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Applications and Real-Time HTTP Digest This document defines HTTP fields that support integrity checksums. The Digest field can be used for the integrity of HTTP representations. The Content-Digest field can be used for the integrity of HTTP message content. Want-Digest and Want-Content-Digest can be used to indicate a sender's desire to receive integrity fields respectively. This document obsoletes RFC 3230. 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.

Introduction HTTP does not define a means to protect the integrity of representations. When HTTP messages are transferred between endpoints, the protocol might choose to make use of features of the lower layer in order to provide some integrity protection; for instance, TCP checksums or TLS records . This document defines two digest integrity mechanisms for HTTP. First, representation data integrity, which acts on representation data (). Second, content digest integrity, which acts on conveyed content (). Both mechanisms operate independent of transport integrity, offering the potential to detect programming errors and corruption of data in flight or at rest. They can be used across multiple hops in order to provide end-to-end integrity guarantees, which can aid fault diagnosis when resources are transferred across hops and system boundaries. Finally, they can be used to validate integrity when reconstructing a resource fetched using different HTTP connections. This document obsoletes .

Document Structure This document is structured as follows: describes concepts related to representation digests, defines the Digest request and response header and trailer field, defines the Content-Digest request and response header and trailer field, defines the Want-Digest and Want-Content-Digest request and response header and trailer field, describes algorithms and their relation to Digest, details computing representation digests, and provide examples of using Digest and Want-Digest.

Concept Overview This document defines the Digest request and response header and trailer field; see . At a high level, the value contains a checksum, computed over selected representation data (), that the recipient can use to validate integrity. Basing Digest on the selected representation makes it straightforward to apply it to use-cases where the transferred data requires some sort of manipulation to be considered a representation or conveys a partial representation of a resource, such as Range Requests (see ). To support use-cases where a simple checksum of the content bytes is required, this document introduces the Content-Digest request and response header and trailer field; see . Digest and Content-Digest support algorithm agility. The Want-Digest and Want-Content-Digest fields allows endpoints to express interest in Digest and Content-Digest respectively, and preference of algorithms in either. Digest field calculations are tied to the Content-Encoding and Content-Type header fields. Therefore, a given resource may have multiple different checksum values when transferred with HTTP. Digest fields do not provide integrity for HTTP messages or fields. However, they can be combined with other mechanisms that protect metadata, such as digital signatures, in order to protect the phases of an HTTP exchange in whole or in part. This specification does not define means for authentication, authorization or privacy.

Replacing RFC 3230 Historically, the Content-MD5 header field provided an HTTP integrity mechanism but HTTP/1.1 () obsoleted it due to inconsistent handling of partial responses. defined the concept of "instance" digests and a more flexible integrity scheme to help address issues with Content-MD5. It first introduced the Digest and Want-Digest fields. HTTP terminology has evolved since was published. The concept of "instance" has been superseded by selected representation. This document replaces . The changes described in the following paragraphs are intended to be semantically compatible with existing implementations where possible. The Digest and Want-Digest field definitions are updated to align with the terms and notational conventions in . Negotiation of Content-MD5 is deprecated and has been replaced by Content-Digest negotiation via Want-Content-Digest. Sections and of defined field parameters. This document obsoletes the usage of parameters with Digest because this feature has not been widely deployed and complicates field-value processing. intended field parameters to provide a common way to attach additional information to a representation-data-digest. However, if parameters are used as an input to validate the checksum, an attacker could alter them to steer the validation behavior. A digest-algorithm can still be parameterized by defining its own way to encode parameters into the representation-data-digest, in such a way as to mitigate security risks related to its computation. The algorithm table has been updated to reflect the current state of the art, (see ).

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 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 and the "qvalue" rule defined in . The definitions "representation", "selected representation", "representation data", "representation metadata", and "content" in this document are to be interpreted as described in . Algorithm names respect the casing used in their definition document (e.g. SHA-1, CRC32c) whereas digest-algorithm tokens are quoted (e.g. "sha", "crc32c").

