Repr-Digest

The Repr-Digest response or request header provides a digest of the selected representation of the target resource. It is invariant under e.g., Content-Encoding or Content-Range, which do affect the Content-Digest. Furthermore, Content Negotiation can result in different selected representations with different representation digests.

Header type	Representation header
Forbidden header name	no

Syntax

Repr-Digest describes an RFC8941 dictionary with its keys being names of digest algorithms and its values being the digest in bytes (or an integer digest for legacy digest algorithms).

Note: In contrast to earlier drafts of the specification, the standard-base64-encoded digest bytes are wrapped in colons (:, ASCII 0x3A) as part of the dictionary syntax.

Directives

Digest algorithms

Only two digest algorithms are currently registered and not considered insecure: sha-512 and sha-256.

The registered insecure digest algorithms are: md5, sha (SHA-1), unixsum, unixcksum, adler (ADLER32) and crc32c.

Usage of digest algorithms which are considered insecure is discouraged as collisions can realistically be forced, rendering the digest's usefulness weak. Unless when working with legacy systems (which is unlikely since most will expect the legacy Digest header and not understand this specification), consider not emitting a Repr-Digest instead of emitting one using an insecure digest algorithm.

Examples

HTTP response where `Repr-Digest` and `Content-Digest` coincide

An HTTP server may send content octets equivalent to the selected representation's octets:

http

...
Repr-Digest: sha-256=:AEGPTgUMw5e96wxZuDtpfm23RBU3nFwtgY5fw4NYORo=:
Content-Digest: sha-256=:AEGPTgUMw5e96wxZuDtpfm23RBU3nFwtgY5fw4NYORo=:
...
Content-Type: text/yaml
Content-Encoding: identity
Content-Length: 38054
Content-Range: 0-38053/38054
...

HTTP responses where `Repr-Digest` and `Content-Digest` diverge

A static file server may however choose to compress an HTML page, resulting in differing Content-Digest and Repr-Digest header values:

http

...
Repr-Digest: sha-256=:AEGPTgUMw5e96wxZuDtpfm23RBU3nFwtgY5fw4NYORo=:, sha-512=:U59TCCaZPA9Qio3CzHJVAgDnIAut53t5Sgkj2Gv4BvDd0b+OX9QpIdgWkzdXLmBsmvBrf3t5PBt+UrVK6k5dkw==:
Content-Digest: sha-256=:293wcr5IoFAsDCzdoDXR1Qppgf2yxOPO1bvQ3nZQtuI=:, unixsum=54809
...
Content-Type: text/html; charset=utf-8
Content-Encoding: br
...

...

http

...
Repr-Digest: sha-256=:AEGPTgUMw5e96wxZuDtpfm23RBU3nFwtgY5fw4NYORo=:, sha-512=:U59TCCaZPA9Qio3CzHJVAgDnIAut53t5Sgkj2Gv4BvDd0b+OX9QpIdgWkzdXLmBsmvBrf3t5PBt+UrVK6k5dkw==:
Content-Digest: sha-256=:rv9Jivc4TmcacLUshzN3OdX7Hz+ORnQRaiTaIKZQ0zk=:
...
Content-Type: text/html; charset=utf-8
Content-Encoding: deflate, deflate, deflate
...

...

Successful HTTP request-response employing `Want-Repr-Digest`, `Repr-Digest`, and `Content-Digest`

http

PUT /api/transact HTTP/1.1
Want-Repr-Digest: sha-256=8
Content-Type: text/json
...

http

HTTP/1.1 201 Created
Repr-Digest: sha-256=:W8oN3H3CmE/CBpV6ZPNozV2AIDzzQpWL7CCOXyDyDzI=:
Content-Encoding: br
Content-Digest: sha-256=:2IBI7hQn83oTCgB3Z/6apOl91WGoctRfRj/F9gkvVo8=:
...

Unsuccessful HTTP request-response employing `Repr-Digest`

http

GET /api/last-transaction HTTP/1.1
Accept: text/json
Repr-Digest: sha-256=:2IBI7hQn83oTCgB3Z/6apOl91WGoctRfRj/F9gkvVo8=:
...

http

HTTP/1.1 406 Not Acceptable
Repr-Digest: sha-256=:W8oN3H3CmE/CBpV6ZPNozV2AIDzzQpWL7CCOXyDyDzI=:
...

Specifications

Specification
Digest Fields

Browser compatibility

This header has no specification-defined browser integration ("browser compatibility" does not apply). Developers can set and get HTTP headers using fetch() in order to provide application-specific implementation behavior.