[TLS] Commentary on the client authentication presentation slides

[Ilari Liusvaara <[e-mail]>]

Some commentary on client authentication slides (there is no linked draft
nor other material yet).

- Mechanism like proposed looks dangerous when combined with HTTP/2.
  Multiplexed protocols are in general not safe to authenticate without
  application-layer signaling (which can be implicit via separate
  connections), especially if dealing with something like web
  environment.
- Regarding last point about interleaving: Assuming the scheme works
  in 1RTT (and I see no reason for requiring more rounds), you can't
  prevent application_data transmission after certificate_request.
  The best that can be done is to require the client to send all
  the authentication-related data in one go.
- The certificate_types field in CertificateRequest is pretty much
  useless, since all supported algorithms are of signature type.
- One can't just remove fields without breaking parse compatiblity,
  but adding field breaks parse compatiblity too, so removing
  field at the same time isn't a problem.
- How does extension_values work? If multiple values for one
  OID are allowed, then the OID/value pair is repeated, once for
  each value?



-Ilari