Digest Fields

1.1. Document Structure

This document is structured as follows:¶

• Section 2 defines the Repr-Digest request and response header and trailer field,¶
• Section 3 defines the Content-Digest request and response header and trailer field,¶
• Section 4 defines the Want-Repr-Digest and Want-Content-Digest request and response header and trailer field,¶
• Section 5 describes algorithms and their relation to the fields defined in this document,¶
• Section 2.1 details computing representation digests,¶
• Appendix B and Appendix C provide examples of using Repr-Digest and Want-Repr-Digest.¶

1.2. Concept Overview

The HTTP fields defined in this document can be used for HTTP integrity. Senders choose a hashing algorithm and calculate a digest from an input related to the HTTP message, the algorithm identifier and digest are transmitted in an HTTP field. Receivers can validate the digest for integrity purposes. Hashing algorithms are registered in the "Hash Algorithms for HTTP Digest Fields" (see Section 5).¶

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.¶

This document defines the Repr-Digest request and response header and trailer field (Section 2) that 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]).¶

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 Section 3.¶

Repr-Digest and Content-Digest support hashing algorithm agility. The Want-Repr-Digest and Want-Content-Digest fields allows endpoints to express interest in Repr-Digest and Content-Digest respectively, and preference of algorithms in either.¶

Repr-Digest and Content-Digest are collectively termed Integrity fields. Want-Repr-Digest and Want-Content-Digestare 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.¶

This specification does not define means for authentication, authorization or privacy.¶

1.3. Obsoleting RFC 3230

[RFC3230] defined the Digest and Want-Digest HTTP fields for HTTP integrity. It also coined the 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 (Section 2 and Section 3) and Integrity preference fields (Section 4) defined in this document are better aligned with current HTTP semantics and have names that more clearly articulate the intended usages.¶

1.4. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

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

This document uses the Boolean, Byte Sequence, Dictionary, Integer and List types from [STRUCTURED-FIELDS] along with the sf-dictionary and sf-list ABNF rules.¶

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 Repr-Digest and Content-Digest¶

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

2.1. Using Repr-Digest in State-Changing Requests

When the representation enclosed in a state-changing request does not describe the target resource, the representation digest MUST be computed on the representation data. This is the only possible choice because representation digest requires complete representation metadata (see Section 2).¶

In responses,¶

• if the representation describes the status of the request, Repr-Digest MUST be computed on the enclosed representation (see Appendix B.8 );¶
• if there is a referenced resource Repr-Digest MUST be computed on the selected representation of the referenced resource even if that is different from the target resource. 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.¶

2.2. Repr-Digest and Content-Location in Responses

When a state-changing method returns the Content-Location header field, the enclosed representation refers to the resource identified by its value and Repr-Digest is computed accordingly. An example is given in Appendix B.7.¶

6.1. HTTP Messages Are Not Protected In Full

This document specifies a data integrity mechanism that protects HTTP "representation data" or content, but not HTTP header and trailer fields, from certain kinds of corruption.¶

Integrity fields are not intended to be a general protection against malicious tampering with HTTP messages. This can be achieved by combining it with other approaches such as transport-layer security or digital signatures.¶

6.2. End-to-End Integrity

Integrity fields can help detect "representation data" or content modification due to implementation errors, undesired "transforming proxies" (see Section 7.7 of [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 Section 6.3.¶

6.3. Usage in Signatures

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

Digests explicitly depend on the "representation metadata" (e.g. the values of Content-Type, Content-Encoding etc). A signature that protects Integrity fields but not other "representation metadata" can expose the communication to tampering. For example, an actor could manipulate the Content-Type field-value and cause a digest validation failure at the recipient, preventing the application from accessing the representation. Such an attack consumes the resources of both endpoints. See also Section 2.2.¶

Signatures are likely to be deemed an adversarial setting when applying Integrity fields; see Section 5. 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.¶

6.4. Usage in Trailer Fields

Before sending Integrity fields in a trailer section, the sender should consider that intermediaries are explicitly allowed to drop any trailer (see Section 6.5.2 of [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]).¶

6.5. 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.¶

6.6. Algorithm Agility

The security properties of hashing algorithms are not fixed. Algorithm Agility (see [RFC7696]) is achieved by providing implementations with flexibility to choose hashing algorithms from the IANA Hash Algorithms for HTTP Digest Fields registry; see Section 7.2.¶

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-Repr-Digest or Want-Content-Digest (see Section 4) 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 Section 2).¶

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.¶

6.7. 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.¶

7.1. HTTP Field Name Registration

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

7.2. Establish the Hash Algorithms for HTTP Digest Fields Registry

This memo sets this specification to be the establishing document for the Hash Algorithms for HTTP Digest Fields registry defined in Section 5.¶

IANA is asked to initialize the registry with the entries in Table 1.¶

B.1. Server Returns Full Representation Data

In this example, the message content conveys complete representation data. This means that in the response, Repr-Digest and Content-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

NOTE: '\' line wrapping per RFC 8792

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

{"hello": "world"}

B.2. Server Returns No Representation Data

In this example, a HEAD request is used to retrieve the checksum of a resource.¶

The response Repr-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

NOTE: '\' line wrapping per RFC 8792

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

B.3. Server Returns Partial Representation Data

In this example, the client makes a range request and the server responds with partial content.¶

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

NOTE: '\' line wrapping per RFC 8792

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

"hello"

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=:

B.4. Client and Server Provide Full Representation Data

The request contains a Repr-Digest field-value calculated on the enclosed representation. It also includes an Accept-Encoding: br header field that advertises 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"}

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==

B.5. 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"}

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

B.6. 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"}

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==

B.7. POST Response does not Reference the Request URI

The request Repr-Digest field-value is computed on the enclosed representation (see Section 2.1).¶

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"}

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"
}

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

B.8. POST Response Describes the Request Status

The request Repr-Digest field-value is computed on the enclosed representation (see Section 2.1).¶

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"}

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"
}

B.9. 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"}

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

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

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

B.10. Error responses

In error responses, the "representation data" does not necessarily refer to the target resource. Instead, it refers to the representation of the error.¶

In the following example, a client sends the same request from Figure 25 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
}

B.11. Use with Trailer Fields and Transfer Coding

An origin server sends Repr-Digest as trailer field, so it can calculate digest-value while streaming content and thus mitigate resource consumption. The Repr-Digest field-value is the same as in Appendix B.1 because Repr-Digest is designed to be independent from the use of one or more transfer codings (see Section 2).¶

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

C.1. 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

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

{"hello": "world"}

C.2. Server Selects Algorithm Unsupported by Client

The client requests a "sha" digest because that is the only algorithm it supports. The server is not obliged to produce a response containing a "sha" digest, it instead uses a different algorithm.¶

GET /items/123 HTTP/1.1
Host: foo.example
Want-Repr-Digest: sha=10

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"}

C.3. Server Does Not Support Client Algorithm and Returns an Error

Appendix C.2 is an example where a server ignores the client's preferred digest algorithm. Alternatively a server can also reject the request and return 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

HTTP/1.1 400 Bad Request
Content-Type: application/problem+json

{
  "title": "Bad Request",
  "detail": "Supported hashing algorithms: sha-256, sha-512",
  "status": 400
}

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 [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¶