Repository: TheAlgorithms/Java (java)

Issue:
[FEATURE REQUEST] Addition of DES to ciphers
### What would you like to Propose?

I feel DES is an important cipher from educational standpoint and thus should be included. Happy to take this up if considered valuable, so please let me know.

### Issue details

Addition of the standard DES Algorithm to Ciphers: https://en.wikipedia.org/wiki/Data_Encryption_Standard

### Additional Information

_No response_

Pull request:
Des
<!-- For completed items, change [ ] to [x] -->

- [x] I have read [CONTRIBUTING.md](https://github.com/TheAlgorithms/Java/blob/master/CONTRIBUTING.md).
- [x] This pull request is all my own work -- I have not plagiarized it.
- [x] All filenames are in PascalCase.
- [x] All functions and variable names follow Java naming conventions.
- [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
 
This closes #4167 


Patch:
diff --git a/DIRECTORY.md b/DIRECTORY.md
index fb49d163..6f246ff1 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -30,6 +30,7 @@
             * [Blowfish](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Blowfish.java)
             * [Caesar](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Caesar.java)
             * [ColumnarTranspositionCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/ColumnarTranspositionCipher.java)
+            * [DES](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/DES.java)
             * [HillCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/HillCipher.java)
             * [Polybius](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Polybius.java)
             * [ProductCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/ProductCipher.java)
@@ -151,14 +152,15 @@
               * [AVLSimple](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/AVLSimple.java)
               * [AVLTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/AVLTree.java)
               * [BinaryTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/BinaryTree.java)
+              * [BSTFromSortedArray](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/BSTFromSortedArray.java)
               * [BSTIterative](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/BSTIterative.java)
               * [BSTRecursive](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/BSTRecursive.java)
               * [BSTRecursiveGeneric](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/BSTRecursiveGeneric.java)
               * [CeilInBinarySearchTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/CeilInBinarySearchTree.java)
+              * [CheckBinaryTreeIsValidBST](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/CheckBinaryTreeIsValidBST.java)
               * [CheckIfBinaryTreeBalanced](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/CheckIfBinaryTreeBalanced.java)
               * [CheckTreeIsSymmetric](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/CheckTreeIsSymmetric.java)
               * [CreateBinaryTreeFromInorderPreorder](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/CreateBinaryTreeFromInorderPreorder.java)
-              * [CreateBSTFromSortedArray](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/CreateBSTFromSortedArray.java)
               * [FenwickTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/FenwickTree.java)
               * [GenericTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/GenericTree.java)
               * [InorderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/InorderTraversal.java)
@@ -166,7 +168,6 @@
               * [LazySegmentTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/LazySegmentTree.java)
               * [LCA](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/LCA.java)
               * [LevelOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/LevelOrderTraversal.java)
-              * [LevelOrderTraversalHelper](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/LevelOrderTraversalHelper.java)
               * [nearestRightKey](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/nearestRightKey.java)
               * [PostOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/PostOrderTraversal.java)
               * [PreOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/PreOrderTraversal.java)
@@ -175,9 +176,7 @@
               * [SameTreesCheck](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SameTreesCheck.java)
               * [SegmentTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java)
               * [TreeRandomNode](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/TreeRandomNode.java)
-              * [TreeTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/TreeTraversal.java)
               * [TrieImp](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/TrieImp.java)
-              * [ValidBSTOrNot](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/ValidBSTOrNot.java)
               * [VerticalOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/VerticalOrderTraversal.java)
               * [ZigzagTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/ZigzagTraversal.java)
           * devutils
@@ -202,6 +201,7 @@
             * [BoundaryFill](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/BoundaryFill.java)
             * [BruteForceKnapsack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/BruteForceKnapsack.java)
             * [CatalanNumber](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/CatalanNumber.java)
+            * [ClimbingStairs](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/ClimbingStairs.java)
             * [CoinChange](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/CoinChange.java)
             * [CountFriendsPairing](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/CountFriendsPairing.java)
             * [DiceThrow](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/DiceThrow.java)
@@ -382,6 +382,7 @@
             * [KochSnowflake](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/KochSnowflake.java)
             * [Krishnamurthy](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/Krishnamurthy.java)
             * [LinearCongruentialGenerator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/LinearCongruentialGenerator.java)
+            * [LineSweep](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/LineSweep.java)
             * [LowestBasePalindrome](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/LowestBasePalindrome.java)
             * [Luhn](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/Luhn.java)
             * [Mandelbrot](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/Mandelbrot.java)
@@ -403,7 +404,6 @@
             * [StackPostfixNotation](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/StackPostfixNotation.java)
             * [StringMatchFiniteAutomata](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/StringMatchFiniteAutomata.java)
             * [Sudoku](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/Sudoku.java)
-            * [ThreeSum](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/ThreeSum.java)
             * [TopKWords](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/TopKWords.java)
             * [TowerOfHanoi](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/TowerOfHanoi.java)
             * [TwoPointers](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/others/TwoPointers.java)
@@ -526,6 +526,7 @@
               * [LFSRTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/a5/LFSRTest.java)
             * [BlowfishTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/BlowfishTest.java)
             * [CaesarTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/CaesarTest.java)
+            * [DESTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/DESTest.java)
             * [PolybiusTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/PolybiusTest.java)
             * [RSATest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/RSATest.java)
             * [SimpleSubCipherTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/SimpleSubCipherTest.java)
@@ -575,7 +576,10 @@
               * [PriorityQueuesTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/PriorityQueuesTest.java)
             * trees
               * [BinaryTreeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/BinaryTreeTest.java)
+              * [BSTFromSortedArrayTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/BSTFromSortedArrayTest.java)
+              * [BSTIterativeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/BSTIterativeTest.java)
               * [CeilInBinarySearchTreeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/CeilInBinarySearchTreeTest.java)
+              * [CheckBinaryTreeIsValidBSTTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/CheckBinaryTreeIsValidBSTTest.java)
               * [CheckTreeIsSymmetricTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/CheckTreeIsSymmetricTest.java)
               * [InorderTraversalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/InorderTraversalTest.java)
               * [KDTreeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/KDTreeTest.java)
@@ -585,7 +589,6 @@
               * [PreOrderTraversalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/PreOrderTraversalTest.java)
               * [SameTreesCheckTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/SameTreesCheckTest.java)
               * [TreeTestUtils](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/TreeTestUtils.java)
-              * [ValidBSTOrNotTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/ValidBSTOrNotTest.java)
               * [VerticalOrderTraversalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/VerticalOrderTraversalTest.java)
               * [ZigzagTraversalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/trees/ZigzagTraversalTest.java)
           * divideandconquer
@@ -593,6 +596,7 @@
             * [StrassenMatrixMultiplicationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/divideandconquer/StrassenMatrixMultiplicationTest.java)
           * dynamicprogramming
             * [CatalanNumberTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/CatalanNumberTest.java)
+            * [climbStairsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/climbStairsTest.java)
             * [EggDroppingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/EggDroppingTest.java)
             * [KnapsackMemoizationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/KnapsackMemoizationTest.java)
             * [LevenshteinDistanceTests](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/LevenshteinDistanceTests.java)
@@ -674,6 +678,7 @@
             * [CRCAlgorithmTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/CRCAlgorithmTest.java)
             * [FirstFitCPUTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/FirstFitCPUTest.java)
             * [KadaneAlogrithmTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/KadaneAlogrithmTest.java)
+            * [LineSweepTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/LineSweepTest.java)
             * [LinkListSortTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/LinkListSortTest.java)
             * [NewManShanksPrimeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/NewManShanksPrimeTest.java)
             * [NextFitTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/others/NextFitTest.java)
@@ -731,6 +736,7 @@
             * [CheckAnagramsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/CheckAnagramsTest.java)
             * [CheckVowelsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/CheckVowelsTest.java)
             * [HammingDistanceTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/HammingDistanceTest.java)
+            * [HorspoolSearchTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/HorspoolSearchTest.java)
             * [IsomorphicTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/IsomorphicTest.java)
             * [LetterCombinationsOfPhoneNumberTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/LetterCombinationsOfPhoneNumberTest.java)
             * [longestNonRepeativeSubstringTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/longestNonRepeativeSubstringTest.java)
diff --git a/src/main/java/com/thealgorithms/ciphers/DES.java b/src/main/java/com/thealgorithms/ciphers/DES.java
new file mode 100644
index 00000000..8b226e28
--- /dev/null
+++ b/src/main/java/com/thealgorithms/ciphers/DES.java
@@ -0,0 +1,344 @@
+package com.thealgorithms.ciphers;
+
+/**
+ * This class is build to demonstrate the application of the DES-algorithm 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();
+    }
+
+    private String createPaddedString(String s, int desiredLength, char pad) {
+        int i, l = s.length();
+        StringBuilder paddedString = new StringBuilder();
+        int diff = desiredLength - l;
+        for (i = 0; i < diff; i++) {
+            paddedString.append(pad);
+        }
+        return paddedString.toString();
+    }
+
+    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;
+            message = padLast(message, 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++) {
+                bitBlock.append(pad(Integer.toBinaryString(bytes[j]), 8));
+            }
+            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
+    }
+
+}
\ No newline at end of file


