| RFC 0 | TLS Flag - Request mTLS | October 2023 |
| Hoyland | Informational | [Page] |
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.¶
This document is not an Internet Standards Track specification; it is published for informational purposes.¶
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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.¶
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.¶
A server receiving this flag MAY send a CertificateRequest message.¶
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.¶
This document requests IANA to add an entry to the TLS Flags registry in the TLS namespace with the following values:¶
TODO acknowledge.¶