RSA

import java.math.BigInteger;
import java.util.Random;
import java.util.Scanner;

public class RSA {
    public static int bitlength = 256;
    public static Random r = new Random();
    public static BigInteger p = BigInteger.probablePrime(bitlength, r);
    public static BigInteger q = BigInteger.probablePrime(bitlength, r);
    public static BigInteger n = p.multiply(q);
    public static BigInteger phi = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
    public static BigInteger d;
    public static BigInteger e = BigInteger.probablePrime(bitlength / 2, r);

    public static void main(String args[]) {
        // While gcd(phi,e) > 1 and e < phi
        while (phi.gcd(e).compareTo(BigInteger.ONE) > 0 && e.compareTo(phi) < 0) {
            e.add(BigInteger.ONE);
        }

        d = e.modInverse(phi);
        System.out.println("Public key : " + e);
        System.out.println("Private key : " + d);
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter plaintext :");
        String plaintext = sc.nextLine();
        sc.close();
        System.out.println("\nPlaintext : " + plaintext);
        byte encrypted[] = encrypt(plaintext.getBytes());

        System.out.println("\nPlaintext in bytes : " + bytestoString(plaintext.getBytes()));
        System.out.println("\nCiphertext in bytes : " + bytestoString(encrypted));

        byte decrypted[] = decrypt(encrypted);
        System.out.println("\nPlaintext after decrypting : " + new String(decrypted));
    }

    private static String bytestoString(byte[] encrypted) {
        StringBuilder ciphertext = new StringBuilder();
        for (byte b : encrypted) {
            ciphertext.append(Byte.toString(b));
        }
        return ciphertext.toString();
    }

    private static byte[] encrypt(byte[] message) {
        return (new BigInteger(message)).modPow(e, n).toByteArray();
    }

    private static byte[] decrypt(byte[] message) {
        return (new BigInteger(message)).modPow(d, n).toByteArray();
    }
}
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import java.util.*;
import java.math.*;
 
class RSA
{
 public static void main(String args[])
 {
 Scanner sc=new Scanner(System.in);
 int p,q,n,z,d=0,e,i;
 System.out.println("Enter the number to be encrypted and decrypted");
 int msg=sc.nextInt();
 double c;
 BigInteger msgback; 
 System.out.println("Enter 1st prime number p");
 p=sc.nextInt();
 System.out.println("Enter 2nd prime number q");
 q=sc.nextInt();
 
 n=p*q;
 z=(p-1)*(q-1);
 System.out.println("the value of z = "+z); 
 
 for(e=2;e<z;e++)
 {
 if(gcd(e,z)==1)            // e is for public key exponent
 { 
 break;
 }
 }
 System.out.println("the value of e = "+e); 
 for(i=0;i<=9;i++)
 {
 int x=1+(i*z);
 if(x%e==0)      //d is for private key exponent
 {
 d=x/e;
 break;
 }
 }
 System.out.println("the value of d = "+d); 
 c=(Math.pow(msg,e))%n;
 System.out.println("Encrypted message is : -");
 System.out.println(c);
                //converting int value of n to BigInteger
 BigInteger N = BigInteger.valueOf(n);
 //converting float value of c to BigInteger
 BigInteger C = BigDecimal.valueOf(c).toBigInteger();
 msgback = (C.pow(d)).mod(N);
 System.out.println("Derypted message is : -");
 System.out.println(msgback);
 
 }
 
 static int gcd(int e, int z)
 {
 if(e==0)
 return z; 
 else
 return gcd(z%e,e);
 }
}