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Applications and Real-Time HTTP Digest This document defines the HTTP Digest and Want-Digest fields, thus allowing client and server to negotiate an integrity checksum of the exchanged resource representation data. This document obsoletes RFC 3230. It replaces the term "instance" with "representation", which makes it consistent with the HTTP Semantic and Context defined in draft-ietf-httpbis-semantics. RFC EDITOR: please remove this section before publication Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/. The source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions.

The core specification of HTTP does not define a means to protect the integrity of resources. When HTTP messages are transferred between endpoints, the protocol might choose to make use of features of the lower layer in order to provide some integrity protection; for instance TCP checksums or TLS records . However, there are cases where relying on this alone is insufficient. An HTTP-level integrity mechanism that operates independent of transfer can be used to detect programming errors and/or corruption of data at rest, be used across multiple hops in order to provide end-to-end integrity guarantees, aid fault diagnosis across hops and system boundaries, and can be used to validate integrity when reconstructing a resource fetched using different HTTP connections. This document defines a mechanism that acts on HTTP representation-data. It can be combined with other mechanisms that protect representation-metadata, such as digital signatures, in order to protect the desired parts of an HTTP exchange in whole or in part.

The Content-MD5 header field was originally introduced to provide integrity, but HTTP/1.1 () obsoleted it: The Content-MD5 header field has been removed because it was inconsistently implemented with respect to partial responses. provided a more flexible solution introducing the concept of "instance", and the fields Digest and Want-Digest.

The concept of selected representation defined in makes definitions inconsistent with current HTTP semantics. This document updates the Digest and Want-Digest field definitions to align with concepts. Basing Digest on the selected representation makes it straightforward to apply it to use-cases where the transferred data does require some sort of manipulation to be considered a representation, or conveys a partial representation of a resource eg. Range Requests (see ). Changes are semantically compatible with existing implementations and better cover both the request and response cases. The value of Digest is calculated on selected representation, which is tied to the value contained in any Content-Encoding or Content-Type header fields. Therefore, a given resource may have multiple different digest values. To allow both parties to exchange a Digest of a representation with no content codings (see ) two more digest-algorithms are added ("id-sha-256" and "id-sha-512").

The goals of this proposal are: Digest coverage for either the resource's representation data or selected representation data communicated via HTTP. Support for multiple digest-algorithms. Negotiation of the use of digests. The goals do not include: The digest mechanism described here does not cover the full HTTP message nor its semantic, as representation metadata are not included in the checksum. The digest mechanisms described here cover only representation and selected representation data, and do not protect the integrity of associated representation metadata or other message fields. The digest mechanisms described here are not meant to support authentication of the source of a digest or of a message or anything else. These mechanisms, therefore, are not a sufficient defense against many kinds of malicious attacks. Digest mechanisms do not provide message privacy. The digest mechanisms described here are not meant to support authorization or other kinds of access controls.

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 ( and ) when, and only when, they appear in all capitals, as shown here. This document uses the Augmented BNF defined in and updated by along with the "#rule" extension defined in . The definitions "representation", "selected representation", "representation data", "representation metadata", and "payload body" in this document are to be interpreted as described in . Algorithm names respect the casing used in their definition document (eg. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (eg. "sha", "crc32c").

The representation digest is an integrity mechanism for HTTP resources which uses a checksum that is calculated independently of the payload body (see ). It uses the representation data (see ), that can be fully or partially contained in the payload body, or not contained at all:

representation-data := Content-Encoding( Content-Type( bits ) )

This takes into account the effect of the HTTP semantics on the messages; for example the payload body can be affected by Range Requests or methods such as HEAD, while the way the payload body is transferred "on the wire" is dependent on other transformations (eg. transfer codings for HTTP/1.1 see 6.1 of ): contains several examples to help illustrate those effects. A representation digest consists of the value of a checksum computed on the entire selected representation data (see ) of a resource identified according to together with an indication of the algorithm used

representation-data-digest = digest-algorithm "=" <encoded digest output>

The checksum is computed using one of the digest-algorithms listed in and then encoded in the associated format. The example below shows the "sha-256" digest-algorithm which uses base64 encoding.

sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

The Digest field contains a list of one or more representation digest values as defined in . It can be used in both request and response.

