On Mon, Oct 23, 2023 at 11:36:10AM -0400, David Benjamin wrote:
> Would you expect a browser user to send this flag? On the browser side, we
> don't know until the CertificateRequest whether a client certificate is
> configured. We have to do a moderately expensive query, dependent on
> information on the CertificateRequest of the OS's cert and key stores to
> get this information. This query may even call into things like 3p
> smartcard drivers, which may do arbitrarily disruptive things like showing
> UI.
One sensible use-case for such a signal is in mail user agent (MUA) to
mail submission agent (MSA) communication.
- When the submission client is a bot, a client cert is a fairly
natural choice of credential.
- Some Java TLS libraries (used to?) fail the handshake when the
client has no configured certs, or the list of issuer CA DN hints
does include any of its available (typically just zero or one)
certificates.
They could just proceed without a certificate, or return a default
one, but they don't.
- Most MTAs and MSAs don't request certificates, avoiding the
potential interoperability issue.
It would be useful to be able to request certificates conditioned on the
client promising to not fail just because it is unable or unwilling to
offer one.
Hence, I would like to see this flag move forward, though I'd also,
perhaps separately, like to see an extension, that either combined with
this flag, or alone, conveys an additional DNS name, where the server
might find the client's TLSA records, allowing the client to establish
a binding to that name. Something like, given:
_smtp-client.example.com. IN TLSA 3 1 1 ...
send a hint of "example.com", with the "_smtp-client" prefix implied by
the application protocol (prepended by the server).
https://datatracker.ietf.org/doc/html/draft-huque-tls-dane-clientid
--
Viktor.
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