| HTTP Working Group | R. Polli |
| Internet-Draft | Team Digitale, Italian Government |
| Obsoletes: 3230 (if approved) | L. Pardue |
| Intended status: Standards Track | Cloudflare |
| Expires: October 15, 2021 | April 13, 2021 |
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.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on October 15, 2021.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
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 [RFC2818].¶
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 in flight or at rest, be used across multiple hops in order to provide end-to-end integrity guarantees, can 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 ([RFC7231], Appendix B) obsoleted it:¶
The concept of selected representation defined in Section 3.2 of [SEMANTICS] makes [RFC3230] definitions inconsistent with current HTTP semantics. This document updates the Digest and Want-Digest field definitions to align with [SEMANTICS] 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 Section 14.2 of [SEMANTICS]).¶
This document replaces [RFC3230] to better align with [SEMANTICS] and to provide more detailed description of Digest usage in request and response cases. Changes are intended to be semantically compatible with existing implementations but note that negotiation of Content-MD5 is deprecated Section 7, Digest field parameters are obsoleted Section 8, "md5" and "sha" digest-algorithms are obsoleted Section 12.2 and the "adler32" algorithm is deprecated Section 12.3.¶
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 Section 8.4.1 of [SEMANTICS]) two more digest-algorithms are added ("id-sha-256" and "id-sha-512").¶
The goals of this proposal are:¶
The goals do not include:¶
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] along with the "#rule" extension defined in Section 5.6.1 of [SEMANTICS].¶
The definitions "representation", "selected representation", "representation data", "representation metadata", and "content" in this document are to be interpreted as described in [SEMANTICS].¶
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 content (see Section 6.4 of [SEMANTICS]). It uses the representation data (see Section 8.1 of [SEMANTICS]), that can be fully or partially contained in the content, 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 content can be affected by Range Requests or methods such as HEAD, while the way the content is transferred "on the wire" is dependent on other transformations (e.g. transfer codings for HTTP/1.1 - see Section 6.1 of [HTTP11]). To help illustrate how such things affect Digest, several examples are provided in Appendix A.¶
A representation digest consists of the value of a checksum computed on the entire selected representation data (see Section 8.1 of [SEMANTICS]) of a resource identified according to Section 6.4.2 of [SEMANTICS] 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 Section 12.6).¶
The checksum is computed using one of the digest-algorithms listed in Section 5 and then encoded in the associated format.¶
The example below shows the "sha-256" digest-algorithm that uses base64 encoding.¶
sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
The Digest field contains a list of one or more representation digest values as defined in Section 2. It can be used in both requests and responses.¶
Digest = 1#representation-data-digest
For example:¶
Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm
AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
The relationship between Content-Location (see Section 8.7 of [SEMANTICS]) and Digest is demonstrated in Section 10.7. A comprehensive set of examples showing the impacts of representation metadata, payload transformations and HTTP methods on Digest is provided in Section 10 and Section 11.¶
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 Section 12.9).¶
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
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 an incremental digest-algorithms is used, the sender and the receiver can dynamically compute the digest value while streaming the content.¶
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 = 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 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 and MUST not be used:¶
See the references above for further information.¶
To allow sender and recipient to provide a checksum which is independent from Content-Encoding, the following additional digest-algorithms are defined:¶
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 Section 2).¶
In responses,¶
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 Section 2 of [PATCH]).¶
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 POST, but with the resource's own semantic partly implied by the method and by the patch document.¶
Section 4.1.1 and 4.2 of [RFC3230] 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.¶
[RFC3230] 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.¶
Subresource Integrity [SRI] 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 Section 12. 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 Section 2). The major differences are in serialization format.¶
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 Section 10.3).¶
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.¶
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.¶
Digest is media-type agnostic and does not provide canonicalization algorithms for specific formats. Examples of Digest are calculated inclusive of any space.¶
Request:¶
GET /items/123 HTTP/1.1 Host: foo.example
Response:¶
HTTP/1.1 200 OK
Content-Type: application/json
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
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.¶
Request:¶
HEAD /items/123 HTTP/1.1 Host: foo.example
Response:¶
HTTP/1.1 200 OK Content-Type: application/json Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
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"}.¶
Request:¶
GET /items/123 HTTP/1.1 Host: foo.example 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-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.¶
Request:¶
PUT /items/123 HTTP/1.1
Host: foo.example
Content-Type: application/json
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Response:¶
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==
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.¶
Request:¶
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"}
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:¶
As the response body contains non-printable characters, it is displayed as a base64-encoded string.¶
Request:¶
PUT /items/123 HTTP/1.1
Host: foo.example
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
Content-Location: /items/123
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=,
id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
The representation enclosed in the response refers to the resource identified by Content-Location (see [SEMANTICS], Section 6.4.2). Digest is thus computed on the enclosed representation.¶