Representation Digest The representation digest is an integrity mechanism for HTTP resources which uses a checksum that is calculated independently of the content (see ). It uses the representation data (see ), that can be fully or partially contained in the content, or not contained at all. This takes into account the effect of the HTTP semantics on the messages; for example, the content can be affected by Range Requests or methods such as HEAD, while the way the content is transferred "on the wire" is dependent on other transformations (e.g. transfer codings for HTTP/1.1 - see ). To help illustrate how such things affect Digest, several examples are provided in . 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>

When a message has no representation data it is still possible to assert that no representation data was sent computing the representation digest on an empty string (see ). The checksum is computed using one of the digest-algorithms listed in the HTTP Digest Algorithm Values Registry (see ) and then encoded in the associated format.

The Digest Field The Digest field contains a comma-separated list of one or more representation digest values as defined in . It can be used in both requests and responses.

Digest = 1#representation-data-digest

For example:

Digest: sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

A Digest field MAY contain multiple representation-data-digest values. For example, a server may provide representation-data-digest values using different algorithms, allowing it to support a population of clients with different evolving capabilities; this is particularly useful in support of transitioning away from weaker algorithms should the need arise (see ).

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

A recipient MAY ignore any or all of the representation-data-digests in a Digest field. This allows the recipient to choose which digest-algorithm(s) to use for validation instead of verifying every received representation-data-digest. A sender MAY send a representation-data-digest using a digest-algorithm without knowing whether the recipient supports the digest-algorithm, or even knowing that the recipient will ignore it. Digest can be sent in a trailer section. In this case, Digest MAY be merged into the header section; see . When an incremental digest-algorithm is used, the sender and the receiver can dynamically compute the digest value while streaming the content. A non-comprehensive set of examples showing the impacts of representation metadata, payload transformations and HTTP methods on Digest is provided in and .

The Content-Digest Field The Content-Digest field contains a comma-separated list of one or more content digest values. A content digest value is computed by applying a digest-algorithm to the actual message content (see ). It can be used in both requests and responses.

Content-Digest = 1#content-digest content-digest = digest-algorithm "=" <encoded digest output>

For example:

Content-Digest: sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==

A Content-Digest field MAY contain multiple content-digest values, similarly to Digest (see )

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

A recipient MAY ignore any or all of the content-digests in a Content-Digest field. This allows the recipient to choose which digest-algorithm(s) to use for validation instead of verifying every received content-digest. A sender MAY send a content-digest using a digest-algorithm without knowing whether the recipient supports the digest-algorithm, or even knowing that the recipient will ignore it. Content-Digest can be sent in a trailer section. In this case, Content-Digest MAY be merged into the header section; see . When an incremental digest-algorithm is used, the sender and the receiver can dynamically compute the digest value while streaming the content.

Want-Digest and Want-Content-Digest Fields Senders can indicate their integrity checksum preferences using the Want-Digest or Want-Content-Digest fields. These can be used in both requests and responses. Want-Digest indicates the sender's desire to receive a representation digest on messages associated with the request URI and representation metadata, using the Digest field. Want-Content-Digest indicates the sender's desire to receive a content digest on messages associated with the request URI and representation metadata, using the Content-Digest field.

Want-Digest = 1#want-digest-value Want-Content-Digest = 1#want-digest-value want-digest-value = digest-algorithm [ ";" "q" "=" qvalue]

qvalue indicates the sender's digest-algorithm preferences. ) describes qvalue usage and semantics. Senders can provide multiple digest-algorithm items with the same qvalue. Examples:

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

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

digest-algorithm = token

All digest-algorithm token values are case-insensitive but lower case is preferred; digest-algorithm token values MUST be compared in a case-insensitive fashion. Every digest-algorithm defines its computation procedure and encoding output. Unless specified otherwise, comparison of encoded output is case-sensitive. The "HTTP Digest Algorithm Values Registry", maintained by IANA at https://www.iana.org/assignments/http-dig-alg/ registers digest-algorithm values. Registrations MUST include the following fields: Digest algorithm: the token value. The registry can be used to reserve token values Status: the status of the algorithm. Use "standard" for standardized algorithms without known problems; "experimental" or some other appropriate value e.g. according to the type and status of the primary document in which the algorithm is defined; "insecure" when the algorithm is insecure; "reserved" when Digest algorithm references a reserved token value Description: the description of the digest-algorithm and its encoding Reference: a set of pointers to the primary documents defining the digest-algorithm The associated encoding for new digest-algorithms MUST either be represented as a quoted string or MUST NOT include ";" or "," in the character sets used for the encoding. Insecure digest algorithms MAY be used to preserve integrity against accidental change, but MUST NOT be used in a potentially adversarial setting; for example, when signing the digest of content for authenticity. The registry is initialized with the tokens listed below.

