ex2

public class ex2 {
    static char[] sTable = {
            '3','4','1','2','0','9','7','6','5','8','D','C','F','A','B','E'
    };

    static char[] inverseSTable = new char[16];
    static {
        char[] hex = {'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
        for (int i = 0; i < 16; i++) {
            char mapped = sTable[i];
            int idx = Character.digit(mapped, 16);
            inverseSTable[idx] = hex[i];
        }
    }

    public static String encrypt(String block) {
        StringBuilder sb = new StringBuilder();
        for (char c : block.toCharArray()) {
            int index = Character.digit(c, 16);
            sb.append(sTable[index]);
        }
        return sb.toString();
    }

    public static String decrypt(String block) {
        StringBuilder sb = new StringBuilder();
        for (char c : block.toCharArray()) {
            int index = Character.digit(c, 16);
            sb.append(inverseSTable[index]);
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        String block = "FA01AD";
        String original = block;
        System.out.println("Original block: " + block);

        int count = 0;
        do {
            block = encrypt(block);
            count++;
            System.out.println("After " + count + " substitution(s): " + block);
        } while (!block.equals(original));

        System.out.println("\nIt took " + count + " substitutions to return to the original block!");
    }
}