Buffer for remaining plaintext.
Pointer to store the number of bytes written.
The MAC to verify (must be aegis128x4_ABYTES_MIN to aegis128x4_ABYTES_MAX).
Length of the MAC.
0 on success, -1 if MAC verification fails, or another negative error code on failure.
Finalizes decryption in detached mode, verifying the MAC and producing final plaintext.