sha-512

Digest Algorithm: sha-512 Description: The SHA-512 algorithm . The output of this algorithm is encoded using the base64 encoding . Reference: , , this document. Status: standard

sha-256

Digest Algorithm: sha-256 Description: The SHA-256 algorithm . The output of this algorithm is encoded using the base64 encoding . Reference: , , this document. Status: standard

md5

Digest Algorithm: md5 Description: The MD5 algorithm, as specified in . The output of this algorithm is encoded using the base64 encoding . This digest-algorithm is now vulnerable to collision attacks. See and . Reference: , , this document. Status: insecure

sha

Digest Algorithm: sha Description: The SHA-1 algorithm . The output of this algorithm is encoded using the base64 encoding . This digest-algorithm is now vulnerable to collision attacks. See and . Reference: , , , this document. Status: insecure

unixsum

Digest Algorithm: unixsum Description: The algorithm computed by the UNIX "sum" command, as defined by the Single UNIX Specification, Version 2 . 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: insecure

unixcksum

Digest Algorithm: unixcksum 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: insecure

adler32

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. Reference: , this document. Status: insecure

crc32c

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

Using Digest in State-Changing Requests When the representation enclosed in a state-changing request does not describe the target resource, the representation digest MUST be computed on the representation-data. This is the only possible choice because representation digest requires complete representation metadata (see ). 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 is done according to the HTTP semantics of the given method, for example using the Content-Location header field (see ). In contrast, the Location header field does not affect Digest because it is not representation metadata. For example, in PATCH requests, the representation digest will be computed on the patch document because the representation metadata refers to the patch document and not to the target resource (see ). In responses, instead, the representation digest will be computed on the selected representation of the patched resource.

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

Security Considerations

Digest Does Not Protect the Full HTTP Message This document specifies a data integrity mechanism that protects HTTP representation data or content, but not HTTP header and trailer fields, from certain kinds of accidental corruption. Digest fields are not intended to be a general protection against malicious tampering with HTTP messages. This can be achieved by combining it with other approaches such as transport-layer security or digital signatures.

Digest for End-to-End Integrity Digest fields can help detect representation data or content modification due to implementation errors, undesired "transforming proxies" (see ) or other actions as the data passes across multiple hops or system boundaries. Even a simple mechanism for end-to-end representation data integrity is valuable because user-agent can validate that resource retrieval succeeded before handing off to a HTML parser, video player etc. for parsing. Note that using digest fields alone does not provide end-to-end integrity of HTTP messages over multiple hops, since metadata could be manipulated at any stage. Methods to protect metadata are discussed in .

Usage in Signatures 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 digest fields are hashes of resource representations, they explicitly depend on the representation metadata (e.g. the values of Content-Type, Content-Encoding etc). A signature that protects Digest but not other representation metadata can expose the communication to tampering. For example, an actor could manipulate the Content-Type field-value and cause a digest validation failure at the recipient, preventing the application from accessing the representation. Such an attack consumes the resources of both endpoints. See also . Digest fields SHOULD always be used over a connection that provides integrity at the transport layer that protects HTTP fields. A Digest field using NOT RECOMMENDED digest-algorithms SHOULD NOT be used in signatures. Using signatures to protect the checksum of an empty representation allows receiving endpoints to detect if an eventual payload has been stripped or added. Any mangling of digest fields, including de-duplication of representation-data-digest values or combining different field values (see ) might affect signature validation.

Usage in Trailer Fields Before sending digest fields in a trailer section, the sender should consider that intermediaries are explicitly allowed to drop any trailer (see ). When digest fields are used in a trailer section, the field-values are received after the content. Eager processing of content before the trailer section prevents digest validation, possibly leading to processing of invalid data. Not every digest-algorithm is suitable for use in the trailer section, some may require to pre-process the whole payload before sending a message (e.g. see ).