Digest = "Digest" ":" OWS 1#representation-data-digest

The relationship between Content-Location (see ) and Digest is demonstrated in . A comprehensive set of examples showing the impacts of representation metadata, payload transformations and HTTP methods on Digest is provided in and . A Digest field MAY contain multiple representation-data-digest values. For example, a server may provide representation-data-digest values using different algorithms, allowing it to support a population of clients with different evolving capabilities; this is particularly useful in support of transitioning away from weaker algorithms should the need arise (see ). A recipient MAY ignore any or all of the representation-data-digests in a Digest field. This allows the recipient to choose which digest-algorithm(s) to use for validation instead of verifying every received representation-data-digest. A sender MAY send a representation-data-digest using a digest-algorithm without knowing whether the recipient supports the digest-algorithm, or even knowing that the recipient will ignore it. Digest can be sent in a trailer section. When using incremental digest-algorithms this allows the sender and the receiver to dynamically compute the digest value while streaming the content. Two examples of its use are

Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew== Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=, id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

The Want-Digest field indicates the sender's desire to receive a representation digest on messages associated with the request URI and representation metadata.

Want-Digest = "Want-Digest" ":" OWS 1#want-digest-value want-digest-value = digest-algorithm [ ";" "q" "=" qvalue] qvalue = ( "0"  [ "."  0*1DIGIT ] ) /   ( "1"  [ "."  0*1( "0" ) ] )

If a digest-algorithm is not accompanied by a "qvalue", it is treated as if its associated "qvalue" were 1.0. The sender is willing to accept a digest-algorithm if and only if it is listed in a Want-Digest field of a message, and its "qvalue" is non-zero. If multiple acceptable digest-algorithm values are given, the sender's preferred digest-algorithm is the one (or ones) with the highest "qvalue". Two examples of its use are

Want-Digest: sha-256 Want-Digest: sha-512;q=0.3, sha-256;q=1, unixsum;q=0

Digest-algorithm values are used to indicate a specific digest computation.

digest-algorithm = token

All digest-algorithm values are case-insensitive but the lower case is preferred. The Internet Assigned Numbers Authority (IANA) acts as a registry for digest-algorithm values. The registry contains the tokens listed below. Some digest-algorithms, although registered, rely on vulnerable algorithms: the "md5" digest-algorithm MUST NOT be used due to collision attacks and the "sha" digest-algorithm MUST NOT be used due to collision attacks . Description: The SHA-256 algorithm . The output of this algorithm is encoded using the base64 encoding . Reference: , , this document. Status: standard Description: The SHA-512 algorithm . The output of this algorithm is encoded using the base64 encoding . Reference: , , this document. Status: standard Description: The MD5 algorithm, as specified in . The output of this algorithm is encoded using the base64 encoding . This digest-algorithm MUST NOT be used as it's now vulnerable to collision attacks . Reference: , , this document. Status: deprecated Description: The SHA-1 algorithm . The output of this algorithm is encoded using the base64 encoding . This digest-algorithm MUST NOT be used as it's now vulnerable to collision attacks . Reference: , , , this document. Status: deprecated Description: The algorithm computed by the UNIX "sum" command, as defined by the Single UNIX Specification, Version 2 . The output of this algorithm is an ASCII decimal-digit string representing the 16-bit checksum, which is the first word of the output of the UNIX "sum" command. Reference: , this document. Status: standard Description: The algorithm computed by the UNIX "cksum" command, as defined by the Single UNIX Specification, Version 2 . The output of this algorithm is an ASCII digit string representing the 32-bit CRC, which is the first word of the output of the UNIX "cksum" command. Reference: , this document. Status: standard To allow sender and recipient to provide a checksum which is independent from Content-Encoding, the following additional digest-algorithms are defined: Description: The sha-512 digest of the representation-data of the resource when no content coding is applied Reference: , , this document. Status: standard Description: The sha-256 digest of the representation-data of the resource when no content coding is applied Reference: , , this document. Status: standard If other digest-algorithm values are defined, the associated encoding MUST either be represented as a quoted string, or MUST NOT include ";" or "," in the character sets used for the encoding.

