HTTP                                                           L. PardueInternet-Draft                                                CloudflareUpdates: 9530 (if approved)                                      M. WestIntended status: Standards Track                                  GoogleExpires: 15 June 2026                                   12 December 2025                         HTTP Unencoded Digest                 draft-ietf-httpbis-unencoded-digest-03Abstract   The Repr-Digest and Content-Digest integrity fields are subject to   HTTP content coding considerations.  There are some use cases that   benefit from the unambiguous exchange of integrity digests of   unencoded representation.  The Unencoded-Digest and Want-Unencoded-   Digest fields complement existing integrity fields for this purpose.   This document updates the terms "Integrity fields" and "Integrity   preference fields" defined in RFC 9530.About This Document   This note is to be removed before publishing as an RFC.   Status information for this document may be found at   https://datatracker.ietf.org/doc/draft-ietf-httpbis-unencoded-   digest/.   Discussion of this document takes place on the HTTP Working Group   mailing list (mailto:ietf-http-wg@w3.org), which is archived at   https://lists.w3.org/Archives/Public/ietf-http-wg/.  Working Group   information can be found at https://httpwg.org/.   Source for this draft and an issue tracker can be found at   https://github.com/httpwg/http-extensions/labels/unecoded-digest.Status of This Memo   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/.Pardue & West             Expires 15 June 2026                  [Page 1]Internet-Draft            HTTP Unencoded Digest            December 2025   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on 15 June 2026.Copyright Notice   Copyright (c) 2025 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 Revised BSD License text as   described in Section 4.e of the Trust Legal Provisions and are   provided without warranty as described in the Revised BSD License.Table of Contents   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4   3.  The Unencoded-Digest Field  . . . . . . . . . . . . . . . . .   4   4.  The Want-Unencoded-Digest Field . . . . . . . . . . . . . . .   6   5.  Messages containing both Unencoded-Digest and           Content-Encoding  . . . . . . . . . . . . . . . . . . . .   6   6.  Integrity Fields are Complementary  . . . . . . . . . . . . .   7   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  111.  Introduction   The Repr-Digest and Content-Digest integrity fields defined in   [DIGEST-FIELDS] are suitable for a range of use cases.  However,   because the fields are subject to HTTP content coding considerations,   it is difficult to support use cases that could benefit from the   exchange of integrity digests of the unencoded representation.Pardue & West             Expires 15 June 2026                  [Page 2]Internet-Draft            HTTP Unencoded Digest            December 2025   As a simple example, an application using HTTP might be presented   with request or response representation data that has been   transparently decoded.  Attempting to verify the integrity of the   data against the Repr-Digest would first require re-encoding that   data using the same coding indicated by the Content-Encoding header   field (Section 8.4 of [HTTP]), which is not always possible (see   Section 6.5 of [DIGEST-FIELDS]).   Although receivers could feasibly re-encode data in order to carry   out Repr-Digest validation, it might be impractical for certain kinds   of environments.  For instance, browsers tend to provide built-in   support for transparent decoding but little support for encoding;   while this could be done via the use of additional libraries it would   create work in JavaScript that could contend with other activities.   Even on the server side, the re-encoding of received data might not   be acceptable; some coding algorithms are optimized towards efficient   decoding at the cost of complex encoding.  A Content-Encoding field   value that indicates a series of encodings adds further complexity.   A more complex example involves HTTP Range Requests (Section 14 of   [HTTP]), where a client issues multiple requests to obtain partial   representations and "stitches" them back into a whole.   Unfortunately, if the responses have different content codings, the   Repr-Digest field will vary by the server's selected encoding (i.e.   the Content-Encoding header field, Section 8.4 of [HTTP]).  This   provides a challenge for a client - in order to verify the integrity   of the pieced-together whole it would need to remove the encoding of   each part, combine them, and then encode the result in order to   compare against one or more Repr-Digests.   The Accept-Encoding header field (Section 12.5.3 of [HTTP]) provides   the means to indicate preferences for content codings.  