Java Program on RSA Algorithm

RSA algorithm is an asymmetric cryptography algorithm. Asymmetric means that it works on two different keys i.e. Public Key and Private Key. As the name suggests that the Public Key is given to everyone and Private Key is kept private.

Algorithm

Step 1 : Choose two prime numbers p and q.

Step 2 : Calculate n = p*q

Step 3 : Calculate ϕ(n) = (p – 1) * (q – 1)

Step 4 : Choose e such that gcd(e , ϕ(n) ) = 1

Step 5 : Calculate d such that e*d mod ϕ(n) = 1

Step 6 : Public Key {e,n} Private Key {d,n}

Step 7 : Cipher text C = P^e mod n where P = plaintext

Step 8 : For Decryption D = D^d mod n where D will give back the plaintext.

If you need a dry run of the program or any other query, then kindly leave a comment in the comment box or mail me, I would be more than happy to help you.

Program

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);
	}
}

CODEDOST

Java Program on RSA Algorithm