POST and PATCH requests can appear to convey partial representations but are semantically acting on resources. The enclosed representation, including its metadata refers to that action. In these requests the representation digest MUST be computed on the representation-data of that action. This is the only possible choice because representation digest requires complete representation metadata (see ). In responses, if the representation describes the status of the request, Digest MUST be computed on the enclosed representation (see ); if there is a referenced resource Digest MUST be computed on the selected representation of the referenced resource even if that is different from the target resource. That might or might not result in computing Digest on the enclosed representation. The latter case might be done according to the HTTP semantics of the given method, for example using the Content-Location header field. In contrast, the Location header field does not affect Digest because it is not representation metadata.

In PATCH requests the representation digest MUST be computed on the patch document because the representation metadata refers to the patch document and not to the target resource (see ). In PATCH responses the representation digest MUST be computed on the selected representation of the patched resource. Digest usage with PATCH is thus very similar to the POST one, but with the resource's own semantic partly implied by the method and by the patch document.

This RFC deprecates the negotiation of Content-MD5 as it has been obsoleted by . The contentMD5 token defined in MUST NOT be used as a digest-algorithm.

This document obsoletes the usage of parameters with Digest introduced in Section and of because this feature has not been widely deployed and complicates field-value processing. Field parameters provided a common way to attach additional information to a representation-data-digest, but if they are used as an input to validate the checksum, an attacker could alter them to steer the validation behavior. A digest-algorithm can still be parameterized defining its own way to encode parameters into the representation-data-digest in such a way as to mitigate security risks related to its computation.

Subresource Integrity is an integrity mechanism that shares some similarities to the present document's mechanism. However, there are differences in motivating factors, threat model and specification of integrity digest generation, signalling and validation. SRI allows a first-party authority to declare an integrity assertion on a resource served by a first or third party authority. This is done via the integrity attribute that can be added to script or link HTML elements. Therefore, the integrity assertion is always made out-of-band to the resource fetch. In contrast, the Digest field is supplied in-band alongside the selected representation, meaning that an authority can only declare an integrity assertion for itself. Methods to improve the security properties of representation digests are presented in . This contrast is interesting because on one hand self-assertion is less likely to be affected by coordination problems such as the first-party holding stale information about the third party, but on the other hand the self-assertion is only as trustworthy as the authority that provided it. The SRI integrity attribute contains a cryptographic hash algorithm and digest value which is similar to representation-data-digest (see ). The major differences are in serialization format. The SRI digest value is calculated over the identity encoding of the resource, not the selected representation (as specified for representation-data-digest in this document). Section 3.4.5 of describes the benefit of the identity approach - the SRI integrity attribute can contain multiple algorithm-value pairs where each applies to a different identity encoded payload. This allows for protection of distinct resources sharing a URL. However, this is a contrast to the design of representation digests, where multiple Digest field-values all protect the same representation. SRI does not specify handling of partial representation data (e.g. Range requests). In contrast, this document specifies handling in terms that are fully compatible with core HTTP concepts (an example is provided in ). SRI specifies strong requirements on the selection of algorithm for generation and validation of digests. In contrast, the requirements in this document are weaker. SRI defines no method for a client to declare an integrity assertion on resources it transfers to a server. In contrast, the Digest field can appear on requests.

The SRI and Representation Digest mechanisms are different and complementary but one is not capable of replacing the other because they have different threat, security and implementation properties. A user agent that supports both mechanisms is expected to apply the rules specified for each but since the two mechanisms are independent, the ordering is not important. However, a user agent supporting both could benefit from performing representation digest validation first because it does not always require a conversion into identity encoding. There is a chance that a user agent supporting both mechanisms may find one validates successfully while the other fails. This document specifies no requirements or guidance for user agents that experience such cases.

The following examples demonstrate interactions where a server responds with a Digest field even though the client did not solicit one using Want-Digest.