It is   possible for an endpoint to indicate a preference for no encoding,   for example by sending the "identity" token.  However, codings often   provide data compression that is advantageous.  Disabling content   coding in order to simplify integrity checking is possibly an   unacceptable trade-off.   For a variety of reasons, decoding and re-encoding content in order   to benefit from HTTP integrity fields is not preferable.  This   specification defines the Unencoded-Digest and Want-Unencoded-Digest   fields to support a simpler validation workflow in some scenarios   where content coding is applied.  These fields complement the other   integrity fields defined in [DIGEST-FIELDS].Pardue & West             Expires 15 June 2026                  [Page 3]Internet-Draft            HTTP Unencoded Digest            December 2025   This document updates the term "Integrity fields" defined in   [DIGEST-FIELDS] to also include the Unencoded-Digest field, and the   term "Integrity preference fields" defined in [DIGEST-FIELDS] to also   include the Want-Unencoded-Digest field.2.  Conventions and Definitions   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 includes the rules: LF (line feed)   This document uses the following terminology from Section 3 of   [STRUCTURED-FIELDS] to specify syntax and parsing: Byte Sequence,   Dictionary, and Integer.   The definitions "representation", "selected representation",   "representation data", "representation metadata", and "content" in   this document are to be interpreted as described in [HTTP].   This document uses the line folding strategies described in   [FOLDING].   The term "digest" is to be interpreted as described in   [DIGEST-FIELDS].3.  The Unencoded-Digest Field   The Unencoded-Digest HTTP field can be used in requests and responses   to communicate digests that are calculated using a hashing algorithm   applied to the entire selected representation data with no content   codings applied (Section 8.4.1 of [HTTP]).   Apart from the content coding concerns, Unencoded-Digest behaves   similarly to Repr-Digest (Section 3 of [DIGEST-FIELDS]).  In the   absence of content codings, Unencoded-Digest is identical to Repr-   Digest.   Unencoded-Digest is a Dictionary (see Section 3.2 of   [STRUCTURED-FIELDS]) where each:   *  key conveys the hashing algorithm (see Section 5 of      [DIGEST-FIELDS]) used to compute the digest;Pardue & West             Expires 15 June 2026                  [Page 4]Internet-Draft            HTTP Unencoded Digest            December 2025   *  value is a Byte Sequence (Section 3.3.5 of [STRUCTURED-FIELDS]),      that conveys an encoded version of the byte output produced by the      digest calculation.      Each Dictionary value can have zero or more Parameters      (Section 3.1.2 of [STRUCTURED-FIELDS]).  This specification does      not define any Parameters; future extensions may do so.  Unknown      Parameters MUST be ignored.   For example:   NOTE: '\' line wrapping per RFC 8792   Unencoded-Digest: \     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:   The Dictionary type can be used, for example, to attach multiple   digests calculated using different hashing algorithms in order to   support a population of endpoints with different or evolving   capabilities.  Such an approach could support transitions away from   weaker algorithms (see Section 6.6 of [DIGEST-FIELDS]).   NOTE: '\' line wrapping per RFC 8792   Unencoded-Digest: \     sha-256=:d435Qo+nKZ+gLcUHn7GQtQ72hiBVAgqoLsZnZPiTGPk=:,\     sha-512=:YMAam51Jz/jOATT6/zvHrLVgOYTGFy1d6GJiOHTohq4yP+pgk4vf2aCs\     yRZOtw8MjkM7iw7yZ/WkppmM44T3qg==:   A recipient MAY ignore any or all digests.  Application-specific   behavior or local policy MAY set additional constraints on the   processing and validation practices of the conveyed digests.   Security considerations related to ignoring digests or validating   multiple digests are presented in Sections 6.6 and 6.7 of   [DIGEST-FIELDS] respectively.   A sender MAY send a digest without knowing whether the recipient   supports a given hashing algorithm.  A sender MAY send a digest if it   knows the recipient will ignore it.   Unencoded-Digest can be sent in a trailer section.  In this case,   Unencoded-Digest MAY be merged into the header section; see   Section 6.5.1 of [HTTP].Pardue & West             Expires 15 June 2026                  [Page 5]Internet-Draft            HTTP Unencoded Digest            December 20254.  The Want-Unencoded-Digest Field   Want-Unencoded-Digest is an integrity preference field; see Section 4   of [DIGEST-FIELDS].  It indicates that the sender would like to   receive (via the Unencoded-Digest field) a representation digest on   messages associated with the request URI and representation metadata   where no content coding is applied.   If Want-Unencoded-Digest is used in a response, it indicates that the   server would like the client to provide the Unencoded-Digest field on   future requests.   