Added one time pad cipher

src/main/java/com/thealgorithms/ciphers/OneTimePadCipher.java

@@ -0,0 +1,72 @@
+package com.thealgorithms.ciphers;
+
+import java.security.SecureRandom;
+
+/**
+ * The One-Time Pad Cipher is a symmetric encryption technique
+ * that XORs plaintext with a truly random key of equal length.
+ * 
+ * ⚠️ Important:
+ *  - The key must be random and used only once.
+ *  - Key reuse makes it insecure.
+ * 
+ * Example:
+ *   Plaintext:  HELLO
+ *   Key:        XMCKL
+ *   Ciphertext: EQNVZ
+ * 
+ * Reference:
+ *   Shannon, C. E. (1949). Communication Theory of Secrecy Systems.
+ */
+public class OneTimePadCipher {
+
+    private static final SecureRandom RANDOM = new SecureRandom();
+
+    /**
+     * Encrypts or decrypts a message using the One-Time Pad method.
+     * 
+     * @param input The input string (plaintext or ciphertext)
+     * @param key The key (must be the same length as input)
+     * @return The resulting encrypted/decrypted string
+     */
+    public static String xorCipher(String input, String key) {
+        if (input.length() != key.length()) {
+            throw new IllegalArgumentException("Input and key lengths must match!");
+        }
+
+        StringBuilder output = new StringBuilder();
+        for (int i = 0; i < input.length(); i++) {
+            char encryptedChar = (char) (input.charAt(i) ^ key.charAt(i));
+            output.append(encryptedChar);
+        }
+        return output.toString();
+    }
+
+    /**
+     * Generates a random key of the same length as the message.
+     * 
+     * @param length The desired key length
+     * @return A random key string
+     */
+    public static String generateRandomKey(int length) {
+        StringBuilder key = new StringBuilder();
+        for (int i = 0; i < length; i++) {
+            // Generate printable ASCII range (32–126)
+            key.append((char) (RANDOM.nextInt(95) + 32));
+        }
+        return key.toString();
+    }
+
+    public static void main(String[] args) {
+        String message = "HELLO WORLD";
+        String key = generateRandomKey(message.length());
+
+        String encrypted = xorCipher(message, key);
+        String decrypted = xorCipher(encrypted, key);
+
+        System.out.println("Message:   " + message);
+        System.out.println("Key:       " + key);
+        System.out.println("Encrypted: " + encrypted);
+        System.out.println("Decrypted: " + decrypted);
+    }
+}

src/main/java/com/thealgorithms/ciphers/OneTimePadCipher.java.txt

@@ -0,0 +1,66 @@
+package com.thealgorithms.ciphers;
+
+import java.security.SecureRandom;
+
+/**
+ * Implementation of the One-Time Pad Cipher.
+ *
+ * <p>The One-Time Pad Cipher is a symmetric encryption technique that XORs
+ * plaintext with a truly random key of equal length. It is considered
+ * theoretically unbreakable when the key is random, used only once, and kept
+ * secret.
+ *
+ * <p>Example:
+ *
+ * <pre>
+ * Plaintext:  HELLO
+ * Key:        XMCKL
+ * Ciphertext: EQNVZ
+ * </pre>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/One-time_pad">Wikipedia: One-time pad</a>
+ */
+public final class OneTimePadCipher {
+
+    private static final SecureRandom RANDOM = new SecureRandom();
+
+    private OneTimePadCipher() {
+        // Utility class; prevent instantiation.
+    }
+
+    /**
+     * Encrypts or decrypts a message using the One-Time Pad method.
+     *
+     * @param input the input string (plaintext or ciphertext)
+     * @param key the key (must be the same length as input)
+     * @return the resulting encrypted/decrypted string
+     * @throws IllegalArgumentException if input and key lengths do not match
+     */
+    public static String xorCipher(String input, String key) {
+        if (input.length() != key.length()) {
+            throw new IllegalArgumentException("Input and key lengths must match!");
+        }
+
+        StringBuilder output = new StringBuilder();
+        for (int i = 0; i < input.length(); i++) {
+            char encryptedChar = (char) (input.charAt(i) ^ key.charAt(i));
+            output.append(encryptedChar);
+        }
+        return output.toString();
+    }
+
+    /**
+     * Generates a random key of the same length as the message.
+     *
+     * @param length the desired key length
+     * @return a random key string
+     */
+    public static String generateRandomKey(int length) {
+        StringBuilder key = new StringBuilder();
+        for (int i = 0; i < length; i++) {
+            // Generate printable ASCII range (32–126)
+            key.append((char) (RANDOM.nextInt(95) + 32));
+        }
+        return key.toString();
+    }
+}