Add DiffieHellman and MonoAlphabetic (#5508)

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

@@ -0,0 +1,38 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.math.BigInteger;
+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 DiffieHellmanTest {
+
+    // Test for public value calculation using instance methods
+    @ParameterizedTest
+    @MethodSource("provideTestData")
+    public void testCalculatePublicValue(BigInteger base, BigInteger secret, BigInteger prime, BigInteger publicExpected, BigInteger sharedExpected) {
+        DiffieHellman dh = new DiffieHellman(base, secret, prime); // Create an instance of DiffieHellman
+        assertEquals(publicExpected, dh.calculatePublicValue()); // Call instance method
+    }
+
+    // Test for shared secret calculation using instance methods
+    @ParameterizedTest
+    @MethodSource("provideTestData")
+    public void testCalculateSharedSecret(BigInteger base, BigInteger secret, BigInteger prime, BigInteger publicExpected, BigInteger sharedExpected) {
+        DiffieHellman dh = new DiffieHellman(base, secret, prime); // Create an instance of DiffieHellman
+        assertEquals(sharedExpected, dh.calculateSharedSecret(publicExpected)); // Call instance method
+    }
+
+    // Provide test data for both public key and shared secret calculation
+    private static Stream<Arguments> provideTestData() {
+        return Stream.of(createTestArgs(5, 6, 23, 8, 13), createTestArgs(2, 5, 13, 6, 2));
+    }
+
+    // Helper method for arguments
+    private static Arguments createTestArgs(long base, long secret, long prime, long publicExpected, long sharedExpected) {
+        return Arguments.of(BigInteger.valueOf(base), BigInteger.valueOf(secret), BigInteger.valueOf(prime), BigInteger.valueOf(publicExpected), BigInteger.valueOf(sharedExpected));
+    }
+}