Want-Unencoded-Digest is only a hint.  The receiver of the field can   ignore it and send an Unencoded-Digest field using any algorithm or   omit the field entirely.  It is not a protocol error if preferences   are ignored.  Applications that use Unencoded-Digest and Want-   Unencoded-Digest can define expectations or constraints that operate   in addition to this specification.  Ignored preferences are an   application-specific concern.   Want-Unencoded-Digest is of type Dictionary where each:   *  key conveys the hashing algorithm;   *  value is an Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) that      conveys an ascending, relative, weighted preference.  It must be      in the range 0 to 10 inclusive. 1 is the least preferred, 10 is      the most preferred, and a value of 0 means "not acceptable".      Each Dictionary value can have zero or more Parameters      (Section 3.1.2 of [STRUCTURED-FIELDS]).  This specification does      not define any Parameters; future extensions may do so.  Unknown      Parameters MUST be ignored.   Examples:   Want-Unencoded-Digest: sha-256=1   Want-Unencoded-Digest: sha-512=3, sha-256=10, unixsum=05.  Messages containing both Unencoded-Digest and Content-Encoding   Digests delivered through Unencoded-Digest apply to the unencoded   representation.  If a message is received with content codings, a   recipient needs to decode the message in order to calculate the   digest that can subsequently be used for validation.  If multiple   content codings are applied, the recipient needs to decode all   encodings in order before validation.Pardue & West             Expires 15 June 2026                  [Page 6]Internet-Draft            HTTP Unencoded Digest            December 2025   Since the digest is calculated on unencoded representation bytes,   validation of a message with content codings (as described above) can   only succeed where the decoded output produces the same byte sequence   as the input.  While Section 8.4.1 of [HTTP] describes content   codings to operate "without loss of information", that doesn't   necessarily mean a byte-for-byte equivalence.  A content coding could   perform semantically-meaningless transformations that nevertheless   result in a decoded byte sequence that does not exactly match the   original unencoded representation.  In order to avoid unintended   validation failures, care is advised when selecting content codings   for use with Unencoded-Digest; that said, most registered content   codings do provide byte-for-byte equivalence and are appropriate.6.  Integrity Fields are Complementary   Integrity fields can be used in combination to address different and   complementary needs, particularly the cases described in Section 1.   In the following examples, the selected representation data with no   content codings applied is: "An unexceptional string" following by an   LF.  For presentation purposes, the response content is displayed as   a sequence of hex-encoded bytes because it contains non-printable   characters.   The first example demonstrates a request that uses content   negotiation.   GET /boringstring HTTP/1.1   Host: example.org   Accept-Encoding: gzip               Figure 1: GET request with content negotiation   The server responds with the full GZIP-encoded representation.  The   Repr-Digest and Unencoded-Digest therefore differ.Pardue & West             Expires 15 June 2026                  [Page 7]Internet-Draft            HTTP Unencoded Digest            December 2025   NOTE: '\' line wrapping per RFC 8792   HTTP/1.1 200 OK   Content-Type: text/plain   Content-Encoding: gzip   Repr-Digest: \     sha-256=:XyjvEuFb1P5rqc2le3vQm7M96DwZhvmOwqHLu2xVpY4=:   Unencoded-Digest: \     sha-256=:5Bv3NIx05BPnh0jMph6v1RJ5Q7kl9LKMtQxmvc9+Z7Y=:   1f 8b 08 00 79 1f 08 64 00 ff   73 cc 53 28 cd 4b ad 48 4e 2d   28 c9 cc cf 4b cc 51 28 2e 29   ca cc 4b e7 02 00 7e af 07 44   18 00 00 00              Figure 2: GET response with GZIP content coding   The second example demonstrates a range request with content   negotiation.   GET /boringstring HTTP/1.1   Host: example.org   Accept-Encoding: gzip   Range: bytes=0-9              Figure 3: Range request with content negotiation   The server responds with a 206 (Partial Content) response using GZIP   content coding, it has three different Integrity fields.  The   Content-Digest relates to the response content that can be used to   validate the integrity of the received part.  Repr-Digest and   Unencoded-Digest can be used later once the entire object is   reconstructed.  The choice of which to use is left to the application   that would consider a range of factors outside the scope of this   document.Pardue & West             Expires 15 June 2026                  [Page 8]Internet-Draft            HTTP Unencoded Digest            December 2025   NOTE: '\' line wrapping per RFC 8792   HTTP/1.1 206 Partial Content   Content-Type: text/plain   Content-Encoding: gzip   Content-Range: bytes 0-9/44   Content-Digest: \     sha-256=:SotB7Pa5A7iHSBdh9mg1Ev/ktAzrxU4Z8ldcCIUyfI4=:   Repr-Digest: \     sha-256=:XyjvEuFb1P5rqc2le3vQm7M96DwZhvmOwqHLu2xVpY4=:   Unencoded-Digest: \     sha-256=:5Bv3NIx05BPnh0jMph6v1RJ5Q7kl9LKMtQxmvc9+Z7Y=:   1f 8b 08 00 79 1f 08 64 00 ff            Figure 4: Partial response with GZIP content coding7.  