Add One-Time Pad Cipher implementation (fixed)

Author: Shewale41

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();
+    }
+}