Request:

GET /items/123

Response:

HTTP/1.1 200 OK Content-Type: application/json Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= {"hello": "world"}

Requests without a payload body can still send a Digest field applying the digest-algorithm to an empty representation. As there is no content coding applied, the "sha-256" and the "id-sha-256" digest-values in the response are the same. Request:

HEAD /items/123 HTTP/1.1 Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

Response:

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

Request:

GET /items/123 Range: bytes=1-7

Response:

HTTP/1.1 206 Partial Content Content-Type: application/json Content-Range: bytes 1-7/18 Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= "hello"

The request contains a Digest field calculated on the enclosed representation. It also includes an Accept-Encoding: br header field that advertises the client supports brotli encoding. The response includes a Content-Encoding: br that indicates the selected representation is brotli encoded. The Digest field-value is therefore different compared to the request. The response body is displayed as a base64-encoded string because it contains non-printable characters. Request:

PUT /items/123 Content-Type: application/json Accept-Encoding: br Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= {"hello": "world"}

Response:

Content-Type: application/json Content-Encoding: br Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

Request Digest value is calculated on the enclosed payload. Response Digest value depends on the representation metadata header fields, including Content-Encoding: br even when the response does not contain a payload body. Request:

PUT /items/123 Content-Type: application/json Content-Length: 18 Accept-Encoding: br Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= {"hello": "world"}

Response:

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

The response contains two digest values: one with no content coding applied, which in this case accidentally matches the unencoded digest-value sent in the request; one taking into account the Content-Encoding. As the response body contains non-printable characters, it is displayed as a base64-encoded string. Request:

PUT /items/123 HTTP/1.1 Content-Type: application/json Accept-Encoding: br Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= {"hello": "world"}

Response:

HTTP/1.1 200 OK Content-Type: application/json Content-Encoding: br Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=, id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

Request Digest value is computed on the enclosed representation (see ). The representation enclosed in the response refers to the resource identified by Content-Location (see ). Digest is thus computed on the enclosed representation. Request:

POST /books HTTP/1.1 Content-Type: application/json Accept: application/json Accept-Encoding: identity Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ= {"title": "New Title"}

Response

HTTP/1.1 201 Created Content-Type: application/json Digest: id-sha-256=BZlF2v0IzjuxN01RQ97EUXriaNNLhtI8Chx8Eq+XYSc= Content-Location: /books/123 {"id": "123", "title": "New Title"}

Note that a 204 No Content response without a payload body but with the same Digest field-value would have been legitimate too.

Request Digest value is computed on the enclosed representation (see ). The representation enclosed in the response describes the status of the request, so Digest is computed on that enclosed representation. Response Digest has no explicit relation with the resource referenced by Location. Request:

POST /books HTTP/1.1 Content-Type: application/json Accept: application/json Accept-Encoding: identity Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ= Location: /books/123 {"title": "New Title"}

Response

HTTP/1.1 201 Created Content-Type: application/json Digest: id-sha-256=0o/WKwSfnmIoSlop2LV/ISaBDth05IeW27zzNMUh5l8= Location: /books/123 { "status": "created", "id": "123", "ts": 1569327729, "instance": "/books/123" }

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 . Digest is calculated on the enclosed payload, which corresponds to the patch document. The response Digest is computed on the complete representation of the patched resource. Request:

PATCH /books/123 HTTP/1.1 Content-Type: application/merge-patch+json Accept: application/json Accept-Encoding: identity Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ= {"title": "New Title"}

Response:

HTTP/1.1 200 OK Content-Type: application/json Digest: id-sha-256=BZlF2v0IzjuxN01RQ97EUXriaNNLhtI8Chx8Eq+XYSc= {"id": "123", "title": "New Title"}

Note that a 204 No Content response without a payload body but with the same Digest field-value would have been legitimate too.

In error responses, the representation-data does not necessarily refer to the target resource. Instead it refers to the representation of the error. In the following example a client attempts to patch the resource located at /books/123. However, the resource does not exist and the server generates a 404 response with a body that describes the error in accordance with . The digest of the response is computed on this enclosed representation. Request:

PATCH /books/123 HTTP/1.1 Content-Type: application/merge-patch+json Accept: application/json Accept-Encoding: identity Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ= {"title": "New Title"}

Response:

HTTP/1.1 404 Not Found Content-Type: application/problem+json Digest: sha-256=UJSojgEzqUe4UoHzmNl5d2xkmrW3BOdmvsvWu1uFeu0= { "title": "Not Found", "detail": "Cannot PATCH a non-existent resource", "status": 404 }

An origin server sends Digest in the HTTP trailer, so it can calculate digest-value while streaming content and thus mitigate resource consumption. The field value is the same as in because Digest is designed to be independent from the use of one or more transfer codings (see ). Request:

GET /items/123

Response:

HTTP/1.1 200 OK Content-Type: application/json Transfer-Encoding: chunked Trailer: Digest 8\r\n {"hello"\r\n 8 : "world\r\n 2\r\n "}\r\n 0\r\n Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

The following examples demonstrate interactions where a client solicits a Digest using Want-Digest.

