Add DiffieHellman and MonoAlphabetic (#5508)

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

@@ -0,0 +1,29 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+public class MonoAlphabeticTest {
+
+    // Test for both encryption and decryption with different keys
+    @ParameterizedTest
+    @MethodSource("provideTestData")
+    public void testEncryptDecrypt(String plainText, String key, String encryptedText) {
+        // Test encryption
+        String actualEncrypted = MonoAlphabetic.encrypt(plainText, key);
+        assertEquals(encryptedText, actualEncrypted, "Encryption failed for input: " + plainText + " with key: " + key);
+
+        // Test decryption
+        String actualDecrypted = MonoAlphabetic.decrypt(encryptedText, key);
+        assertEquals(plainText, actualDecrypted, "Decryption failed for input: " + encryptedText + " with key: " + key);
+    }
+
+    // Provide test data for both encryption and decryption
+    private static Stream<Arguments> provideTestData() {
+        return Stream.of(Arguments.of("HELLO", "MNBVCXZLKJHGFDSAPOIUYTREWQ", "LCGGS"), Arguments.of("JAVA", "MNBVCXZLKJHGFDSAPOIUYTREWQ", "JMTM"), Arguments.of("HELLO", "QWERTYUIOPLKJHGFDSAZXCVBNM", "ITKKG"), Arguments.of("JAVA", "QWERTYUIOPLKJHGFDSAZXCVBNM", "PQCQ"));
+    }
+}