void silc_packet_set_keys(SilcPacketStream stream, SilcCipher send_key,
                              SilcCipher receive_key, SilcHmac send_hmac,
                              SilcHmac receive_hmac, SilcBool rekey);

    Set ciphers and HMACs to be used to encrypt sent packets, and decrypt
    received packets.  This can be called multiple times to change the
    ciphers and HMACs.

    If the `rekey' is TRUE this function will send SILC_PACKET_REKEY_DONE
    to the `stream' and will set the new keys.  If it is FALSE the keys
    are changed but the packet is not changed.

    When changing keys the old cipher and HMACs will be freed.  If the keys
    are not set at all, packets will not be encrypted or decrypted.