[TLS] Draft 18 review : Hello Retry Request and supported groups cache

[Olivier Levillain <[e-mail]>]

Hi list,

I am sorry for the very late answer concerning draft 18, but we
(ANSSI) have several remarks after proof-reading the current
specification.

We are sorry for the multiple long messages.

If the WG is interested by some of our concerns/proposals, we would be
glad to propose some PRs.


= HRR and supported groups cache =

In 4.2.4 (P.41), a server can send a supported_groups extension to
"update the client's view of its preference" in its ServerHello.
Since this behaviour is completely left to the client's discretion, it
does not seem a very relevant policy from the server: either the
server accepts one of the proposed groups, or it sends an HRR.  We do
not think the middle ground (OK for this group, but I would prefer
this other one) is relevant, so the sentence should be removed.

Moreover, as far as I could understand, there is no indication in the
specification that a client should remember the preference of the
server in case it receives a HRR, which there would definitely make
sense.  Such text could go in 4.1.4.

I can propose a PR for this point.


Olivier Levillain