SilcAsyncOperation
    silc_ske_rekey_responder(SilcSKE ske,
                             SilcPacketStream stream,
                             SilcSKERekeyMaterial rekey,
                             SilcPacket packet);

    Starts SILC Key Exchange key regeneration (rekey) protocol as responder.
    The `rekey' is the rekey material received earlier in
    SilcSKECompletionCb.  That same callback is called after the rekey
    protocol is over to deliver new key material and new rekey material.
    When the rekey is completed the SKE library will automatically update
    the new keys into `stream'.  The completion callback is called after
    the new keys has been taken into use.

    The `packet' is the SILC_PACKET_REKEY received to start the rekey
    protocol.  If `packet' is NULL it is assumed that the packet will be
    received from the `stream'.

    This function returns SilcAsyncOperation operation context which can
    be used to control the protocol from the application.  Application may
    for example safely abort the protocol at any point, if needed.  Returns
    NULL on error.