Explanation

KeyFile = files:/key.txt
MsgFile = files:/msg.txt
CphFile = files:/cipher.txt

1. desPlaintextBlock() that takes the plaintext as input and splits it up into 64 bit blocks. If the last block is less than 64 bits, pad it with 0s.

 public String[] desPlaintextBlock(String file_path, int block_size) {

        String plain_text = "";
        try {
            File myObj = new File(file_path);
            Scanner myReader = new Scanner(myObj);

            while (myReader.hasNextLine()) {
                plain_text += myReader.nextLine().toUpperCase();
            }
            myReader.close();
        } catch (FileNotFoundException e) {
            System.out.println("An error occurred.");
            e.printStackTrace();
        }

        return split(plain_text, block_size);
    }

    public String padLeftZeros(String inputString, int length) {
        if (inputString.length() >= length) {
            return inputString;
        }
        StringBuilder sb = new StringBuilder();
        while (sb.length() < length - inputString.length()) {
            sb.append('0');
        }
        sb.append(inputString);

        return sb.toString();
    }

    public String[] split(String src, int len) {
        String[] result = new String[(int) Math.ceil((double) src.length() / (double) len)];

        for (int i = 0; i < result.length; i++)
            result[i] = src.substring(i * len, Math.min(src.length(), (i + 1) * len));

        result[result.length - 1] = padLeftZeros(result[result.length - 1], len);
        return result;
    }

2. permutation() that takes a 64-bit block, performs permutation according to the 8 x 8 IP table and returns the permuted block.

CONSTANTS:

// CONSTANTS
// Initial Permutation Table

int[] IP = {58, 50, 42, 34, 26, 18, 10, 2, 60, 52, 44, 36, 28, 20, 12, 4, 62, 54, 46, 38, 30,
            22, 14, 6, 64, 56, 48, 40, 32, 24, 16, 8, 57, 49, 41, 33, 25, 17, 9, 1, 59, 51, 43, 35,
            27, 19, 11, 3, 61, 53, 45, 37, 29, 21, 13, 5, 63, 55, 47, 39, 31, 23, 15, 7};

METHOD:

 /***
     * per-mutate input hexadecimal
     * according to specified sequence
     *
     * @param sequence permutation table
     * @param input
     * @return
    */
    String permutation(int[] sequence, String input) {
        String output = "";
        input = hextoBin(input);
        for (int i = 0; i < sequence.length; i++)
            output += input.charAt(sequence[i] - 1);
        output = binToHex(output);
        return output;
    }

USAGE:

// Initial permutation
        plainText = permutation(IP, plainText);

3. FinalPermutation that takes a 64-bit block, performs permutation according to the 8 x 8 FP table and returns the permuted block.

    // Inverse Initial Permutation Table
    int[] IP1 = {40, 8, 48, 16, 56, 24, 64, 32, 39, 7, 47, 15, 55, 23, 63, 31, 38, 6, 46, 14, 54,
            22, 62, 30, 37, 5, 45, 13, 53, 21, 61, 29, 36, 4, 44, 12, 52, 20, 60, 28, 35, 3, 43, 11,
            51, 19, 59, 27, 34, 2, 42, 10, 50, 18, 58, 26, 33, 1, 41, 9, 49, 17, 57, 25};



USAGE:

// Final permutation
        plainText = permutation(IP1, plainText);