Method addRoundKey

src/main/java/com/thealgorithms/ciphers/AES.java:2529–2531

@return ciphertext XOR key

(BigInteger ciphertext, BigInteger key)

     * @return ciphertext XOR key
     */
    public static BigInteger addRoundKey(BigInteger ciphertext, BigInteger key) {
        return ciphertext.xor(key);
    }

    /**
     * substitutes 8-Bit long substrings of the input using the S-Box and