Verification of PSK modes #377
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Adds requirement from RFC 8446 section 4.2.9: If clients offer "pre_shared_key" without a "psk_key_exchange_modes" extension, servers MUST abort the handshake. Servers MUST NOT select a key exchange mode that is not listed by the client. This extension also restricts the modes for use with PSK resumption.
Follows recommandation from RFC 8446 section 4.2.9: Servers SHOULD NOT send NewSessionTicket with tickets that are not compatible with the advertised modes; however, if a server does so, the impact will just be that the client's attempts at resumption fail.
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Makes client extension "psk_key_exchange_modes" mandatory for PSK handshake, and verifies adequacy of server-selected cipher.
Sending NST when PSK_DHE_KE is not supported is not useful, the implementation is not able to resume with PSK_KE.