| HTTP | R. Polli |
| Internet-Draft | Team Digitale, Italian Government |
| Intended status: Standards Track | L. Pardue |
| Expires: May 6, 2020 | Cloudflare |
| November 03, 2019 |
Digest Headers
draft-ietf-httpbis-digest-headers-01
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.
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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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 at rest, be used across multiple hops in order to provide end-to-end integrity guarantees, aid fault diagnosis across hops and system boundaries, and can be used to validate integrity when reconstructing a resource fetched using different HTTP connections.
This document defines a mechanism that acts on HTTP representation-data. It can be combined with other mechanisms that protect representation-metadata, such as digital signatures, in order to protect the desired parts of an HTTP exchange in whole or in part.
The Content-MD5 header field was originally introduced to provide integrity, but HTTP/1.1 ([RFC7231], Appendix B) obsoleted it:
[RFC3230] provided a more flexible solution introducing the concept of “instance”, and the header fields 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].
The definition “validator” in this document is to be interpreted as described in Section 7.2 of [RFC7231].
To avoid inconsistencies, an integrity mechanism for HTTP messages should decouple the checksum calculation from:
The following examples show how representation metadata, payload transformations 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 1-7/18 iwgAla3RXA==
Now the method too alters the payload body.
Request:
HEAD /entries/1234 HTTP/1.1 Accept: application/json Accept-Encoding: gzip
Response:
HTTP/1.1 200 OK Content-Type: application/json Content-Encoding: gzip
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-algorithm 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.
Two examples of its use are
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 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.
The Digest value depends on the representation metadata.
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.
Two examples of its use are
Digest: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew== Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=, id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
POST and PATCH requests may 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 3.1).
In responses,
The latter case might be done accordingly to the HTTP semantics of the given method, for example using the Content-Location header field.
Differently from Content-Location, which is representation metadata, the Location header field does not affect Digest.
In PATCH requests the representation digest MUST be computed on the patch document.
This is because the representation metadata refers to the patch document and not to the target resource (see Section 2 of [RFC5789]).
In PATCH responses the representation digest MUST be computed on the selected representation of the patched resource.
Digest usage with PATCH is thus very similar to the POST one, but with the resource’s own semantic partly implied by the method and by the patch document.
This RFC deprecates the negotiation of Content-MD5 as it has been obsoleted by [RFC7231]
The MD5 algorithm MUST NOT be used as it has been found vulnerable to collision attacks [CMU-836068].
The SHA algorithm is NOT RECOMMENDED as it has been found vulnerable to collision attacks [IACR-2019-459].
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 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 header 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 11. 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 3.1). The major differences are in serialization format.
The SRI digest value is calculated over the identity encoding of the resource, not the selected representation (as specified for representation-data-digest in this document). Section 3.4.5 of [SRI] describes the benefit of the identity approach - the SRI integrity attribute can contain multiple algorithm-value pairs where each applies to a different identity encoded payload. This allows for protection of distinct resources sharing a URL. However, this is a contrast to the design of representation digests, where multiple Digest field-values all protect the same representation.
SRI does not specify handling of partial representation data (e.g. Range requests). In contrast, this document specifies handling in terms that are fully compatible with core HTTP concepts (an example is provided in Section 9.3).
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 header field can appear on requests.
The SRI and Representation Digest mechanism are different and complementary but one is not capable of replacing the other because they have 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 the it does not require a conversion to 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 header field even though the client did not solicit one using Want-Digest.
Request:
GET /items/123
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Content-Encoding: identity
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
As there is no content coding applied, the sha-256 and the id-sha-256 digest-values are the same.
Request:
HEAD /items/123
Response:
HTTP/1.1 200 OK Content-Type: application/json Content-Encoding: identity Digest: id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Request:
GET /items/123 Range: bytes=1-7
Response:
HTTP/1.1 206 Partial Content Content-Type: application/json Content-Encoding: identity Content-Range: bytes 1-7/18 Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= "hello"
The request contains a Digest header 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.
Request:
PUT /items/123
Content-Type: application/json
Content-Encoding: identity
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Response:
Content-Type: application/json Content-Encoding: br Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo= iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
Request Digest value is calculated on the enclosed payload. Response Digest value depends on the representation metadata header fields, including Content-Encoding: br even when the response does not contain a payload body.
Request:
PUT /items/123
Content-Type: application/json
Content-Encoding: identity
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:
Request:
PUT /items/123 HTTP/1.1
Content-Type: application/json
Content-Encoding: identity
Accept-Encoding: br
Digest: sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
{"hello": "world"}
Response:
HTTP/1.1 200 OK Content-Type: application/json Content-Encoding: br Digest: sha-256=4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=, id-sha-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= iwiAeyJoZWxsbyI6ICJ3b3JsZCJ9Aw==
Request Digest value is computed on the enclosed representation (see Section 5).
The representation enclosed in the response refers to the resource identified by Content-Location (see [RFC7231] Section 3.1.4.2 and Section 3.1.4.1 point 4).
Digest is thus computed on the enclosed representation.
Request:
POST /books HTTP/1.1
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Response
HTTP/1.1 201 Created
Content-Type: application/json
Digest: id-sha-256=BZlF2v0IzjuxN01RQ97EUXriaNNLhtI8Chx8Eq+XYSc=
Content-Location: /books/123
{"id": "123", "title": "New Title"}
Note that a 204 No Content response without a payload body but with the same Digest field-value would have been legitimate too.
