feat: add ElGamalCipher with Safe Prime generation and stateless design (#7257)

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

@@ -0,0 +1,145 @@
+package com.thealgorithms.ciphers;
+
+import java.math.BigInteger;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+
+/**
+ * Unit tests for ElGamalCipher.
+ * Includes property-based testing (homomorphism), probabilistic checks,
+ * and boundary validation.
+ */
+class ElGamalCipherTest {
+
+    private static ElGamalCipher.KeyPair sharedKeys;
+
+    @BeforeAll
+    static void setup() {
+        // Generate 256-bit keys for efficient unit testing
+        sharedKeys = ElGamalCipher.generateKeys(256);
+    }
+
+    @Test
+    @DisplayName("Test Key Generation Validity")
+    void testKeyGeneration() {
+        Assertions.assertNotNull(sharedKeys.p());
+        Assertions.assertNotNull(sharedKeys.g());
+        Assertions.assertNotNull(sharedKeys.x());
+        Assertions.assertNotNull(sharedKeys.y());
+
+        // Verify generator bounds: 1 < g < p
+        Assertions.assertTrue(sharedKeys.g().compareTo(BigInteger.ONE) > 0);
+        Assertions.assertTrue(sharedKeys.g().compareTo(sharedKeys.p()) < 0);
+
+        // Verify private key bounds: 1 < x < p-1
+        Assertions.assertTrue(sharedKeys.x().compareTo(BigInteger.ONE) > 0);
+        Assertions.assertTrue(sharedKeys.x().compareTo(sharedKeys.p().subtract(BigInteger.ONE)) < 0);
+    }
+
+    @Test
+    @DisplayName("Security Check: Probabilistic Encryption")
+    void testSemanticSecurity() {
+        // Encrypting the same message twice MUST yield different ciphertexts
+        // due to the random ephemeral key 'k'.
+        BigInteger message = new BigInteger("123456789");
+
+        ElGamalCipher.CipherText c1 = ElGamalCipher.encrypt(message, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+        ElGamalCipher.CipherText c2 = ElGamalCipher.encrypt(message, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+
+        // Check that the ephemeral keys (and thus 'a' components) were different
+        Assertions.assertNotEquals(c1.a(), c2.a(), "Ciphertexts must be randomized (Semantic Security violation)");
+        Assertions.assertNotEquals(c1.b(), c2.b());
+
+        // But both must decrypt to the original message
+        Assertions.assertEquals(ElGamalCipher.decrypt(c1, sharedKeys.x(), sharedKeys.p()), message);
+        Assertions.assertEquals(ElGamalCipher.decrypt(c2, sharedKeys.x(), sharedKeys.p()), message);
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideMessages")
+    @DisplayName("Parameterized Test: Encrypt and Decrypt various messages")
+    void testEncryptDecrypt(String messageStr) {
+        BigInteger message = new BigInteger(messageStr.getBytes());
+
+        // Skip if message exceeds the test key size (256 bits)
+        if (message.compareTo(sharedKeys.p()) >= 0) {
+            return;
+        }
+
+        ElGamalCipher.CipherText ciphertext = ElGamalCipher.encrypt(message, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+        BigInteger decrypted = ElGamalCipher.decrypt(ciphertext, sharedKeys.x(), sharedKeys.p());
+
+        Assertions.assertEquals(message, decrypted, "Decrypted BigInteger must match original");
+        Assertions.assertEquals(messageStr, new String(decrypted.toByteArray()), "Decrypted string must match original");
+    }
+
+    static Stream<String> provideMessages() {
+        return Stream.of("Hello World", "TheAlgorithms", "A", "1234567890", "!@#$%^&*()");
+    }
+
+    @Test
+    @DisplayName("Edge Case: Message equals 0")
+    void testMessageZero() {
+        BigInteger zero = BigInteger.ZERO;
+        ElGamalCipher.CipherText ciphertext = ElGamalCipher.encrypt(zero, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+        BigInteger decrypted = ElGamalCipher.decrypt(ciphertext, sharedKeys.x(), sharedKeys.p());
+
+        Assertions.assertEquals(zero, decrypted, "Should successfully encrypt/decrypt zero");
+    }
+
+    @Test
+    @DisplayName("Edge Case: Message equals p-1")
+    void testMessageMaxBound() {
+        BigInteger pMinus1 = sharedKeys.p().subtract(BigInteger.ONE);
+        ElGamalCipher.CipherText ciphertext = ElGamalCipher.encrypt(pMinus1, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+        BigInteger decrypted = ElGamalCipher.decrypt(ciphertext, sharedKeys.x(), sharedKeys.p());
+
+        Assertions.assertEquals(pMinus1, decrypted, "Should successfully encrypt/decrypt p-1");
+    }
+
+    @Test
+    @DisplayName("Negative Test: Message >= p should fail")
+    void testMessageTooLarge() {
+        BigInteger tooLarge = sharedKeys.p();
+        Assertions.assertThrows(IllegalArgumentException.class, () -> ElGamalCipher.encrypt(tooLarge, sharedKeys.p(), sharedKeys.g(), sharedKeys.y()));
+    }
+
+    @Test
+    @DisplayName("Negative Test: Decrypt with wrong private key")
+    void testWrongKeyDecryption() {
+        BigInteger message = new BigInteger("99999");
+        ElGamalCipher.CipherText ciphertext = ElGamalCipher.encrypt(message, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+
+        // Generate a fake private key
+        BigInteger wrongX = sharedKeys.x().add(BigInteger.ONE);
+
+        BigInteger decrypted = ElGamalCipher.decrypt(ciphertext, wrongX, sharedKeys.p());
+
+        Assertions.assertNotEquals(message, decrypted, "Decryption with wrong key must yield incorrect result");
+    }
+
+    @Test
+    @DisplayName("Property Test: Multiplicative Homomorphism")
+    void testHomomorphism() {
+        BigInteger m1 = new BigInteger("50");
+        BigInteger m2 = BigInteger.TEN; // Fix: Replaced new BigInteger("10") with BigInteger.TEN
+
+        ElGamalCipher.CipherText c1 = ElGamalCipher.encrypt(m1, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+        ElGamalCipher.CipherText c2 = ElGamalCipher.encrypt(m2, sharedKeys.p(), sharedKeys.g(), sharedKeys.y());
+
+        // Multiply ciphertexts component-wise: (a1*a2, b1*b2)
+        BigInteger aNew = c1.a().multiply(c2.a()).mod(sharedKeys.p());
+        BigInteger bNew = c1.b().multiply(c2.b()).mod(sharedKeys.p());
+        ElGamalCipher.CipherText cCombined = new ElGamalCipher.CipherText(aNew, bNew);
+
+        BigInteger decrypted = ElGamalCipher.decrypt(cCombined, sharedKeys.x(), sharedKeys.p());
+        BigInteger expected = m1.multiply(m2).mod(sharedKeys.p());
+
+        Assertions.assertEquals(expected, decrypted, "Cipher must satisfy multiplicative homomorphism");
+    }
+}