TLS N. Sullivan
Internet-Draft Cloudflare Inc.
Intended status: Standards Track October 31, 2016
Expires: May 4, 2017

Exported Authenticators in TLS


This document describes a mechanism in Transport Layer Security (TLS) to provide an exportable proof of ownership of a certificate that can be transmitted out of band and verified by the other party.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 4, 2017.

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Table of Contents

1. Introduction

This document provides a way to authenticate one party of a Transport Layer Security (TLS) communication to another using a certificate after the session has been established. This allows both the client and server to prove ownership of additional identities at any time after the handshake has completed. This proof of authentication can be exported and transmitted out of band from one party then validated by the other party.

This mechanism is useful in the following situations:

This document intends to replace much of the functionality of renegotiation in previous versions of TLS. It has the advantages over renegotiation of not requiring additional on-the-wire changes during a connection.

2. Authenticator

Given an established TLS connection, a certificate, and a corresponding private key, an authenticator message can be constructed by either the client or the server. This authenticator uses the message structures from section 4.4. of [I-D.ietf-tls-tls13], but with a different handshake context and finished key. These messages are not encrypted.

The Handshake Context is an [RFC5705] (for TLS 1.2 or earlier) or [I-D.ietf-tls-tls13] exporter value derived using the label “authenticator handshake context” and length 64 bytes. The Finished MAC Key is an exporter value derived using the label “server authenticator finished key” or “client authenticator finished key”, depending on the sender, with length corresponding to the length of the handshake hash.

If the connection is TLS 1.2 or earlier, the master secret MUST have been computed with the extended master secret [RFC7627] to avoid key synchronization attacks.

The certificate to be used for authentication and any supporting certificates in the chain.

The certificate message contains an opaque string called certificate_request_context which MUST be unique for a given connection. Its format should be defined by the application level protocol and MUST be non-zero length.

A signature over the value Hash(Handshake Context + Certificate)
A HMAC over the value Hash(Handshake Context + Certificate + CertificateVerify) using the hash function from the handshake and the Finished MAC Key as a key.

The certificates used in the Certificate message must conform to the requirements of a Certificate message in the version of TLS that is being negotiated as described in section 4.2.3. of [I-D.ietf-tls-tls13].

The exported authenticator message is the sequence: Certificate, CertificateVerify, Finished

3. API considerations

TLS implementations supporting the use of exported authenticators MUST provide application programming interfaces by which clients and servers may request and verify exported authenticator messages.

Given an established connection, the application should be able to obtain an authenticator by providing the following:

Given an established connection and an exported authenticator message, the application should be able to provide the authenticator to the connection. If the Finished and CertificateVerify messages verify, the TLS library should return the following:

4. Security Considerations


5. Acknowledgements

Comments on this proposal were provided by Martin Thomson.

6. Normative References

[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-14, July 2016.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010.
[RFC7627] Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley, A. and M. Ray, "Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension", RFC 7627, DOI 10.17487/RFC7627, September 2015.

Author's Address

Nick Sullivan Cloudflare Inc. EMail: