world"}

{: title="Response with Unsupported Algorithm"}

Server Does Not Support Client Algorithm and Returns an Error

{{ex-server-selects-unsupported-algorithm}} 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" Repr-Digest, and the server returns an error with problem details {{?RFC7807}} 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

{: title="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
}

{: title="Response advertising the supported algorithms"}

Migrating from RFC 3230

HTTP digests are computed by applying a hashing algorithm to input data. RFC 3230 defined the input data as an "instance", a term it also defined. The concept of instance has since been superseded by the HTTP semantic term "representation". It is understood that some implementations of RFC 3230 mistook "instance" to mean HTTP content. Using content for the Digest field is an error that leads to interoperability problems between peers that implement RFC 3230.

For the uncertainty of doubt, RFC 3230 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 Representation-Digest {{representation-digest}}.

While the syntax of Digest and Repr-Digest are different, the considerations and examples this document gives to Repr-Digest apply equally to Digest because they operate on the same input data. See {{state-changing-requests}}, {{security}} and {{usage-in-signatures}}.

RFC 3230 could never communicate the digest of HTTP message content in the Digest field; Content-Digest now provides that capability.

Acknowledgements

{:numbered="false"} This document is based on ideas from [RFC3230], so thanks to J. Mogul and A. Van Hoff for their great work. The original idea of refreshing RFC3230 arose from an interesting discussion with M. Nottingham, J. Yasskin and M. 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.

Code Samples

{:numbered="false"}

RFC Editor: Please remove this section before publication.

How can I generate and validate the Repr-Digest values shown in the examples throughout this document?

The following python3 code can be used to generate digests for JSON objects using SHA algorithms for a range of encodings. Note that these are formatted as base64. This function could be adapted to other algorithms and should take into account their specific formatting rules.

import base64, json, hashlib, brotli, logging
log = logging.getLogger()

def encode_item(item, encoding=lambda x: x):
    indent = 2 if isinstance(item, dict) and len(item) > 1 else None
    json_bytes = json.dumps(item, indent=indent).encode()
    return encoding(json_bytes)

def digest_bytes(bytes_, algorithm=hashlib.sha256):
    checksum_bytes = algorithm(bytes_).digest()
    log.warning("Log bytes: \n[%r]", bytes_)
    return base64.encodebytes(checksum_bytes).strip()

def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256):
    content_encoded = encode_item(item, encoding)
    return digest_bytes(content_encoded, algorithm)


item = {"hello": "world"}

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

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

print("Encoding | hashing algorithm | digest-value")
print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512))
print("Brotli | sha512 |", digest(item, algorithm=hashlib.sha512,
                                    encoding=brotli.compress))
# Encoding | hashing algorithm | digest-value
# Identity | sha512 |b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
#                     '+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew=='
# Brotli | sha512 | b'pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE6'
#                   '0jBCwnMPyA/s3NF3ZO5oIWA7lf8ukk+5KJzm3p5og=='

Changes

{:numbered="false"}

RFC Editor: Please remove this section before publication.

Since draft-ietf-httpbis-digest-headers-07

{:numbered="false"}

• Introduced Repr-Digest and Want-Repr-Digest, and deprecated Digest and Want-Digest. Use of Structured Fields. #1993, #1919
• IANA refactoring. #1983
• No normative text in security considerations. #1972

Since draft-ietf-httpbis-digest-headers-06

{:numbered="false"}

• Remove id-sha-256 and id-sha-512 from the list of supported algorithms #855

Since draft-ietf-httpbis-digest-headers-05

{:numbered="false"}

• Reboot digest-algorithm values registry #1567
• Add Content-Digest #1542
• Remove SRI section #1478

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

{:numbered="false"}

• Improve SRI section #1354
• About duplicate digest-algorithms #1221
• Improve security considerations #852
• md5 and sha deprecation references #1392
• Obsolete 3230 #1395
• Editorial #1362

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

{:numbered="false"}

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

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

{:numbered="false"}

• Deprecate SHA-1 #1154
• Avoid id-* with encrypted content
• Digest is independent from MESSAGING and HTTP/1.1 is not normative #1215
• Identity is not a valid field value for content-encoding #1223
• Mention trailers #1157
• Reference httpbis-semantics #1156
• Add contentMD5 as an obsoleted digest-algorithm #1249
• Use lowercase digest-algorithms names in the doc and in the digest-algorithm IANA table.

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

{:numbered="false"}

• Digest of error responses is computed on the error representation-data #1004
• Effect of HTTP semantics on payload and message body moved to appendix #1122
• Editorial refactoring, moving headers sections up. #1109-#1112, #1116, #1117, #1122-#1124

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

{:numbered="false"}

• Align title with document name
• Add id-sha-* algorithm examples #880
• Reference [RFC6234] and [RFC3174] instead of FIPS-1
• Deprecate MD5
• Obsolete ADLER-32 but don't forbid it #828
• Update CRC32C value in IANA table #828
• Use when acting on resources (POST, PATCH) #853
• Added Relationship with SRI, draft Use Cases #868, #971
• Warn about the implications of Content-Location