Add ElGamalCipher implementing ElGamal encryption and decryption

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.ciphers;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+public class ElGamalCipher {
+    private BigInteger p;
+    private BigInteger g;
+    private BigInteger x;
+    private BigInteger y;
+
+    private SecureRandom random = new SecureRandom();
+
+    // Key generation
+    public void generateKeys(int bitLength) {
+        p = BigInteger.probablePrime(bitLength, random);
+        g = new BigInteger(bitLength - 1, random).mod(p);
+        x = new BigInteger(bitLength - 2, random); // Private key
+        y = g.modPow(x, p); // Public key
+    }
+
+    // Encryption: returns [c1, c2]
+    public BigInteger[] encrypt(BigInteger message) {
+        BigInteger k = new BigInteger(p.bitLength() - 1, random);
+        BigInteger c1 = g.modPow(k, p);
+        BigInteger s = y.modPow(k, p);
+        BigInteger c2 = s.multiply(message).mod(p);
+        return new BigInteger[] {c1, c2};
+    }
+
+    // Decryption: m = c2 * (c1^x)^-1 mod p
+    public BigInteger decrypt(BigInteger c1, BigInteger c2) {
+        BigInteger s = c1.modPow(x, p);
+        BigInteger sInv = s.modInverse(p);
+        return c2.multiply(sInv).mod(p);
+    }
+}