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1Practical No.5: Write a program to implement the ElGamal Cryptosystem to generate keys and perform encryption and decryption.
2
3Source Code:
4package cryptanalysis;
5import java.math.*;
6import java.util.*;
7import java.security.*;
8import java.io.*;
9
10public class ElGamal
11{
12	public static void main(String[] args) throws IOException
13	{
14		BigInteger p, b, c, secretKey;
15		Random sc = new SecureRandom();
16		secretKey = new BigInteger("12345678901234567890");
17		// public key calculation
18		System.out.println("secretKey = " + secretKey);
19		p = BigInteger.probablePrime(64, sc);
20		b = new BigInteger("3");
21		c = b.modPow(secretKey, p);
22		System.out.println("p = " + p);
23		System.out.println("b = " + b);
24		System.out.println("c = " + c);
25		// Encryption
26		System.out.print("Enter your Big Number message -->");
27		Scanner sca=new Scanner(System.in);
28		String s = sca.nextLine();
29		BigInteger X = new BigInteger(s);
30		BigInteger r = new BigInteger(64, sc);
31		BigInteger EC = X.multiply(c.modPow(r, p)).mod(p);
32		BigInteger brmodp = b.modPow(r, p);
33		System.out.println("Plaintext = " + X);
34		System.out.println("r = " + r);
35		System.out.println("EC = " + EC);
36		System.out.println("b^r mod p = " + brmodp);
37		// Decryption
38		BigInteger crmodp = brmodp.modPow(secretKey, p);
39		BigInteger d = crmodp.modInverse(p);
40		BigInteger ad = d.multiply(EC).mod(p);
41		System.out.println("\n\nc^r mod p = " + crmodp);
42		System.out.println("d = " + d);
43		System.out.println("Alice decodes: " + ad);
44	}
45}