SilcBuffer
    silc_attribute_payload_encode_data(SilcBuffer attrs,
                                       SilcAttribute attribute,
                                       SilcAttributeFlags flags,
                                       const unsigned char *data,
                                       SilcUInt32 data_len);

    Same function as silc_attribute_payload_encode except the attribute
    is already encoded into `data' of `data_len' bytes in length.
    Encodes the attribute into the `attrs' buffer and returns pointer
    to the buffer, which may be different in case the buffer was
    reallocated.  If `attrs' is NULL for first attribute this allocates
    the buffer and returns it.  Returns NULL on error.