Function open 

pub fn open(
    ciphertext_with_tag: &[u8],
    nonce: &Nonce12,
    key: &Key,
    aad: &[u8],
) -> Result<Zeroizing<Vec<u8>>>

Decrypts ciphertext_with_tag under key after verifying aad.

Returns the plaintext in zeroizing memory. Fails with Error::Authentication if the password/key is wrong or the data was modified.