| HTTP | R. Polli |
| Internet-Draft | Team Digitale, Italian Government |
| Intended status: Standards Track | L. Pardue |
| Expires: January 5, 2020 | Cloudflare |
| July 04, 2019 |
Resource Digests for HTTP
draft-ietf-httpbis-digest-headers-00
This document defines the Digest and Want-Digest header fields for HTTP, 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 RFC 7231.
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.
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Integrity protection for HTTP content is multi layered and is usually achieved across the protocol stack: TCP checksums and TLS [RFC2818] record to name but some.
The HTTP protocol does not provide means to protect the various message parts. Besides, it might be desirable to add additional guarantees to the ones provided by the transport layer (eg. HTTPS). This may be in order to:
The Content-MD5 header field was originally introduced to provide integrity, but HTTP/1.1 [RFC7231] in appendix-B obsoleted it:
[RFC3230] provided a more flexible solution introducing the concept of “instance”, and the headers Digest and Want-Digest.
The concept of selected representation defined in [RFC7231] made [RFC3230] definitions inconsistent with the current standard. A refresh was then required.
This document updates the Digest and Want-Digest header field definitions to align with [RFC7231] concepts.
This approach can be easily adapted 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 [RFC7233]).
Changes are semantically compatible with existing implementations and better cover both the request and response cases.
The value of Digest is calculated on selected representation, which is tied to the value contained in any Content-Encoding or Content-Type header fields. Therefore, a given resource may have multiple different digest values.
To allow both parties to exchange a Digest of a representation with no content codings two more 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] and [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 7 of [RFC7230].
The definitions “representation”, “selected representation”, “representation data”, “representation metadata” and “payload body” in this document are to be interpreted as described in [RFC7230] and [RFC7231].
To avoid inconsistencies, an integrity mechanism for HTTP messages should decouple the checksum calculation:
The following examples show how representation metadata, payload tranformations and method impacts on the message and payload body.
Here is a gzip-compressed json object
Request:
PUT /entries/1234 HTTP/1.1
Content-Type: application/json
Content-Encoding: gzip
H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
Now the same payload body conveys a malformed json object.
Request:
PUT /entries/1234 HTTP/1.1
Content-Type: application/json
H4sIAItWyFwC/6tWSlSyUlAypANQqgUAREcqfG0AAAA=
A Range-Request alters the payload body, conveying a partial representation.
Request:
GET /entries/1234 HTTP/1.1
Range: bytes=1-7
Response:
HTTP/1.1 206 Partial Content
Content-Encoding: gzip
Content-Type: application/json
Content-Range: bytes=1-7
iwgAla3RXA==
Now the method too alters the payload body.
Request:
HEAD /entries/1234 HTTP/1.1
Accept: application/json
Accept-Encoding: gzip
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: gzip
Digest algorithm values are used to indicate a specific digest computation. For some algorithms, one or more parameters may be supplied.
digest-algorithm = token
The BNF for “parameter” is as is used in [RFC7230]. All digest-algorithm values are case-insensitive.
The Internet Assigned Numbers Authority (IANA) acts as a registry for digest-algorithm values. The registry contains the following tokens.
To allow sender and recipient to provide a checksum which is independent from Content-Encoding, the following additional 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.
A representation digest is the value of the output of a digest algorithm, together with an indication of the algorithm used (and any parameters).
representation-data-digest = digest-algorithm "="
<encoded digest output>
As explained in Section 2 the digest is computed on the entire selected representation data of the resource defined in [RFC7231]:
representation-data := Content-Encoding( Content-Type( bits ) )
The encoded digest output uses the encoding format defined for the specific digest-algorithm.
The sha-256 digest-algorithm uses base64 encoding. Note that digest-algoritm values are case insensitive.
sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
The “UNIXsum” digest-algorithm uses ASCII string of decimal digits.
UNIXsum=30637
The following headers are defined
The Want-Digest message header field indicates the sender’s desire to receive a representation digest on messages associated with the Request- URI and representation metadata.
Want-Digest = "Want-Digest" ":" OWS 1#want-digest-value want-digest-value = digest-algorithm [ ";" "q" "=" qvalue] qvalue = ( "0" [ "." 0*1DIGIT ] ) / ( "1" [ "." 0*1( "0" ) ] )
If a digest-algorithm is not accompanied by a qvalue, it is treated as if its associated qvalue were 1.0.
The sender is willing to accept a digest-algorithm if and only if it is listed in a Want-Digest header 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.
Examples:
Want-Digest: sha-256 Want-Digest: SHA-512;q=0.3, sha-256;q=1, md5;q=0
The Digest header field provides a digest of the representation data
Digest = "Digest" ":" OWS 1#representation-data-digest
Representation data might be:
The resource is specified by the effective Request-URI and any cache-validator contained in the message.
