[TLS] Alerts

[Matt Caswell <[e-mail]>]

Draft-20 currently contains this text in the section "Server
Certificate Selection":

'If the client cannot construct an acceptable chain using the provided
certificates and decides to abort the handshake, then it MUST abort
the handshake with an"unsupported_certificate" alert."'

However, section 6 lists a number of other generic certificate related alerts:
certificate_revoked
certificate_expired
certificate_unknown
unknown_ca

While section 6 provides descriptions of these they are not mentioned
elsewhere in the spec, so there is no guidance as to when they might
be used.

So, given the text in the "Server Certificate Selection" section,
should a client always send "unsupported_certificate" for all
failures? Or should one of the other alerts be used instead if it
seems more appropriate? E.g. If a client attempts to "construct an
acceptable chain", but finds an expired certificate - should it
respond with "unsupported_certificate" or "certificate_expired"?

There seems to be some inconsistency between server certificate
selection and client certificate selection, i.e. in the former it is
mandated that a specific alert be sent, but in the latter there is no
such requirement.

Additionally, there are a number of other alerts mentioned in section
6 (not just certificate related ones), that are never mentioned
anywhere else in the spec. Do we really need all of these alerts? If
they are needed, shouldn't they be explicitly mentioned elsewhere in
the spec in the locations where they are relevant (associated with
some MUST/SHOULD/MAY requirement)? As things stand the interpretation
of when they should be sent seems quite loose.

Matt