Created DiffieHellman.java and MonoAlphabetic.java

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

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+import java.math.BigInteger;
+import java.util.Scanner;
+
+public final class DiffieHellman {
+    private DiffieHellman() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+    public static void main(String[] args) {
+        Scanner read = new Scanner(System.in);
+        System.out.println("Hello User! \nEnter your name:");
+        String name = read.nextLine();
+        read.nextLine();
+        System.out.println("Welcome " + name + "!");
+
+        BigInteger n;
+        BigInteger g;
+        BigInteger x;
+        BigInteger y;
+        BigInteger k1;
+        BigInteger k2;
+        BigInteger a;
+        BigInteger b;
+
+        System.out.println("Enter two prime numbers: ");
+        n = new BigInteger(read.next());
+        g = new BigInteger(read.next());
+
+        System.out.println("Person A : Enter your secret number");
+        x = new BigInteger(read.next());
+        a = g.modPow(x, n);
+
+        System.out.println("Person B : Enter your secret number");
+        y = new BigInteger(read.next());
+        b = g.modPow(y, n);
+
+        k1 = b.modPow(x, n);
+        k2 = a.modPow(y, n);
+
+        System.out.println("A's secret key: " + k1);
+        System.out.println("B's secret key: " + k2);
+    }
+}

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

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+import java.util.Scanner;
+
+public final class MonoAlphabetic {
+    private MonoAlphabetic() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+    public static void main(String[] args) {
+        Scanner read = new Scanner(System.in);
+        System.out.println("Hello User! \nEnter your name:");
+        String name = read.nextLine();
+        read.nextLine();
+        System.out.println("Welcome " + name + "!\nDo you want to encrypt data or decrypt the data?\nFor encryption enter: 1\nFor decryption enter: 2");
+        int x = read.nextInt();
+
+        String key = "MNBVCXZLKJHGFDSAPOIUYTREWQ";
+
+        switch (x) {
+        case 1:
+            System.out.println("\nPlease enter the data that is to be encrypted, we will be using MonoAlphabetic Cipher to encrypt the data.");
+            String data = read.nextLine().toUpperCase();
+            String encryptedData = encrypt(data, key);
+            System.out.println("Encrypted data: " + encryptedData);
+            break;
+        case 2:
+            System.out.println("\nPlease enter the data that is to be decrypted, we will be using MonoAlphabetic Cipher to decrypt the data.");
+            data = read.nextLine().toUpperCase();
+            String decryptedData = decrypt(data, key);
+            System.out.println("Decrypted data: " + decryptedData);
+            break;
+        default:
+            System.out.println("The input was invalid. Kindly restart.");
+            break;
+        }
+    }
+
+    public static String encrypt(String data, String key) {
+        int idx;
+        char c;
+        StringBuffer sb = new StringBuffer(data);
+
+        for (int i = 0; i < sb.length(); i++) {
+            idx = sb.charAt(i) - 65;
+            c = key.charAt(idx);
+            sb.setCharAt(i, c);
+        }
+        return new String(sb);
+    }
+
+    public static String decrypt(String data, String key) {
+        int idx;
+        char c;
+        StringBuffer sb = new StringBuffer(data);
+
+        for (int i = 0; i < sb.length(); i++) {
+            c = sb.charAt(i);
+            idx = getIndex(c, key);
+            c = (char) (idx + 65);
+            sb.setCharAt(i, c);
+        }
+        return new String(sb);
+    }
+
+    public static int getIndex(char c, String key) {
+        int idx = -1;
+        for (int i = 0; i < key.length(); i++) {
+            if (key.charAt(i) == c) {
+                idx = i;
+            }
+        }
+        return idx;
+    }
+}