Security Considerations   All the same considerations documented in [DIGEST-FIELDS] apply.   This document introduces a further consideration related to the   process of validation when an HTTP message contains both Content-   Encoding and Unencoded-Digest (Section 5).  In order to validate the   Unencoded-Digest, encoded content needs to be decoded.  This provides   an opportunity for an attacker to direct malicious data into a   decoder.  One possible mitigation would be to also provide a Content-   Digest or Repr-Digest in the message, allowing for validation of the   received bytes before further processing.  An attacker that can   substitute various parts of an HTTP message presents several risks;   Sections 6.1, 6.2 and 6.3 of [DIGEST-FIELDS] describe relevant   considerations and mitigations.   A content coding may provide encryption capabilities, for example   "aes128gcm" ([RFC8188]).  Using Unencoded-Digest with such content   codings can leak information about the original data because header   fields are visible to anyone who can read the HTTP message.  This   could be used as a side channel.8.  IANA Considerations   IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field   Name Registry" [HTTP] as shown in the table below:Pardue & West             Expires 15 June 2026                  [Page 9]Internet-Draft            HTTP Unencoded Digest            December 2025    +=======================+===========+============+===============+    | Field Name            | Status    | Structured | Reference     |    |                       |           | Type       |               |    +=======================+===========+============+===============+    | Unencoded-Digest      | permanent | Dictionary | Section 3 of  |    |                       |           |            | this document |    +-----------------------+-----------+------------+---------------+    | Want-Unencoded-Digest | permanent | Dictionary | Section 4 of  |    |                       |           |            | this document |    +-----------------------+-----------+------------+---------------+          Table 1: Hypertext Transfer Protocol (HTTP) Field Name                             Registry Update9.  References9.1.  Normative References   [DIGEST-FIELDS]              Polli, R. and L. Pardue, "Digest Fields", RFC 9530,              DOI 10.17487/RFC9530, February 2024,              <https://www.rfc-editor.org/rfc/rfc9530>.   [FOLDING]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,              "Handling Long Lines in Content of Internet-Drafts and              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,              <https://www.rfc-editor.org/rfc/rfc8792>.   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,              Ed., "HTTP Semantics", STD 97, RFC 9110,              DOI 10.17487/RFC9110, June 2022,              <https://www.rfc-editor.org/rfc/rfc9110>.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/rfc/rfc2119>.   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", STD 68, RFC 5234,              DOI 10.17487/RFC5234, January 2008,              <https://www.rfc-editor.org/rfc/rfc5234>.   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",              RFC 7405, DOI 10.17487/RFC7405, December 2014,              <https://www.rfc-editor.org/rfc/rfc7405>.Pardue & West             Expires 15 June 2026                 [Page 10]Internet-Draft            HTTP Unencoded Digest            December 2025   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.   [STRUCTURED-FIELDS]              Nottingham, M. and P. Kamp, "Structured Field Values for              HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,              <https://www.rfc-editor.org/rfc/rfc9651>.9.2.  Informative References   [RFC8188]  Thomson, M., "Encrypted Content-Encoding for HTTP",              RFC 8188, DOI 10.17487/RFC8188, June 2017,              <https://www.rfc-editor.org/rfc/rfc8188>.Acknowledgments   Early drafts of [DIGEST-FIELDS] included a mechanism to support the   exchange of digests where no content coding is applied, which was   removed before publication.  While the design here is different, it   is motivated by discussion of the previous design in the HTTP WG.   The motivating use cases still mostly apply identically.   The following people provided detailed feedback on the document: Mike   Bishop, Roberto Polli, and Martin Thomson.Authors' Addresses   Lucas Pardue   Cloudflare   Email: lucas@lucaspardue.com   Mike West   Google   Email: mkwst@google.comPardue & West             Expires 15 June 2026                 [Page 11]