import java.util.Scanner;

public class SDES {

    static int[] P10 = {3, 5, 2, 7, 4, 10, 1, 9, 8, 6};
    static int[] P8  = {6, 3, 7, 4, 8, 5, 10, 9};

    static int[] IP  = {2, 6, 3, 1, 4, 8, 5, 7};
    static int[] IP_INV = {4, 1, 3, 5, 7, 2, 8, 6};

    static int[] EP = {4, 1, 2, 3, 2, 3, 4, 1};
    static int[] P4 = {2, 4, 3, 1};

    static int[][] S0 = {
            {1, 0, 3, 2},
            {3, 2, 1, 0},
            {0, 2, 1, 3},
            {3, 1, 3, 2}
    };

    static int[][] S1 = {
            {0, 1, 2, 3},
            {2, 0, 1, 3},
            {3, 0, 1, 0},
            {2, 1, 0, 3}
    };

    static String K1, K2;

    static String permute(String input, int[] table) {
        StringBuilder result = new StringBuilder();

        for (int position : table) {
            result.append(input.charAt(position - 1));
        }

        return result.toString();
    }

    static String leftShift(String input, int shifts) {
        shifts %= input.length();
        return input.substring(shifts) + input.substring(0, shifts);
    }

    static String xor(String a, String b) {
        StringBuilder result = new StringBuilder();

        for (int i = 0; i < a.length(); i++) {
            result.append(a.charAt(i) == b.charAt(i) ? '0' : '1');
        }

        return result.toString();
    }

    static void generateKeys(String key) {

        String p10 = permute(key, P10);

        String left = p10.substring(0, 5);
        String right = p10.substring(5);

        left = leftShift(left, 1);
        right = leftShift(right, 1);

        K1 = permute(left + right, P8);

        left = leftShift(left, 2);
        right = leftShift(right, 2);

        K2 = permute(left + right, P8);
    }

    static String sBox(String input, int[][] box) {

        int row = Integer.parseInt(
                "" + input.charAt(0) + input.charAt(3), 2
        );

        int col = Integer.parseInt(
                "" + input.charAt(1) + input.charAt(2), 2
        );

        int value = box[row][col];

        return String.format("%2s",
                Integer.toBinaryString(value)).replace(' ', '0');
    }

    static String functionF(String right, String key) {

        String expanded = permute(right, EP);

        String xorResult = xor(expanded, key);

        String left4 = xorResult.substring(0, 4);
        String right4 = xorResult.substring(4);

        String s0Result = sBox(left4, S0);
        String s1Result = sBox(right4, S1);

        return permute(s0Result + s1Result, P4);
    }

    static String fk(String bits, String key) {

        String left = bits.substring(0, 4);
        String right = bits.substring(4);

        String fResult = functionF(right, key);

        String newLeft = xor(left, fResult);

        return newLeft + right;
    }

    static String switchHalves(String bits) {
        return bits.substring(4) + bits.substring(0, 4);
    }

    static String encrypt(String plaintext) {

        String temp = permute(plaintext, IP);

        temp = fk(temp, K1);

        temp = switchHalves(temp);

        temp = fk(temp, K2);

        return permute(temp, IP_INV);
    }

    static String decrypt(String ciphertext) {

        String temp = permute(ciphertext, IP);

        temp = fk(temp, K2);

        temp = switchHalves(temp);

        temp = fk(temp, K1);

        return permute(temp, IP_INV);
    }

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        System.out.print("Enter 10-bit key: ");
        String key = sc.nextLine();

        System.out.print("Enter 8-bit plaintext: ");
        String plaintext = sc.nextLine();

        if (!key.matches("[01]{10}") ||
                !plaintext.matches("[01]{8}")) {

            System.out.println("Invalid input.");
            return;
        }

        generateKeys(key);

        System.out.println("\nK1 = " + K1);
        System.out.println("K2 = " + K2);

        String ciphertext = encrypt(plaintext);

        System.out.println("Plaintext  = " + plaintext);
        System.out.println("Ciphertext = " + ciphertext);

        String decrypted = decrypt(ciphertext);

        System.out.println("Decrypted  = " + decrypted);

        sc.close();
    }
}