@Override
    public byte[] getMac() {
        return cipher.getMac();
    }
Minimal adapter to make ChaCha20Poly1305 usable as an AEADBlockCipher (it's technically
an AEAD stream cipher, but this is not exposed in the BouncyCastle type system).

@author vekterli
security-utils/src/main/java/com/yahoo/security/ChaCha20Poly1305AeadBlockCipherAdapter.java