Usage with Encryption Digest fields may expose details of encrypted payload when the checksum is computed on the unencrypted data. The checksum of an encrypted payload can change between different messages depending on the encryption algorithm used; in those cases its value could not be used to provide a proof of integrity "at rest" unless the whole (e.g. encoded) content is persisted.

Algorithm Agility The security properties of digest-algorithms are not fixed. Algorithm Agility (see ) is achieved by providing implementations with flexibility choose digest-algorithms from the IANA Digest Algorithm Values registry in . To help endpoints distinguish weaker algorithms from stronger ones, this document adds to the IANA Digest Algorithm Values registry a new "Status" field containing the most recent appraisal of the digest-algorithm. An endpoint might have a preference for algorithms, such as preferring "standard" algorithms over "insecure" ones. Transition from weak algorithms is supported by negotiation of digest-algorithm using Want-Digest or Want-Content-Digest (see ) 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 ).

Duplicate digest-algorithm in field value An endpoint might receive multiple representation-data-digest values (see ) that use the same digest-algorithm with different or identical digest-values. For example:

Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=, sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

A receiver is permitted to ignore any representation-data-digest value, so validation of duplicates is left as an implementation decision. Endpoints might select all, some, or none of the values for checksum comparison and, based on the intersection of those results, conditionally pass or fail digest validation.

Resource exhaustion Digest fields validation consumes computational resources. In order to avoid resource exhaustion, implementations can restrict validation of the algorithm types, number of validations, or the size of content.

IANA Considerations

Establish the HTTP Digest Algorithm Values Registry This memo sets this specification to be the establishing document for the HTTP Digest Algorithm Values registry. IANA is asked to update the "Reference" for this registry to refer this document and to inizialize the registry with the tokens defined in . This registry uses the Specification Required policy ().

Obsolete "contentMD5" token in Digest Algorithm 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

Changes Compared to RFC3230 The contentMD5 digest-algorithm token defined in has been added to the HTTP Digest Algorithm Values Registry with the "obsoleted" status. All digest-algorithms defined in are now "insecure".

Changes Compared to RFC5843 The digest-algorithm tokens for "MD5", "SHA", "SHA-256", "SHA-512" have been updated to lowercase. The status of "MD5" and "SHA" has been updated to "insecure", and their description has been modified accordingly.

Want-Digest Field Registration 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

Digest Field Registration 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

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

Content-Digest Field Registration This section registers the Content-Digest field in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" . Field name: Content-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 Adobe Fastly greenbytes GmbH Adobe Fastly greenbytes GmbH Mozilla Google Fastly The Varnish Cache Project

Resource Representation and Representation-Data The following examples show how representation metadata, payload transformations and method impacts on the message and content. When the content contains non-printable characters (e.g. when it is compressed) it is shown as a Base64-encoded string.

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

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

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=

A Range-Request alters the content, conveying a partial representation.

GET /entries/1234 HTTP/1.1 Host: foo.example Range: bytes=1-7

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

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

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, 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"}

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

Examples of Unsolicited Digest The following examples demonstrate interactions where a server responds with a Digest or Content-Digest fields even though the client did not solicit one using Want-Digest or Want-Content-Digest. Some examples include JSON objects in the content. For presentation purposes, objects that fit completely within the line-length limits are presented on a single line using compact notation with no leading space. Objects that would exceed line-length limits are presented across multiple lines (one line per key-value pair) with 2 spaced of leading indentation. Checksum mechanisms defined in this document are media-type agnostic and do not provide canonicalization algorithms for specific formats. Examples are calculated inclusive of any space. While examples can include both fields, Digest and Content-Digest can be returned independently.

Server Returns Full Representation Data In this example, the message content conveys complete representation data, so Digest and Content-Digest have the same value.

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

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

Server Returns No Representation Data In this example, a HEAD request is used to retrieve the checksum of a resource. The response Digest field-value is calculated over the JSON object {"hello": "world"}, which is not shown because there is no payload data. Content-Digest is computed on empty content.

