feat: implement One-Time Pad cipher (#6941) (#7096)

* feat: implement One-Time Pad cipher (#6941)

* style: format OneTimePadCipher with clang-format

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

@@ -0,0 +1,49 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.nio.charset.StandardCharsets;
+import org.junit.jupiter.api.Test;
+
+class OneTimePadCipherTest {
+
+    @Test
+    void encryptAndDecryptWithRandomKeyRestoresPlaintext() {
+        String plaintext = "The quick brown fox jumps over the lazy dog.";
+        byte[] plaintextBytes = plaintext.getBytes(StandardCharsets.UTF_8);
+
+        byte[] key = OneTimePadCipher.generateKey(plaintextBytes.length);
+
+        byte[] ciphertext = OneTimePadCipher.encrypt(plaintextBytes, key);
+        byte[] decrypted = OneTimePadCipher.decrypt(ciphertext, key);
+
+        assertArrayEquals(plaintextBytes, decrypted);
+        assertEquals(plaintext, new String(decrypted, StandardCharsets.UTF_8));
+    }
+
+    @Test
+    void generateKeyWithNegativeLengthThrowsException() {
+        assertThrows(IllegalArgumentException.class, () -> OneTimePadCipher.generateKey(-1));
+    }
+
+    @Test
+    void encryptWithMismatchedKeyLengthThrowsException() {
+        byte[] data = "hello".getBytes(StandardCharsets.UTF_8);
+        byte[] shortKey = OneTimePadCipher.generateKey(2);
+
+        assertThrows(IllegalArgumentException.class, () -> OneTimePadCipher.encrypt(data, shortKey));
+    }
+
+    @Test
+    void decryptWithMismatchedKeyLengthThrowsException() {
+        byte[] data = "hello".getBytes(StandardCharsets.UTF_8);
+        byte[] key = OneTimePadCipher.generateKey(data.length);
+        byte[] ciphertext = OneTimePadCipher.encrypt(data, key);
+
+        byte[] wrongSizedKey = OneTimePadCipher.generateKey(data.length + 1);
+
+        assertThrows(IllegalArgumentException.class, () -> OneTimePadCipher.decrypt(ciphertext, wrongSizedKey));
+    }
+}