Request:¶
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"}
Response:¶
HTTP/1.1 201 Created
Content-Type: application/json
Content-Location: /books/123
Location: /books/123
Digest: id-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.¶
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
Host: foo.example
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=2LBp5RKZGpsSNf8BPXlXrX4Td4Tf5R5bZ9z7kdi5VvY=
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.¶
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.¶
Request:¶
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"}
Response:¶
HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-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 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 [RFC7807].¶
The response Digest field-value is computed on this enclosed representation.¶
Request:¶
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"}
Response:¶
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
}
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 Section 10.1 because Digest is designed to be independent from the use of one or more transfer codings (see Section 2).¶
Request:¶
GET /items/123 HTTP/1.1 Host: foo.example
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.¶
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.¶
Digest is media-type agnostic and does not provide canonicalization algorithms for specific formats. Examples of Digest are calculated inclusive of any space.¶
The client requests a digest, preferring "sha". The server is free to reply with "sha-256" anyway.¶
Request:¶
GET /items/123 HTTP/1.1 Host: foo.example 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 HTTP/1.1 Host: foo.example 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 "sha-256" and "sha-512".¶
Request:¶
GET /items/123 HTTP/1.1 Host: foo.example 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 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.¶
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 [CMU-836068]. The "md5" and "sha" digest-algorithms are vulnerable to collisions attacks, so they MUST NOT be used with Digest.¶
Digest only covers the representation data and not the representation metadata. Digest could help protect the representation data from buggy manipulation, 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 user-agent can validate that resource retrieval succeeded before handing off to a HTML parser, video player etc. for parsing.¶
Identity digest-algorithms (e.g. "id-sha-256" and "id-sha-512") are particularly useful for end-to-end integrity because they allow piecing together a resource from different sources with different HTTP messaging characteristics. For example, different servers that apply different content codings.¶
Note that using Digest 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 12.6.¶
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 [NIST800-32]. 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 Section 12.5.¶
Digest 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 Digest of an empty representation allows receiving endpoints to detect if an eventual payload has been stripped or added.¶
Any mangling of Digest, including de-duplication of representation-data-digest values or combining different field values (see Section 5.2 of [SEMANTICS]) might affect signature validation.¶
When Digest is used in trailer fields, the receiver gets the digest value after the content and may thus be tempted to process the data before validating the digest value. It is prefereable that data is 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 Section 6.5.1 of [SEMANTICS]).¶
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 (eg. see [I-D.thomson-http-mice]).¶
Digest may expose details of encrypted payload when the checksum is computed on the unencrypted data. For example, the use of the "id-sha-256" digest-algorithm in conjunction with the encrypted content-coding [RFC8188].¶
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) content is persisted.¶
The security properties of digest-algorithms are not fixed. Algorithm Agility (see [RFC7696]) is achieved by providing implementations with flexibility choose digest-algorithms from the IANA Digest Algorithm Values registry in Section 13.1.¶
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 Section 13.2.¶
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 Section 4) 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 Section 3).¶
An endpoint might receive multiple representation-data-digest values (see Section 3) 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.¶
Digest 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.¶
This memo sets this specification to be the establishing document for the HTTP Digest Algorithm Values registry.¶
This memo adds the field "Status" to the HTTP Digest Algorithm Values registry. The allowed values for the "Status" fields are described below.¶
This memo updates the "MD5" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo updates the "UNIXsum" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo updates the "UNIXcksum" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo updates the "CRC32c" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo updates the "SHA" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo updates the "ADLER32" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo adds the "contentMD5" token in the HTTP Digest Algorithm Values registry:¶
This memo registers the "id-sha-256" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
This memo registers the "id-sha-512" digest-algorithm in the HTTP Digest Algorithm Values registry:¶
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" [SEMANTICS].¶
Field name: Want-Digest ¶
Status: permanent¶
This section registers the Digest field in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" [SEMANTICS].¶
Field name: Digest ¶
Status: permanent¶
The following examples show how representation metadata, payload transformations and method impacts on the message and content. When the content 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
Host: foo.example
Content-Type: application/json
{"hello": "world"}
Here is a gzip-compressed JSON object using a content coding.¶
Request:¶
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.¶
Request:¶
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.¶
Request:¶
GET /entries/1234 HTTP/1.1 Host: foo.example 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 content.¶
Request:¶
HEAD /entries/1234 HTTP/1.1 Host: foo.example 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
Host: foo.example
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"}
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))
# Encoding | digest-algorithm | digest-value
# Identity | sha512 | b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm'
# '+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew=='
RFC Editor: Please remove this section before publication. ¶