decode_packet
retval = decode_packet(ctx, secret, buffer, &req);
retval = decode_packet(ctx, secret, buffer, rsppkt);
int decode_packet(int want, unsigned char **pptr, size_t nbytes,
rc = decode_packet(0, &ptr, nbytes, (decoder_t)decode_key,
rc = decode_packet(0, &ptr, (size_t)rc,
rc = decode_packet(2, &ptr, sbytes, (decoder_t)decode_sig, sig);