Add DiffieHellman and MonoAlphabetic (#5508)

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

@@ -0,0 +1,36 @@
+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

@@ -0,0 +1,48 @@
+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
+    }
+}