Request Digest value is computed on the enclosed representation (see Section 5).
The representation enclosed in the response describes the status of the request, so Digest is computed on that enclosed representation.
Response Digest has no explicit relation with the resource referenced by Location.
Request:
POST /books HTTP/1.1
Content-Type: application/json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
Location: /books/123
{"title": "New Title"}
Response
HTTP/1.1 201 Created
Content-Type: application/json
Digest: id-sha-256=0o/WKwSfnmIoSlop2LV/ISaBDth05IeW27zzNMUh5l8=
Location: /books/123
{"status": "created", "id": "123", "ts": 1569327729, "instance": "/books/123"}
This case is analogous to a POST request where the target resource reflects the effective request URI.
The PATCH request uses the application/merge-patch+json media type defined in [RFC7396].
Digest is calculated on the enclosed payload, which corresponds to the patch document.
The response Digest is computed on the complete representation of the patched resource.
Request:
PATCH /books/123 HTTP/1.1
Content-Type: application/merge-patch+json
Accept: application/json
Accept-Encoding: identity
Digest: sha-256=bWopGGNiZtbVgHsG+I4knzfEJpmmmQHf7RHDXA3o1hQ=
{"title": "New Title"}
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Digest: id-sha-256=BZlF2v0IzjuxN01RQ97EUXriaNNLhtI8Chx8Eq+XYSc=
{"id": "123", "title": "New Title"}
Note that a 204 No Content response without a payload body but with the same Digest field-value would have been legitimate too.
The following examples demonstrate interactions where a client solicits a Digest using Want-Digest.
The client requests a digest, preferring sha. The server is free to reply with sha-256 anyway.
Request:
GET /items/123 HTTP/1.1 Want-Digest: sha-256;q=0.3, sha;q=1
Response:
HTTP/1.1 200 OK
Content-Type: application/json
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: id-sha-512=WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==
{"hello": "world"}
The client requests a sha Digest, the server advises for sha-256 and sha-512
Request:
GET /items/123 Want-Digest: sha;q=1
Response:
HTTP/1.1 400 Bad Request Want-Digest: sha-256, sha-512
This document specifies a data integrity mechanism that protects HTTP representation data, but not HTTP representation metadata header fields, from certain kinds of accidental corruption.
Digest is not intended as general protection against malicious tampering with HTTP messages, this can be achieved by combining it with other approaches such as transport-layer security or digital signatures.
Cryptographic algorithms are intended to provide a proof of integrity suited towards cryptographic constructions such as signatures.
However, these rely on collision-resistance for their security proofs [CMU-836068]. The MD5 and SHA-1 algorithms are vulnerable to collisions attacks, so MD5 MUST NOT be used and SHA-1 is NOT RECOMMENDED for use with Digest.
The ADLER32 algorithm defined in [RFC1950] has been deprecated by [RFC3309] because under certain conditions it provides weak detection of errors and is now NOT RECOMMENDED for use with Digest.
Digest alone does not provide end-to-end integrity of HTTP messages over multiple hops, as it just covers the representation data and not the representation metadata.
Besides, it allows to protect representation data from buggy manipulation, buggy compression, etc.
Moreover identity digest algorithms (eg. ID-SHA-256 and ID-SHA-512) allow piecing together a resource from different sources (e.g. different servers that perhaps apply different content codings) enabling the user-agent to detect that the application-layer tasks completed properly, before handing off to say the HTML parser, video player etc.
Even a simple mechanism for end-to-end validation is thus valuable.
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 header fields and there are additional considerations when Digest is included in this set.
Since the Digest header 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 may 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.
Digest SHOULD always be used over a connection which provides integrity at transport layer that protects HTTP header fields.
A Digest header field 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 “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 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 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 “obsoleted”, and its description states that this algorithm is NOT RECOMMENDED.
The status for “CRC32C” 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 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. |
| [RFC5789] | Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789, DOI 10.17487/RFC5789, March 2010. |
| [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", W3C Recommendation REC-SRI-20160623, June 2016. |
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.
RFC Editor: Please remove this section before publication.
How can I generate and validate the Digest values shown in the examples throughout this document?
The following python3 code can be used to generate digests for json objects using SHA algorithms for a range of encodings. Note that these are formatted as base64. This function could be adapted to other algorithms and should take into account their specific formatting rules.
import base64, json, hashlib, brotli
def digest(item, encoding=lambda x: x, algorithm=hashlib.sha256):
json_bytes = json.dumps(item).encode()
content_encoded = encoding(json_bytes)
checksum_bytes = algorithm(content_encoded).digest()
return base64.encodebytes(checksum_bytes).strip()
item = {"hello": "world"}
print("Identity encoding, sha256", digest(item))
# Out: Identity encoding, sha256 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
print("Brotli encoding, sha256", digest(item, encoding=brotli.compress))
# Out: Brotli encoding, sha256 4REjxQ4yrqUVicfSKYNO/cF9zNj5ANbzgDZt3/h3Qxo=
print("Identity encoding, sha512", digest(item, algorithm=hashlib.sha512))
# Out: Identity encoding, sha512 b'WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwE\nmTHWXvJwew==\n'
RFC Editor: Please remove this section before publication.