Function aes256gcm_open 

pub fn aes256gcm_open(
    key: &[u8; 32],
    nonce: &[u8; 12],
    aad: &[u8],
    ciphertext: &[u8],
) -> Option<Vec<u8>>

AES-256-GCM open; ciphertext carries the trailing 16-byte tag.