SilcBuffer
    silc_message_signed_payload_encode(const unsigned char *message_payload,
                                       SilcUInt32 message_payload_len,
                                       SilcPublicKey public_key,
                                       SilcPrivateKey private_key,
                                       SilcHash hash);

    Encodes the SilcMessageSignedPayload Payload and computes the
    digital signature.  The `message_payload' is the message data that
    is used in the signature computation.  The encoding of the buffer
    is specified in the SILC protocol.  If `public_key' is provided
    then the public key included in the payload.  The `private_key'
    is used to produce the signature.  This function returns the encoded
    payload with the signature or NULL on error.  Caller must free the
    returned buffer.  The `hash' SHOULD be SHA-1 hash function.
    
    Application usually does not need to call this since the function
    silc_message_payload_encode calls this automatically if the caller
    wants to sign the message.