private byte[] getAad() {
        if (aadBuf == null) {
            return EmptyArray.BYTE;
        }
        if (aadBuf.array().length == aadBuf.size()) {
            // no need to copy.
            return aadBuf.array();
        }
        return aadBuf.toByteArray();
    }
Returns whether reusing nonces is allowed (aka, whether this is nonce misuse-resistant).
Most AEAD ciphers are not, but some are specially constructed so that reusing a key/nonce
pair is safe.
common/src/main/java/org/conscrypt/OpenSSLAeadCipher.java