El-Gamal algoritmida maʼlumotni shifrlovchi dasturiy vositani ishlab chiqish

O‘ZBEKISTON RESPUBLIKASI AXBOROT TEXNOLOGIYALARI VA KOMMUNIKATSIYALARINI RIVOJLANTIRISH VAZIRLIGI
MUHAMMAD AL-XORAZMIY NOMIDAGI
TOSHKENT AXBOROT TEXNOLOGIYALARI UNIVERSITETI

“Kriptografiya 2”

fanidan


amaliy ish 4


El-Gamal algoritmida maʼlumotni shifrlovchi dasturiy vositani ishlab chiqish
Bajardi: Axborot xavfsizligi ta’lim yo‘nalishi
714-19-guruh
Eshmamatov Obidjon
talabaning F.I.Sh.

Qabul qildi: Mardiyev U

Toshkent 2021



package com.epam.esm;

import java.util.*;

import java.math.BigInteger;

public class ElGamal {

public static void main(String[] args) {

Scanner stdin = new Scanner(System.in);

Random r = new Random();

// Get user input for p.

System.out.println("Enter the approximate value of the prime number for your El Gamal key.");
BigInteger p = getNextPrime(stdin.next());

// Calculate a generator.

BigInteger g = getGenerator(p, r);

// We found a generator, so let's do the rest of it.

if (g != null) {

// Pick a secret a, not 0 or 1.

BigInteger a = new BigInteger(p.bitLength()+1, r);
a = a.mod(p);
if (a.equals(BigInteger.ZERO) || a.equals(BigInteger.ONE))
a = a.add(new BigInteger("2"));

// Calculate the corresponding public b.

BigInteger b = g.modPow(a, p);

// Print out our public keys.

System.out.println("Post p = "+p+" g = "+g+" b = "+b);

// When we send a message, the sender picks a random k.

// I make sure it's never 0 or 1.
BigInteger k = new BigInteger(p.bitLength()+1, r);
k = k.mod(p);
if (k.equals(BigInteger.ZERO) || k.equals(BigInteger.ONE))
k = k.add(new BigInteger("2"));

// Here, the sender starts calculating parts of the ciphertext that

// don't involve the actual message.
BigInteger c1 = g.modPow(k, p);
BigInteger c2 = b.modPow(k, p);

// Here we get the message from the user.

System.out.println("Please enter your message. It should be in between 1 and "+p);
BigInteger m = new BigInteger(stdin.next());

// Now, we can calculate the rest of the second ciphertext.

c2 = c2.multiply(m);
c2 = c2.mod(p);

// Print out the two ciphertexts.

System.out.println("The corresponding cipher texts are c1 = "+c1+" c2 = "+c2);

// First, determine the inverse of c1 raised to the a power mod p.

BigInteger temp = c1.modPow(a,p);
temp = temp.modInverse(p);

// Print this out.

System.out.println("Here is c1^ -a = "+temp);

// Now, just multiply this by the second ciphertext

BigInteger recover = temp.multiply(c2);
recover = recover.mod(p);

// And this will give us our original message back!

System.out.println("The original message = "+recover);
}

// My sorry message!

else
System.out.println("Sorry, a generator for your prime couldn't be found.");

}

// Incrementally tries each BigInteger starting at the value passed
// in as a parameter until one of them is tests as being prime.
public static BigInteger getNextPrime(String ans) {
BigInteger one = new BigInteger("1");
BigInteger test = new BigInteger(ans);
while (!test.isProbablePrime(99))
test = test.add(one);
return test;
}

// Precondition - p is prime and it's reasonably small, say, no more than

// 5,000,000. If it's larger, this method will be quite
// time-consuming.
// Postcondition - if a generator for p can be found, then it is returned
// if no generator is found after 1000 tries, null is
// returned.
public static BigInteger getGenerator(BigInteger p, Random r) {

int numtries = 0;

// Try finding a generator at random 100 times.
while (numtries < 1000) {

// Here's what we're trying as the generator this time.

BigInteger rand = new BigInteger(p.bitLength()+1,r);
rand = rand.mod(p);

BigInteger exp = BigInteger.ONE;

BigInteger next = rand;

// These are bad choices!

if (next.equals(BigInteger.ZERO) || next.equals(BigInteger.ONE))
continue;

// We exponentiate our generator until we get 1 mod p.

while (!next.equals(BigInteger.ONE)) {
next = (next.multiply(rand)).mod(p);
exp = exp.add(BigInteger.ONE);
}

// If the first time we hit 1 is the exponent p-1, then we have

// a generator.
if (exp.equals(p.subtract(BigInteger.ONE)))
return rand;

numtries++;

}

// None of the 1000 values we tried was a generator.

return null;

}

}