Test patch:
diff --git a/src/test/java/com/thealgorithms/ciphers/DESTest.java b/src/test/java/com/thealgorithms/ciphers/DESTest.java
new file mode 100644
index 00000000..a0c529d5
--- /dev/null
+++ b/src/test/java/com/thealgorithms/ciphers/DESTest.java
@@ -0,0 +1,51 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+//Test example taken from https://page.math.tu-berlin.de/~kant/teaching/hess/krypto-ws2006/des.htm
+public class DESTest {
+
+    DES des;
+
+    @BeforeEach
+    public void setUp() {
+        des = new DES("0000111000110010100100100011001011101010011011010000110101110011");
+    }
+
+    @Test
+    void testEncrypt() {
+        //given
+        String plainText = "Your lips are smoother than vaseline\r\n";
+        //This is equal to c0999fdde378d7ed727da00bca5a84ee47f269a4d6438190d9d52f78f5358499828ac9b453e0e653 in hexadecimal
+        String expectedOutput = "11000000100110011001111111011101111000110111100011010111111" +
+        "011010111001001111101101000000000101111001010010110101000010011101110010001111111001" +
+        "001101001101001001101011001000011100000011001000011011001110101010010111101111000111" +
+        "101010011010110000100100110011000001010001010110010011011010001010011111000001110011001010011";
+
+        //when
+        String cipherText = des.encrypt(plainText);
+
+        //then
+        assertEquals(expectedOutput, cipherText);
+    }
+
+    @Test
+    void testDecrypt() {
+        //given
+       //This is equal to c0999fdde378d7ed727da00bca5a84ee47f269a4d6438190d9d52f78f5358499828ac9b453e0e653 in hexadecimal
+       String cipherText = "11000000100110011001111111011101111000110111100011010111111" +
+       "011010111001001111101101000000000101111001010010110101000010011101110010001111111001" +
+       "001101001101001001101011001000011100000011001000011011001110101010010111101111000111" +
+       "101010011010110000100100110011000001010001010110010011011010001010011111000001110011001010011";
+        String expectedOutput = "Your lips are smoother than vaseline\r\n";;
+
+        //when
+        String plainText = des.decrypt(cipherText);
+
+        //then
+        assertEquals(expectedOutput, plainText);
+    }
+}