RFC 0 TLS Flag - Request mTLS October 2023
Hoyland Expires 3 May 2024 [Page]
Network Working Group
Intended Status:
J. Hoyland

TLS Flag - Request mTLS


Normally in TLS there is no way for the client to signal to the server that it has been configured with a certificate suitable for mTLS. This document defines a TLS Flag [I-D.ietf-tls-tlsflags] that enables clients to provide this hint.

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://jhoyla.github.io/draft-jhoyla-req-mtls-flag/draft-jhoyla-req-mtls-flag.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-jhoyla-req-mtls-flag/.

Source for this draft and an issue tracker can be found at https://github.com/jhoyla/draft-jhoyla-req-mtls-flag.

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/.

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 3 April 2024.

Table of Contents

1. Introduction

This document specifies a TLS Flag that indicates to the server that the client supports mTLS. Sometimes a server does not want to negotiate mTLS with every client, but might wish to authenticate a subset of them. In TLS 1.3 this may be done with post-handshake auth, however this adds an extra round-trip, and requires negotiation at the application layer. A client sending the request mTLS flag in the ClientHello allows the server to request authentication during the initial handshake only when it receives a hint the client supports it. This enables a number of use cases, for example allowing bots to authenticate themselves when mixed in with general traffic.

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. Flag specification

A server receiving this flag MAY send a CertificateRequest message.

4. Security Considerations

This flag should have no effect on the security of TLS, as the server may always send a CertificateRequest message during the handshake. This flag merely provides a hint that the client will be able to fulfil the request. If the client sets this flag but then fails to provide a certificate the server MAY terminate the connection with a bad_certificate error.

5. IANA Considerations

This document requests IANA to add an entry to the TLS Flags registry in the TLS namespace with the following values:

6. Normative References

Nir, Y., "A Flags Extension for TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-tlsflags-12, , <https://datatracker.ietf.org/doc/html/draft-ietf-tls-tlsflags-12>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.


TODO acknowledge.

Author's Address

Jonathan Hoyland