| Internet-Draft | HTTP Signature Component for TLS Channel | October 2025 |
| Hoyland & Patton | Expires 23 April 2026 | [Page] |
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.¶
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.¶
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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:¶
the components that need to be signed depend on the security considerations of the application; and¶
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 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?¶
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.¶
@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.¶
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.¶
TODO Define a channel binding and say why it prevents replays between CDNs.¶
TODO Update the "HTTP Signature Derived Component Names" registry.¶
TODO acknowledge.¶