Add One-Time Pad Cipher implementation

Author: Nihhaar Saini

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

@@ -0,0 +1,39 @@
+package com.thealgorithms.ciphers;
+
+import java.security.SecureRandom;
+import java.util.Objects;
+
+public final class OneTimePadCipher {
+
+    private static final SecureRandom RANDOM = new SecureRandom();
+
+    private OneTimePadCipher() {}
+
+    public static byte[] generateKey(final int length) {
+        if (length <= 0) {
+            throw new IllegalArgumentException("Key length must be positive");
+        }
+        byte[] key = new byte[length];
+        RANDOM.nextBytes(key);
+        return key;
+    }
+
+    public static byte[] encrypt(final byte[] plaintext, final byte[] key) {
+        Objects.requireNonNull(plaintext, "plaintext");
+        Objects.requireNonNull(key, "key");
+
+        if (plaintext.length != key.length) {
+            throw new IllegalArgumentException("Plaintext and key must have the same length");
+        }
+
+        byte[] ciphertext = new byte[plaintext.length];
+        for (int i = 0; i < plaintext.length; i++) {
+            ciphertext[i] = (byte) (plaintext[i] ^ key[i]);
+        }
+        return ciphertext;
+    }
+
+    public static byte[] decrypt(final byte[] ciphertext, final byte[] key) {
+        return encrypt(ciphertext, key);
+    }
+}

src/test/java/com/thealgorithms/ciphers/OneTimePadCipherTest.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.Assert.*;
+
+import java.nio.charset.StandardCharsets;
+import org.junit.Test;
+
+public class OneTimePadCipherTest {
+
+    @Test
+    public void encryptDecryptWorks() {
+        String original = "OTP Test ";
+        byte[] plaintext = original.getBytes(StandardCharsets.UTF_8);
+        byte[] key = OneTimePadCipher.generateKey(plaintext.length);
+
+        byte[] encrypted = OneTimePadCipher.encrypt(plaintext, key);
+        byte[] decrypted = OneTimePadCipher.decrypt(encrypted, key);
+
+        assertEquals(original, new String(decrypted, StandardCharsets.UTF_8));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void throwsIfDifferentLength() {
+        byte[] plaintext = "Hi".getBytes(StandardCharsets.UTF_8);
+        byte[] key = new byte[] { 1, 2, 3 }; // wrong length
+
+        OneTimePadCipher.encrypt(plaintext, key);
+    }
+}