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

HTTP/1.1 200 OK Content-Type: application/json Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= Content-Digest: sha-256=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

Server Returns Partial Representation Data In this example, the client makes a range request and the server responds with partial content. The Digest field-value represents the entire JSON object {"hello": "world"}, while the Content-Digest field-value is computed on the message content "hello".

GET /items/123 HTTP/1.1 Host: foo.example Range: bytes=1-7

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

Client and Server Provide Full Representation Data The request contains a Digest field-value calculated on the enclosed representation. It also includes an Accept-Encoding: br header field that advertises the client supports Brotli encoding. The response includes a Content-Encoding: br that indicates the selected representation is Brotli-encoded. The Digest field-value is therefore different compared to the request. For presentation purposes, the response body is displayed as a Base64-encoded string because it contains non-printable characters.

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

HTTP/1.1 200 OK Content-Type: application/json Content-Location: /items/123 Content-Encoding: br Content-Length: 22 Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

Client Provides Full Representation Data, Server Provides No Representation Data The request Digest field-value is calculated on the enclosed payload. The response Digest field-value depends on the representation metadata header fields, including Content-Encoding: br even when the response does not contain content.

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

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

Client and Server Provide Full Representation Data The response contains two digest values using different algorithms. As the response body contains non-printable characters, it is displayed as a base64-encoded string.

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

HTTP/1.1 200 OK Content-Type: application/json Content-Encoding: br Content-Location: /items/123 Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=, sha-512=pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE6 0jBCwnMPyA/s3NF3ZO5oIWA7lf8ukk+\n5KJzm3p5og== iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==

POST Response does not Reference the Request URI The request Digest field-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.

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

HTTP/1.1 201 Created Content-Type: application/json Content-Location: /books/123 Location: /books/123 Digest: sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE= { "id": "123", "title": "New Title" }

Note that a 204 No Content response without content but with the same Digest field-value would have been legitimate too. In that case, Content-Digest would have been computed on an empty content.

POST Response Describes the Request Status The request Digest field-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.

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

HTTP/1.1 201 Created Content-Type: application/json Digest: sha-256=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY= Location: /books/123 { "status": "created", "id": "123", "ts": 1569327729, "instance": "/books/123" }

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

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

HTTP/1.1 200 OK Content-Type: application/json Digest: sha-256=yxOAqEeoj+reqygSIsLpT0LhumrNkIds5uLKtmdLyYE= { "id": "123", "title": "New Title" }

Note that a 204 No Content response without content but with the same 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 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 response Digest field-value is computed on this enclosed representation.

HTTP/1.1 404 Not Found Content-Type: application/problem+json Digest: sha-256=KPqhVXAT25LLitV1w0O167unHmVQusu+fpxm65zAsvk= { "title": "Not Found", "detail": "Cannot PATCH a non-existent resource", "status": 404 }

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

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

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=

Examples of Want-Digest Solicited Digest The following examples demonstrate interactions where a client solicits a Digest using Want-Digest. The behavior of Content-Digest and Want-Content-Digest is identical. 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-Digest: sha-256;q=0.3, sha;q=1

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

Server Selects Algorithm Unsupported by Client The client requests a only "sha" digest because that is the only algorithm it supports. The server is not obliged to produce a response containing a "sha" digest, it instead uses a different algorithm.

GET /items/123 HTTP/1.1 Host: foo.example Want-Digest: sha;q=1

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

Server Does Not Support Client Algorithm and Returns an Error is an example where a server ignores the client's preferred digest algorithm. Alternatively a server can also reject the request and return an error. In this example, the client requests a "sha" Digest, and the server returns an error with problem details contained in the content. The problem details contain a list of the digest algorithms that the server supports. This is purely an example, this specification does not define any format or requirements for such content.

GET /items/123 HTTP/1.1 Host: foo.example Want-Digest: sha;q=1

HTTP/1.1 400 Bad Request Content-Type: application/problem+json { "title": "Bad Request", "detail": "Supported digest-algorithms: sha-256, sha-512", "status": 400 }

Acknowledgements 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. Thanks to Julian Reschke for his valuable contributions to this document, and to the following contributors that have helped improve this specification by reporting bugs, asking smart questions, drafting or reviewing text, and evaluating open issues: Mike Bishop, Brian Campbell, Matthew Kerwin, James Manger, Tommy Pauly, Sean Turner, and Erik Wilde.

