Method getAd

app/src/main/java/eu/faircode/email/CloudSync.java:786–793

(String key, long revision)

    }

    private static byte[] getAd(String key, long revision) throws NoSuchAlgorithmException {
        byte[] k = MessageDigest.getInstance("SHA256").digest(key.getBytes());
        byte[] ad = ByteBuffer.allocate(8 + 8)
                .putLong(revision) // 8 bytes
                .put(Arrays.copyOfRange(k, 0, 8))
                .array();
        return ad;
    }

    private static String transform(String value, byte[] key, byte[] iv, byte[] ad, boolean encrypt)
            throws InvalidCipherTextException, IOException {