For example, in a response to a HEAD request, the digest is calculated using the representation data that would have been enclosed in the payload body if the same request had been a GET.
Digest can be used in requests too. Returned value depends on the representation metadata headers.
A Digest header field MAY contain multiple representation-data-digest values. This could be useful for responses expected to reside in caches shared by users with different browsers, for example.
A recipient MAY ignore any or all of the representation-data-digests in a Digest header field. This allows the recipient to chose 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.
…
This RFC deprecates the negotiation of Content-MD5 as this header has been obsoleted by [RFC7231]
The MD5 algorithm is NOT RECOMMENDED as it’s now vulnerable to collision attacks [CMU-836068].
The SHA algorithm is NOT RECOMMENDED as it’s now vulnerable to collision attacks [IACR-2019-459].
Request:
GET /items/123
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
Content-Encoding: identity
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Request: HEAD /items/123 Response: HTTP/1.1 200 Ok Content-Type: application/json Content-Encoding: identity Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Request: GET /items/123 Range: bytes=1-7 Response: HTTP/1.1 206 Partial Content Content-Type: application/json Content-Encoding: identity Content-Range: bytes 1-7/18 Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= "hello"
Digest can be used in requests too. Returned value depends on the representation metadata headers.
Request:
PUT /items/123
Content-Type: application/json
Content-Encoding: identity
Accept-Encoding: br
Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
{"hello": "world"}
Response:
Content-Type: application/json
Content-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
The client requests a digest, preferring sha. The server is free to reply with sha-256 anyway.
Request:
GET /items/123
Want-Digest: sha-256;q=0.3, sha;q=1
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
Content-Encoding: identity
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
The client requests a sha digest only. The server is currently free to reply with a Digest containing an unsupported algorithm
Request:
GET /items/123
Want-Digest: sha;q=1
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
Content-Encoding: identity
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
The client requests a sha Digest, the server advises for sha-256 and sha-512
Request: GET /items/123 Want-Digest: sha;q=1 Response: HTTP/1.1 400 Bad Request Want-Digest: sha-256, sha-512
…
This document specifies a data integrity mechanism that protects HTTP representation data, but not HTTP representation metadata headers, from certain kinds of accidental corruption.
While it is not intended as general protection against malicious tampering with HTTP messages, this goal can be achieved using Digest together with a transport-layer security mechanism and digital signatures.
Cryptogrphic alorithms 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-1 algorithms are vulnerable to collisions attacks and they are NOT RECOMMENDED.
Digest alone does not provide end-to-end integrity of HTTP messages over multiple hops, as it just covers the representation data and not the representation metadata.
Besides, it allows to protect representation data from buggy manipulation, buggy compression, etc.
Moreover identity digest algorithms (eg. ID-SHA-256 and ID-SHA-512) allow piecing together a resource from different sources (e.g. different servers that perhaps apply different content codings) enabling the user-agent to detect that the application-layer tasks completed properly, before handing off to say the HTML parser, video player etc.
Even a simple mechanism for end-to-end validation is thus valuable.
Digital signatures are widely used together with checksums to provide the certain identification of the origin of a message [NIST800-32].
It’s important to note that, being the Digest header an hash of a resource representation, signing only the Digest header, without all the representation metatada (eg. the values of Content-Type and Content-Encoding) may expose the communication to tampering.
Digest SHOULD always be used over a connection which provides integrity at transport layer that protects HTTP headers.
A Digest header using NOT RECOMMENDED digest-algorithms SHOULD NOT be used in signatures.
…
…
This memo sets this spec to be the establishing document for the HTTP Digest Algorithm Values
This memo adds the field “Status” to the HTTP Digest Algorithm Values registry. The allowed values for the “Status” fields are described below.
This memo updates the “MD5” 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 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 status has been updated to “obsoleted” for both “SHA” and “MD5”, and their descriptions states that those algorithms are NOT RECOMMENDED.
The status for all other algorithms have 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 header field in the “Permanent Message Header Field Names” registry ([RFC3864]).
Header field name: Want-Digest
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 4.1 of this document
This section registers the Digest header field in the “Permanent Message Header Field Names” registry ([RFC3864]).
Header field name: Digest
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 4.2 of this document
| [RFC2818] | Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000. |
| [RFC7396] | Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396, DOI 10.17487/RFC7396, October 2014. |
| [SRI] | Akhawe, D., Braun, F., Marier, F. and J. Weinberger, "Subresource Integrity", June 2016. |
RFC EDITOR PLEASE DELETE THIS SECTION.
The vast majority of this document is inherited from [RFC3230], 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.