Subject: [TLS] psk_key_exchange_mode question

From: Daiki Ueno <[email-masked]>
Date: Mon, 23 Apr 2018 13:45:34 +0200

Hello,

I have a question about handling the psk_key_exchange_mode extension.

4.2.9. Pre-Shared Key Exchange Modes says:

  This extension also restricts the modes for use with PSK resumption;
  servers SHOULD NOT send NewSessionTicket with tickets that are not
  compatible with the advertised modes

Is this compatibility defined externally to the protocol, or does it
depend on the initial handshake?

To take an example, suppose the server uses the ticket construction
mechanism described in RFC 5077.

If the former is the case, the server requires PSK with (EC)DHE when the
ticket encryption key is required to be forward secret.  From the
implementation point of view, that would be provided as an option in the
server configuration.

On the other hand, if the latter is the case, the server requires PSK
with (EC)DHE when the initial handshake chose (EC)DHE key exchange,
because the ticket is tied to resumption_master_secret derived from the
(EC)DHE secret.

Since the above paragraph is followed by:

  however, if a server does so, the impact will just be that the
  client’s attempts at resumption fail.

I thought the latter is more plausible; however, in that case psk_ke
would only be meaningful when the initial handshake is PSK-only.

Regards,
-- 
Daiki Ueno