The client requests a digest, preferring "sha". The server is free to reply with "sha-256" anyway. Request:

GET /items/123 HTTP/1.1 Want-Digest: sha-256;q=0.3, sha;q=1

Response:

HTTP/1.1 200 OK Content-Type: application/json Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= {"hello": "world"}

The client requests a sha digest only. The server is currently free to reply with a Digest containing an unsupported algorithm. Request:

GET /items/123 Want-Digest: sha;q=1

Response:

HTTP/1.1 200 OK Content-Type: application/json Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm +AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew== {"hello": "world"}

The client requests a sha Digest, the server advises for sha-256 and sha-512 Request:

GET /items/123 Want-Digest: sha;q=1

Response:

HTTP/1.1 400 Bad Request Want-Digest: sha-256, sha-512

This document specifies a data integrity mechanism that protects HTTP representation data, but not HTTP representation metadata fields, from certain kinds of accidental corruption. Digest is not intended as 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.

Cryptographic algorithms are intended to provide a proof of integrity suited towards cryptographic constructions such as signatures. However, these rely on collision-resistance for their security proofs . The "md5" and "sha" digest-algorithms are vulnerable to collisions attacks, so they MUST NOT be used with Digest.

The ADLER32 algorithm defined in has been deprecated by because under certain conditions it provides weak detection of errors and is now NOT RECOMMENDED for use with Digest.

Digest alone does not provide end-to-end integrity of HTTP messages over multiple hops, as it just covers the representation data and not the representation metadata. Besides, it allows to protect representation data from buggy manipulation, buggy compression, etc. Moreover identity digest-algorithms (eg. "id-sha-256" and "id-sha-512") allow piecing together a resource from different sources (e.g. different servers that perhaps apply different content codings) enabling the user-agent to detect that the application-layer tasks completed properly, before handing off to say the HTML parser, video player etc. Even a simple mechanism for end-to-end validation is thus valuable.

When a state-changing method returns the Content-Location header field, the enclosed representation refers to the resource identified by its value and Digest is computed accordingly.

Digital signatures are widely used together with checksums to provide the certain identification of the origin of a message . Such signatures can protect one or more HTTP fields and there are additional considerations when Digest is included in this set. Since the Digest field is a hash of a resource representation, it explicitly depends on the representation metadata (eg. the values of Content-Type, Content-Encoding etc). A signature that protects Digest but not other representation metadata can expose the communication to tampering. For example, an actor could manipulate the Content-Type field-value and cause a digest validation failure at the recipient, preventing the application from accessing the representation. Such an attack consumes the resources of both endpoints. See also . Digest SHOULD always be used over a connection which provides integrity at the transport layer that protects HTTP fields. A Digest field using NOT RECOMMENDED digest-algorithms SHOULD NOT be used in signatures. Using signatures to protect the Digest of an empty representation allows receiving endpoints to detect if an eventual payload has been stripped or added.

When used in trailers, the receiver gets the digest value after the payload body and may thus be tempted to process the data before validating the digest value. Instead, data should only be processed after validating the Digest. If received in trailers, Digest MUST NOT be discarded; instead it MAY be merged in the header section (See ). Not every digest-algorithm is suitable for trailers, as they may require to pre-process the whole payload before sending a message (eg. see ).

Digest may expose information details of encrypted payload when the checksum is computed on the unencrypted data. An example of that is the use of the "id-sha-256" digest-algorithm in conjunction with the encrypted content-coding . The representation-data-digest of an encrypted payload can change between different messages depending on the encryption algorithm used; in those cases its value could not be used to provide a proof of integrity "at rest" unless the whole (e.g. encoded) payload body is persisted.

