Nov 12, 2025 · Nov 12, 2025 · Nov 12, 2025 · Nov 12, 2025 · Nov 12, 2025 · Nov 15, 2025
diff --git a/src/main/java/com/thealgorithms/ciphers/ElGamalCipher.java b/src/main/java/com/thealgorithms/ciphers/ElGamalCipher.java
 package com.thealgorithms.ciphers;

 import java.math.BigInteger;
 import java.security.SecureRandom;

 public class ElGamalCipher {
    private BigInteger p, g, x, 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);
    }

    // Example usage
    public static void main(String[] args) {
        ElGamalCipher elgamal = new ElGamalCipher();
        elgamal.generateKeys(256);

        BigInteger message = new BigInteger("12345");
        BigInteger[] cipher = elgamal.encrypt(message);
        BigInteger decrypted = elgamal.decrypt(cipher[0], cipher[1]);

        System.out.println("Original: " + message);
        System.out.println("Encrypted: c1=" + cipher[0] + ", c2=" + cipher[1]);
        System.out.println("Decrypted: " + decrypted);
    }
 }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,47 @@
		package com.thealgorithms.ciphers;

		import java.math.BigInteger;
		import java.security.SecureRandom;

		public class ElGamalCipher {
		private BigInteger p, g, x, 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);
		}

		// Example usage
		public static void main(String[] args) {
		ElGamalCipher elgamal = new ElGamalCipher();
		elgamal.generateKeys(256);

		BigInteger message = new BigInteger("12345");
		BigInteger[] cipher = elgamal.encrypt(message);
		BigInteger decrypted = elgamal.decrypt(cipher[0], cipher[1]);

		System.out.println("Original: " + message);
		System.out.println("Encrypted: c1=" + cipher[0] + ", c2=" + cipher[1]);
		System.out.println("Decrypted: " + decrypted);
		}
		}