#### DES

A
```package com.thealgorithms.ciphers;

/**
* This class is build to demonstrate the application of the DES-algorithm (https://en.wikipedia.org/wiki/Data_Encryption_Standard) on a
* plain English message. The supplied key must be in form of a 64 bit binary String.
*/
public class DES {

private String key;
private String subKeys[];

private void sanitize(String key) {
int length = key.length();
if (length != 64) {
throw new IllegalArgumentException("DES key must be supplied as a 64 character binary string");
}
}

DES(String key) {
sanitize(key);
this.key = key;
subKeys = getSubkeys(key);
}

public String getKey() {
return this.key;
}

public void setKey(String key) {
sanitize(key);
this.key = key;
}

//Permutation table to convert initial 64 bit key to 56 bit key
private static int[] PC1 =
{
57, 49, 41, 33, 25, 17,  9,
1, 58, 50, 42, 34, 26, 18,
10,  2, 59, 51, 43, 35, 27,
19, 11,  3, 60, 52, 44, 36,
63, 55, 47, 39, 31, 23, 15,
7, 62, 54, 46, 38, 30, 22,
14,  6, 61, 53, 45, 37, 29,
21, 13,  5, 28, 20, 12,  4
};

//Lookup table used to shift the initial key, in order to generate the subkeys
private static int[] KEY_SHIFTS =
{
1,  1,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2,  2,  1
};

//Table to convert the 56 bit subkeys to 48 bit subkeys
private static int[] PC2 =
{
14, 17, 11, 24,  1,  5,
3, 28, 15,  6, 21, 10,
23, 19, 12,  4, 26,  8,
16,  7, 27, 20, 13,  2,
41, 52, 31, 37, 47, 55,
30, 40, 51, 45, 33, 48,
44, 49, 39, 56, 34, 53,
46, 42, 50, 36, 29, 32
};

//Initial permutatation of each 64 but message block
private static 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
};

//Expansion table to convert right half of message blocks from 32 bits to 48 bits
private static int[] expansion =
{
32,  1,  2,  3,  4,  5,
4,  5,  6,  7,  8,  9,
8,  9, 10, 11, 12, 13,
12, 13, 14, 15, 16, 17,
16, 17, 18, 19, 20, 21,
20, 21, 22, 23, 24, 25,
24, 25, 26, 27, 28, 29,
28, 29, 30, 31, 32,  1
};

//The eight substitution boxes are defined below
private static int[][] s1 = {
{14, 4, 13,  1,  2, 15, 11,  8,  3, 10,  6, 12,  5,  9,  0,  7},
{0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11,  9,  5,  3,  8},
{4, 1, 14,  8, 13,  6, 2, 11, 15, 12,  9,  7,  3, 10,  5,  0},
{15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13}
};

private static int[][] s2 = {
{15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10},
{3, 13,  4, 7, 15,  2,  8, 14, 12,  0, 1, 10,  6,  9, 11,  5},
{0, 14, 7, 11, 10,  4, 13,  1,  5,  8, 12,  6,  9,  3,  2, 15},
{13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14,  9}
};

private static int[][] s3 = {
{10, 0, 9, 14, 6, 3, 15, 5,  1, 13, 12, 7, 11, 4, 2,  8},
{13, 7, 0, 9, 3,  4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1},
{13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14,  7},
{1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12}
};

private static int[][] s4 = {
{7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15},
{13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14,  9},
{10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4},
{3, 15, 0, 6, 10, 1, 13, 8, 9,  4, 5, 11, 12, 7, 2, 14}
};

private static int[][] s5 = {
{2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9},
{14, 11, 2, 12,  4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6},
{4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14},
{11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3}
};

private static int[][] s6 = {
{12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11},
{10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8},
{9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6},
{4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13}
};

private static int[][] s7 = {
{4, 11, 2, 14, 15,  0, 8, 13 , 3, 12, 9 , 7,  5, 10, 6, 1},
{13 , 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6},
{1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2},
{6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12}
};

private static int[][] s8 = {
{13, 2, 8,  4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7},
{1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6 ,11, 0, 14, 9, 2},
{7, 11, 4, 1, 9, 12, 14, 2,  0, 6, 10 ,13, 15, 3, 5, 8},
{2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6 ,11}
};

private static int[][][] s = {s1, s2, s3, s4, s5, s6, s7, s8};

//Permutation table, used in the feistel function post s-box usage
static int[] permutation =
{
16,  7, 20, 21,
29, 12, 28, 17,
1, 15, 23, 26,
5, 18, 31, 10,
2,  8, 24, 14,
32, 27,  3,  9,
19, 13, 30,  6,
22, 11,  4, 25
};

//Table used for final inversion of the message box after 16 rounds of Feistel Function
static int[] IPinverse =
{
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
};

private String[] getSubkeys(String originalKey) {
StringBuilder permutedKey = new StringBuilder(); //Initial permutation of keys via PC1