The security properties of digest-algorithms are not fixed. Algorithm Agility (see ) is achieved by providing implementations flexibility in their choice of digest-algorithm from the IANA Digest Algorithm Values registry in . To help endpoints understand weaker algorithms from stronger ones, this document adds to the IANA Digest Algorithm Values registry a new "Status" field containing the most-recent appraisal of the digest-algorithm; the allowed values are specified in . An endpoint might have a preference for algorithms, such as preferring "standard" algorithms over "deprecated" ones. Transition from weak algorithms is supported by negotiation of digest-algorithm using Want-Digest (see ) or by sending multiple representation-data-digest values from which the receiver chooses. Endpoints are advised that sending multiple values consumes resources, which may be wasted if the receiver ignores them (see ).

This memo sets this spec to be the establishing document for the HTTP Digest Algorithm Values

This memo adds the field "Status" to the HTTP Digest Algorithm Values registry. The allowed values for the "Status" fields are described below. "standard" for standardized algorithms without known problems; "experimental", "obsoleted" or some other appropriate value - e.g. according to the type and status of the primary document in which the algorithm is defined; "deprecated" when the algorithm is insecure or otherwise undesirable.

This memo updates the "MD5" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: md5 Description: As specified in . Status: As specified in .

This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: As specified in . Description: As specified in . Status: As specified in .

This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: As specified in . Description: As specified in . Status: As specified in .

This memo updates the "CRC32c" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: crc32c Description: The CRC32c algorithm is a 32-bit cyclic redundancy check. It achieves a better hamming distance (for better error-detection performance) than many other 32-bit CRC functions. Other places it is used include iSCSI and SCTP. The 32-bit output is encoded in hexadecimal (using between 1 and 8 ASCII characters from 0-9, A-F, and a-f; leading 0's are allowed). For example, crc32c=0a72a4df and crc32c=A72A4DF are both valid checksums for the 3-byte message "dog". Reference: appendix B, this document. Status: standard.

This memo updates the "SHA" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: sha Description: As specified in . Status: As specified in .

This memo updates the "ADLER32" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: adler32 Description: The ADLER32 algorithm is a checksum specified in "ZLIB Compressed Data Format". The 32-bit output is encoded in hexadecimal (using between 1 and 8 ASCII characters from 0-9, A-F, and a-f; leading 0's are allowed). For example, adler32=03da0195 and adler32=3DA0195 are both valid checksums for the 4-byte message "Wiki". This algorithm is obsoleted and SHOULD NOT be used. Status: obsoleted

This memo adds the "contentMD5" token in the HTTP Digest Algorithm Values registry: Digest Algorithm: contentMD5 Description: defined the "contentMD5" token to be used only in Want-Digest. This token is obsoleted and MUST NOT be used. Reference: of this document, . Status: obsoleted

This memo registers the "id-sha-256" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: id-sha-256 Description: As specified in . Status: As specified in .

This memo registers the "id-sha-512" digest-algorithm in the HTTP Digest Algorithm Values registry: Digest Algorithm: id-sha-512 Description: As specified in . Status: As specified in .

The digest-algorithm values for "MD5", "SHA", "SHA-256", "SHA-512", "UNIXcksum", "UNIXsum", "ADLER32" and "CRC32c" have been updated to lowercase. The status of "MD5" has been updated to "deprecated", and its description states that this algorithm MUST NOT be used. The status of "SHA" has been updated to "deprecated", and its description states that this algorithm MUST NOT be used. The status for "CRC2c", "UNIXsum" and "UNIXcksum" has been updated to "standard". The "id-sha-256" and "id-sha-512" algorithms have been added to the registry.

This section registers the Want-Digest field in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" . Field name: Want-Digest Status: permanent Specification document(s): of this document

This section registers the Digest field in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" . Field name: Digest Status: permanent Specification document(s): of this document

The Open Group National Institute of Standards and Technology, U.S. Department of Commerce Carnagie Mellon University, Software Engineering Institute Inria, France Nanyang Technological University, Singapore; Temasek Laboratories, Singapore

The following examples show how representation metadata, payload transformations and method impacts on the message and payload body. When the payload body contains non-printable characters (eg. when it is compressed) it is shown as base64-encoded string. A request with a json object without any content coding. Request:

PUT /entries/1234 HTTP/1.1 Content-Type: application/json {"hello": "world"}

Here is a gzip-compressed json object using a content coding. Request:

