SilcMessagePayload 
    silc_message_payload_parse(unsigned char *payload,
                               SilcUInt32 payload_len,
                               bool private_message,
                               bool static_key,
                               SilcCipher cipher,
                               SilcHmac hmac);

    Parses Message Payload returning new payload structure.  This also
    decrypts the payload and checks the MAC.  If the payload is
    channel message then `private_message' is FALSE, and if it is
    private message it is TRUE.  If the private message key is static
    (pre-shared key) then protocol dictates that the IV is present
    and `static_key' must be set to TRUE.  If the key is not static
    (Key Agreement was done for the key) then it MUST be FALSE.  For
    channel messages the `static_key' is ignored.

    If the `hmac' is no provided then the MAC of the channel message is
    not verified.  If the message is private message and `cipher' is NULL
    then this assumes that the packet was decrypted with session keys
    (no private message key) and this merely decodes the payload.