Created DiffieHellman.java and MonoAlphabetic.java

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

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+package com.thealgorithms.ciphers;
+
+import java.math.BigInteger;
+
+public final class DiffieHellman {
+
+    private final BigInteger base;
+    private final BigInteger secret;
+    private final BigInteger prime;
+
+    // Constructor to initialize base, secret, and prime
+    public DiffieHellman(BigInteger base, BigInteger secret, BigInteger prime) {
+        // Check for non-null and positive values
+        if (base == null || secret == null || prime == null || base.signum() <= 0 || secret.signum() <= 0 || prime.signum() <= 0) {
+            throw new IllegalArgumentException("Base, secret, and prime must be non-null and positive values.");
+        }
+        this.base = base;
+        this.secret = secret;
+        this.prime = prime;
+    }
+
+    // Method to calculate public value (g^x mod p)
+    public BigInteger calculatePublicValue() {
+        // Returns g^x mod p
+        return base.modPow(secret, prime);
+    }
+
+    // Method to calculate the shared secret key (otherPublic^secret mod p)
+    public BigInteger calculateSharedSecret(BigInteger otherPublicValue) {
+        if (otherPublicValue == null || otherPublicValue.signum() <= 0) {
+            throw new IllegalArgumentException("Other public value must be non-null and positive.");
+        }
+        // Returns b^x mod p or a^y mod p
+        return otherPublicValue.modPow(secret, prime);
+    }
+}

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

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+package com.thealgorithms.ciphers;
+
+public final class MonoAlphabetic {
+
+    private MonoAlphabetic() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    // Encryption method
+    public static String encrypt(String data, String key) {
+        if (!data.matches("[A-Z]+")) {
+            throw new IllegalArgumentException("Input data contains invalid characters. Only uppercase A-Z are allowed.");
+        }
+        StringBuilder sb = new StringBuilder();
+
+        // Encrypt each character
+        for (char c : data.toCharArray()) {
+            int idx = charToPos(c); // Get the index of the character
+            sb.append(key.charAt(idx)); // Map to the corresponding character in the key
+        }
+        return sb.toString();
+    }
+
+    // Decryption method
+    public static String decrypt(String data, String key) {
+        StringBuilder sb = new StringBuilder();
+
+        // Decrypt each character
+        for (char c : data.toCharArray()) {
+            int idx = key.indexOf(c); // Find the index of the character in the key
+            if (idx == -1) {
+                throw new IllegalArgumentException("Input data contains invalid characters.");
+            }
+            sb.append(posToChar(idx)); // Convert the index back to the original character
+        }
+        return sb.toString();
+    }
+
+    // Helper method: Convert a character to its position in the alphabet
+    private static int charToPos(char c) {
+        return c - 'A'; // Subtract 'A' to get position (0 for A, 1 for B, etc.)
+    }
+
+    // Helper method: Convert a position in the alphabet to a character
+    private static char posToChar(int pos) {
+        return (char) (pos + 'A'); // Add 'A' to convert position back to character
+    }
+}

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

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+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));
+    }
+}

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

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+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"));
+    }
+}