[TLS] Request mTLS Flag

[Jonathan Hoyland <[e-mail]>]

Hey TLSWG,

I've just posted a new draft
<https://www.ietf.org/archive/id/draft-jhoyla-req-mtls-flag-00.html> that
defines a TLS Flag
<https://www.ietf.org/archive/id/draft-ietf-tls-tlsflags-12.html> that
provides a hint to the server that the client supports mTLS / is configured
with a client certificate.

Usually the server has no way to know in advance whether a given inbound
connection is from a client with a certificate. If the server unexpectedly
requests a certificate from a human user, most users wouldn’t know what to
do. To avoid this many servers never send the CertificateRequest message in
the server’s first flight, or set up dedicated endpoints used only by bots.
If client authentication is necessary it can be negotiated later using a
higher layer either through post-handshake auth or with an Exported
Authenticator, but both of those options add round trips to the connection.

At Cloudflare we’re exploring ways to quickly identify clients. Having an
explicit signal from the client that it has an mTLS certificate on offer
reduces round-trips to find out, avoids unnecessarily probing clients that
have no certificate, etc. I think this would be an ideal use case for the
TLS Flags extension.

I have a pair of interoperable implementations (one based on boringssl and
one based on Go TLS) which I plan to open source before Prague. Obviously
these include implementations of the TLS Flags extension, which hopefully
will help drive that work forward too.

Regards,

Jonathan