FAQ RFC Editor: Please remove this section before publication. Why remove all references to content-md5? Those were unnecessary to understanding and using this specification. Why remove references to instance manipulation? Those were unnecessary for correctly using and applying the specification. An example with Range Request is more than enough. This document uses the term "partial representation" which should group all those cases. How to use Digest with PATCH method? See . Why remove references to delta-encoding? Unnecessary for a correct implementation of this specification. The revised specification can be nicely adapted to "delta encoding", but all the references here to delta encoding don't add anything to this RFC. Another job would be to refresh delta encoding. Why remove references to Digest Authentication? This specification 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 this document. 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 specification change supported algorithms? Yes. This RFC updates which is still delegated for all algorithms updates. To simplify a future transition to Structured Fields we suggest to use lowercase for digest-algorithms. What about mid-stream trailer fields? While mid-stream trailer fields 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.

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

import base64, json, hashlib, brotli, logging log = logging.getLogger() def encode_item(item, encoding=lambda x: x): indent = 2 if isinstance(item, dict) and len(item) > 1 else None json_bytes = json.dumps(item, indent=indent).encode() return encoding(json_bytes) def digest_bytes(bytes_, algorithm=hashlib.sha256): checksum_bytes = algorithm(bytes_).digest() log.warning("Log bytes: \n[%r]", bytes_) return base64.encodebytes(checksum_bytes).strip() def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256): content_encoded = encode_item(item, encoding) return digest_bytes(content_encoded, algorithm) item = {"hello": "world"} print("Encoding | digest-algorithm | digest-value") print("Identity | sha256 |", digest(item)) # Encoding | digest-algorithm | digest-value # Identity | sha256 | X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= print("Encoding | digest-algorithm | digest-value") print("Brotli | sha256 |", digest(item, encoding=brotli.compress)) # Encoding | digest-algorithm | digest-value # Brotli | sha256 | 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= print("Encoding | digest-algorithm | digest-value") print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512)) print("Identity | sha512 |", digest(item, algorithm=hashlib.sha512, encoding=brotli.compress)) # Encoding | digest-algorithm | digest-value # Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm' # '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==' # Brotli | sha512 | b'pxo7aYzcGI88pnDnoSmAnaOEVys0MABhgvHY9+VI+ElE6' # '0jBCwnMPyA/s3NF3ZO5oIWA7lf8ukk+\n5KJzm3p5og=='

Changes RFC Editor: Please remove this section before publication.

Since draft-ietf-httpbis-digest-headers-06 Remove id-sha-256 and id-sha-512 from the list of supported algorithms #855

Since draft-ietf-httpbis-digest-headers-05 Reboot digest-algorithm values registry #1567 Add Content-Digest #1542 Remove SRI section #1478

Since draft-ietf-httpbis-digest-headers-04 Improve SRI section #1354 About duplicate digest-algorithms #1221 Improve security considerations #852 md5 and sha deprecation references #1392 Obsolete 3230 #1395 Editorial #1362

Since draft-ietf-httpbis-digest-headers-03 Reference semantics-12 Detail encryption quirks Details on Algorithm agility #1250 Obsolete parameters #850

Since draft-ietf-httpbis-digest-headers-02 Deprecate SHA-1 #1154 Avoid id-* with encrypted content Digest is independent from MESSAGING and HTTP/1.1 is not normative #1215 Identity is not a valid field value for content-encoding #1223 Mention trailers #1157 Reference httpbis-semantics #1156 Add contentMD5 as an obsoleted digest-algorithm #1249 Use lowercase digest-algorithms names in the doc and in the digest-algorithm IANA table.

Since draft-ietf-httpbis-digest-headers-01 Digest of error responses is computed on the error representation-data #1004 Effect of HTTP semantics on payload and message body moved to appendix #1122 Editorial refactoring, moving headers sections up. #1109-#1112, #1116, #1117, #1122-#1124

Since draft-ietf-httpbis-digest-headers-00 Align title with document name Add id-sha-* algorithm examples #880 Reference 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