Method scheduleCore

src/main/java/com/thealgorithms/ciphers/AES.java:2384–2422

Subroutine of the Rijndael key expansion.

(BigInteger t, int rconCounter)

     * Subroutine of the Rijndael key expansion.
     */
    public static BigInteger scheduleCore(BigInteger t, int rconCounter) {
        StringBuilder rBytes = new StringBuilder(t.toString(16));

        // Add zero padding
        while (rBytes.length() < 8) {
            rBytes.insert(0, "0");
        }

        // rotate the first 16 bits to the back
        String rotatingBytes = rBytes.substring(0, 2);
        String fixedBytes = rBytes.substring(2);

        rBytes = new StringBuilder(fixedBytes + rotatingBytes);

        // apply S-Box to all 8-Bit Substrings
        for (int i = 0; i < 4; i++) {
            StringBuilder currentByteBits = new StringBuilder(rBytes.substring(i * 2, (i + 1) * 2));

            int currentByte = Integer.parseInt(currentByteBits.toString(), 16);
            currentByte = SBOX[currentByte];

            // add the current RCON value to the first byte
            if (i == 0) {
                currentByte = currentByte ^ RCON[rconCounter];
            }

            currentByteBits = new StringBuilder(Integer.toHexString(currentByte));

            // Add zero padding
            while (currentByteBits.length() < 2) {
                currentByteBits.insert(0, '0');
            }

            // replace bytes in original string
            rBytes = new StringBuilder(rBytes.substring(0, i * 2) + currentByteBits + rBytes.substring((i + 1) * 2));
        }

        return new BigInteger(rBytes.toString(), 16);
    }

    /**
     * Returns an array of 10 + 1 round keys that are calculated by using