PUT /entries/1234 HTTP/1.1 Content-Type: application/json Content-Encoding: gzip H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

Now the same payload body conveys a malformed json object. Request:

PUT /entries/1234 HTTP/1.1 Content-Type: application/json H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=

A Range-Request alters the payload body, conveying a partial representation. Request:

GET /entries/1234 HTTP/1.1 Range: bytes=1-7

Response:

HTTP/1.1 206 Partial Content Content-Encoding: gzip Content-Type: application/json Content-Range: bytes 1-7/18 iwgAla3RXA==

Now the method too alters the payload body. Request:

HEAD /entries/1234 HTTP/1.1 Accept: application/json Accept-Encoding: gzip

Response:

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

Finally the semantics of an HTTP response might decouple the effective request URI from the enclosed representation. In the example response below, the Content-Location header field indicates that the enclosed representation refers to the resource available at /authors/123. Request:

POST /authors/ HTTP/1.1 Accept: application/json Content-Type: application/json {"author": "Camilleri"}

Response:

HTTP/1.1 201 Created Content-Type: application/json Content-Location: /authors/123 Location: /authors/123 {"id": "123", "author": "Camilleri"}

Why remove all references to content-md5? Those were unnecessary to understanding and using this spec. Why remove references to instance manipulation? Those were unnecessary for correctly using and applying the spec. An example with Range Request is more than enough. This doc uses the term "partial representation" which should group all those cases. How to use Digest with PATCH method? See . Why remove references to delta-encoding? Unnecessary for a correct implementation of this spec. The revised spec can be nicely adapted to "delta encoding", but all the references here to delta encoding don't add anything to this RFC. Another job would be to refresh delta encoding. Why remove references to Digest Authentication? This RFC seems to me completely unrelated to Digest Authentication but for the word "Digest". What changes in Want-Digest? The contentMD5 token defined in is deprecated by . To clarify that Digest and Want-Digest can be used in both requests and responses - carefully uses sender and receiver in their definition - we added examples on using Want-Digest in responses to advertise the supported digest-algorithms and the inability to accept requests with unsupported digest-algorithms. Does this spec changes supported algorithms? This RFC updates which is still delegated for all algorithms updates, and adds two more algorithms: "id-sha-256" and "id-sha-512" which allows to send a checksum of a resource representation with no content codings applied. To simplify a future transition to Structured Fields we suggest to use lowercase for digest-algorithms. What about mid-stream trailers? While mid-stream trailers are interesting, since this specification is a rewrite of we do not think we should face that. As a first thought, nothing in this document precludes future work that would find a use for mid-stream trailers, for example an incremental digest-algorithm. A document defining such a digest-algorithm is best positioned to describe how it is used.

The vast majority of this document is inherited from , so thanks to J. Mogul and A. Van Hoff for their great work. The original idea of refreshing this document arose from an interesting discussion with M. Nottingham, J. Yasskin and M. Thomson when reviewing the MICE content coding.

RFC Editor: Please remove this section before publication. How can I generate and validate the Digest values shown in the examples throughout this document? The following python3 code can be used to generate digests for json objects using SHA algorithms for a range of encodings. Note that these are formatted as base64. This function could be adapted to other algorithms and should take into account their specific formatting rules.

import base64, json, hashlib, brotli def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256): json_bytes = json.dumps(item).encode() content_encoded = encoding(json_bytes) checksum_bytes = algorithm(content_encoded).digest() return base64.encodebytes(checksum_bytes).strip() item = {"hello": "world"} print("Encoding | digest-algorithm | digest-value") print("Identity | sha256 |", digest(item)) # Encoding | digest-algorithm | digest-value # Identity | sha256 | 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= print("Encoding | digest-algorithm | digest-value") print("Brotli | sha256 |", digest(item, encoding=brotli.compress)) # Encoding | digest-algorithm | digest-value # Brotli , sha256 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= print("Encoding | digest-algorithm | digest-value") print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512)) # Encoding | digest-algorithm | digest-value # Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2s vX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==\n'

RFC Editor: Please remove this section before publication.

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

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.

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

Align title with document name Add id-sha-* algorithm examples #880 Reference and 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