



HTTP                                                          J. Hoyland
Internet-Draft                                                          
Intended status: Informational                                 C. Patton
Expires: 23 April 2026                                        Cloudflare
                                                         20 October 2025


            HTTP Signature Component for TLS Channel Binding
             draft-hoypat-httpbis-message-signatures-ekm-00

Abstract

   A derived component is specified for HTTP Message Signatures that
   binds the signature to the underlying secure channel (TLS over TCP or
   QUIC), thereby ensuring a signed message transmitted over one channel
   cannot be retransmitted over another.  The component consists of key
   material exported from TLS.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://cjpatton.github.io/draft-hoypat-httpbis-message-signatures-
   ekm/draft-hoypat-httpbis-message-signatures-ekm.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-hoypat-httpbis-message-
   signatures-ekm/.

   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/.

   Source for this draft and an issue tracker can be found at
   https://github.com/cjpatton/draft-hoypat-httpbis-message-signatures-
   ekm.

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/.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The @ekm Derived Component  . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   HTTP Message Signatures [RFC9421] allow various components of an HTTP
   message to the authenticated by the sender either using a digital
   signature or a message authentication code (MAC).  The exact set of
   components to be signed may very depending upon the application:

   1.  the components that need to be signed depend on the security
       considerations of the application; and

   2.  some components of the message may not available at the time of
       signing or verification.

   To accommodate these limitations, HTTP Message Signatures defines a
   number of HTTP Message Components (Section 2 of [RFC9421]) and
   specifies rules for transforming components into the input to the



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   signature algorithm (Section 3.1 of [RFC9421]).  The value of most
   components are extracted directly from the bytes of the HTTP message;
   others are derived from the message through a well-specified process.

   All components are derived from the HTTP messages themselves.
   Consequentially, an on-path attacker with access to the HTTP messages
   transmitted between the client and server can replay a signed message
   at will.  This is described in Section 7.2.2 of [RFC9421].

   The nonce parameter provides some defense against replay attacks, but
   this mechanism is not applicable in all deployment scenarios.  For
   example, it is common for two TLS servers to be authoritative for the
   same DNS name.  (This setup is commonly referred to as "multi-CDN".)
   In this scenario, the first server can intercept a signed request
   from a client, then replay that request to the second server, thereby
   impersonating the client.

   The goal of this document is to make replay protection more robust.
   A new derived component is defined for HTTP Message Signatures whose
   value is set to key material exported from TLS as defined in
   Section 7.5 of [RFC8446].  This binds the signed message to the
   underlying TLS channel, thereby ensuring the signature is never
   accepted outside of that channel.

      OPEN ISSUE Would it be better do define exported key material as a
      signature parameter instead of as a derived component?

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.

3.  The @ekm Derived Component

   A new derived component is defined with the name @ekm.

   The contents of this component are the output of a call to the
   exporter interface of the underlying TLS connection as defined in
   Section 7.5 of [RFC8446], encoded in base64 [RFC4648].  The label
   parameter of the exporter function is set to "http-sig-ekm" and the
   context value is the version of TLS.  For TLS 1.3 (i.e., [RFC8446])
   its value is 0x0304.






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   TLS 1.3 or later is REQUIRED.  This derived component is not
   compatible with HTTP messages sent in plaintext or over TLS 1.2 and
   below.

      NOTE We could in principal specify this for TLS 1.2, if we need
      to.

   If the signer and verifier do not agree on the value of @ekm, then
   the signature will not verify.  If the signer and verifier share a
   TLS connection between them, then they will compute the same value.
   If they do not share a direct TLS connection, it is possible to
   architect a system such that the verifier does not directly call the
   exporter interface, but is simply provided its output on a trusted
   channel.  This behaviour works, but requires the verifier and caller
   to trust each other.

      OPEN ISSUE How do we negotiate usage of @ekm?  Both the signer and
      verifier need to support the new derived component into generate
      message signatures that use it, but the signer might not know if
      the verifier uses it.

4.  Security Considerations

      TODO Define a channel binding and say why it prevents replays
      between CDNs.

5.  IANA Considerations

      TODO Update the "HTTP Signature Derived Component Names" registry.

6.  Normative References

   [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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.



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   [RFC9421]  Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
              Message Signatures", RFC 9421, DOI 10.17487/RFC9421,
              February 2024, <https://www.rfc-editor.org/rfc/rfc9421>.

Acknowledgments

   TODO acknowledge.

Authors' Addresses

   Jonathan Hoyland
   Email: jonathan.hoyland@gmail.com


   Christopher Patton
   Cloudflare
   Email: chrispatton+ietf@gmail.com


































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