We extract cipher suites candidates from the client's proposition.

def should_check_ciphersuites(self):
        """
        We extract cipher suites candidates from the client's proposition.
        """
        if isinstance(self.mykey, PrivKeyRSA):
            kx = "RSA"
        elif isinstance(self.mykey, PrivKeyECDSA):
            kx = "ECDSA"
        if get_usable_ciphersuites(self.cur_pkt.ciphers, kx):
            return
        raise self.NO_USABLE_CIPHERSUITE()