int i, j;
for (i = 0; i < 56; i++) {
permutedKey.append(originalKey.charAt(PC1[i] - 1));
}
String subKeys[] = new String[16];
String initialPermutedKey = permutedKey.toString();
String C0 = initialPermutedKey.substring(0, 28), D0 = initialPermutedKey.substring(28);

//We will now operate on the left and right halves of the permutedKey
for (i = 0; i < 16; i++) {
String Cn = C0.substring(KEY_SHIFTS[i]) + C0.substring(0, KEY_SHIFTS[i]);
String Dn = D0.substring(KEY_SHIFTS[i]) + D0.substring(0, KEY_SHIFTS[i]);
subKeys[i] = Cn + Dn;
C0 = Cn; //Re-assign the values to create running permutation
D0 = Dn;
}

//Let us shrink the keys to 48 bits (well, characters here) using PC2
for (i = 0; i < 16; i++) {
String key = subKeys[i];
permutedKey.setLength(0);
for (j = 0; j < 48; j++) {
permutedKey.append(key.charAt(PC2[j] - 1));
}
subKeys[i] = permutedKey.toString();
}

return subKeys;
}

private String XOR(String a, String b) {
int i, l = a.length();
StringBuilder xor = new StringBuilder();
for (i = 0; i < l; i++) {
int firstBit = a.charAt(i) - 48; // 48 is '0' in ascii
int secondBit = b.charAt(i) - 48;
xor.append((firstBit ^ secondBit));
}
return xor.toString();
}

int i, l = s.length();
int diff = desiredLength - l;
for (i = 0; i < diff; i++) {
}
}

private String pad(String s, int desiredLength) {
return createPaddedString(s, desiredLength, '0') + s;
}

private String padLast(String s, int desiredLength) {
return s + createPaddedString(s, desiredLength, '\u0000');
}

private String feistel(String messageBlock, String key) {
int i;
StringBuilder expandedKey = new StringBuilder();
for (i = 0; i < 48; i++) {
expandedKey.append(messageBlock.charAt(expansion[i] - 1));
}
String mixedKey = XOR(expandedKey.toString(), key);
StringBuilder substitutedString = new StringBuilder();

//Let us now use the s-boxes to transform each 6 bit (length here) block to 4 bits
for (i = 0; i < 48; i += 6) {
String block = mixedKey.substring(i, i + 6);
int row = (block.charAt(0) - 48) * 2 + (block.charAt(5) - 48);
int col = (block.charAt(1) - 48) * 8 + (block.charAt(2) - 48) * 4 + (block.charAt(3) - 48) * 2 + (block.charAt(4) - 48);
String substitutedBlock = pad(Integer.toBinaryString(s[i / 6][row][col]), 4);
substitutedString.append(substitutedBlock);
}

StringBuilder permutedString = new StringBuilder();
for (i = 0; i < 32; i++) {
permutedString.append(substitutedString.charAt(permutation[i] - 1));
}

return permutedString.toString();
}

private String encryptBlock(String message, String keys[]) {
StringBuilder permutedMessage = new StringBuilder();
int i;
for (i = 0; i < 64; i++) {
permutedMessage.append(message.charAt(IP[i] - 1));
}
String L0 = permutedMessage.substring(0, 32), R0 = permutedMessage.substring(32);

//Iterate 16 times
for (i = 0; i < 16; i++) {
String Ln = R0; // Previous Right block
String Rn = XOR(L0, feistel(R0, keys[i]));
L0 = Ln;
R0 = Rn;
}

String combinedBlock = R0 + L0; //Reverse the 16th block
permutedMessage.setLength(0);
for (i = 0; i < 64; i++) {
permutedMessage.append(combinedBlock.charAt(IPinverse[i] - 1));
}
return permutedMessage.toString();
}

//To decode, we follow the same process as encoding, but with reversed keys
private String decryptBlock(String message, String keys[]) {
String reversedKeys[] = new String[keys.length];
for (int i = 0; i < keys.length; i++) {
reversedKeys[i] = keys[keys.length - i - 1];
}
return encryptBlock(message, reversedKeys);
}

/**
* @param message Message to be encrypted
* @return The encrypted message, as a binary string
*/
public String encrypt(String message) {
StringBuilder encryptedMessage = new StringBuilder();
int l = message.length(), i, j;
if (l % 8 != 0) {
int desiredLength = (l / 8 + 1) * 8;
l = desiredLength;
}

for (i = 0; i < l; i+= 8) {
String block = message.substring(i, i + 8);
StringBuilder bitBlock = new StringBuilder();
byte[] bytes = block.getBytes();
for (j = 0; j < 8; j++) {
}
encryptedMessage.append(encryptBlock(bitBlock.toString(), subKeys));
}
return encryptedMessage.toString();
}

/**
* @param message The encrypted string. Expects it to be a multiple of 64 bits, in binary format
* @return The decrypted String, in plain English
*/
public String decrypt(String message) {
StringBuilder decryptedMessage = new StringBuilder();
int l = message.length(), i, j;
if (l % 64 != 0) {
throw new IllegalArgumentException("Encrypted message should be a multiple of 64 characters in length");
}
for (i = 0; i < l; i+= 64) {
String block = message.substring(i, i + 64);
String result = decryptBlock(block.toString(), subKeys);
byte res[] = new byte[8];
for (j = 0; j < 64; j+=8) {
res[j / 8] = (byte)Integer.parseInt(result.substring(j, j + 8), 2);
}
decryptedMessage.append(new String(res));
}
return decryptedMessage.toString().replace("\0", ""); // Get rid of the null bytes used for padding
}

}
```