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Web and Internet Transport HTTP next generation unicorn sparkling distributed ledger 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 The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the HTTP Working Group mailing list (), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction HTTP Message Signatures 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 () and specifies rules for transforming components into the input to the signature algorithm (). 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 . 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 . 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?

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 when, and only when, they appear in all capitals, as shown here.

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 , encoded in base64 . 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., ) 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.

Security Considerations

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

IANA Considerations

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

Normative References This document describes a mechanism for creating, encoding, and verifying digital signatures or message authentication codes over components of an HTTP message. This mechanism supports use cases where the full HTTP message may not be known to the signer and where the message may be transformed (e.g., by intermediaries) before reaching the verifier. This document also describes a means for requesting that a signature be applied to a subsequent HTTP message in an ongoing HTTP exchange. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK]

Acknowledgments TODO acknowledge.