#### 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 final 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 final 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 final 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 final 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 permutation of each 64 but message block
private static final 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 final 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 final 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 final 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 final 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 final 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 final 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 final 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 final 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 final 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 final int[][][] S = {S1, S2, S3, S4, S5, S6, S7, S8};

// Permutation table, used in the Feistel function post s-box usage
static final 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 final int[] IP_INVERSE = {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;
int 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);
String 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;
int 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;
int 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 e0 = permutedMessage.substring(0, 32);
String f0 = permutedMessage.substring(32);

// Iterate 16 times
for (i = 0; i < 16; i++) {
String eN = f0; // Previous Right block
String fN = xOR(e0, feistel(f0, keys[i]));
e0 = eN;
f0 = fN;
}

String combinedBlock = f0 + e0; // Reverse the 16th block
permutedMessage.setLength(0);
for (i = 0; i < 64; i++) {
permutedMessage.append(combinedBlock.charAt(IP_INVERSE[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();
int i;
int 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();
int i;
int 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, 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
}
}
```