Notes

import java.math.BigInteger;
import java.io.*;

public class RSA
{
// BigInteger is used to store and manipulate very large numbers
BigInteger p, q, d, e, n, z;
// p and q -> Large Prime Numbers
// e -> Public Key
// d -> Private Key
BufferedReader keyin = new BufferedReader(new InputStreamReader(System.in));
String msg, rmsg, code;
// msg -> String to Encrypt
//code -> Encrypted String
// rmsg -> Decrypted String
int size;
//Size of the Message
BigInteger m, c;
// m and c -> used to store the ASCII values of individual characters in BigInteger format to encrypt or decrypt

void read()throws IOException
{
System.out.println("Enter the large prime numbers(p and q: Such that p*q > 127):");
p = new BigInteger(keyin.readLine()); // Read p
q = new BigInteger(keyin.readLine()); // Read q
n = p.multiply(q); // n = p*q
z = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE)); // z = (p-1)(q-1)
System.out.println("Enter the public exponent (e):");
e = new BigInteger(keyin.readLine()); // Read Public Key (e)
d = new BigInteger("0"); // Set Private Key (d) to 0
BigInteger temp;
// do-while loop to generate private key (d) such that (e*d) mod z = 1
do
{
d = d.add(BigInteger.ONE); // d++
temp = (d.multiply(e)).mod(z); // computing (e*d) mod z
}while(!temp.equals(BigInteger.ONE)); // the value of d is accepted if temp==1
System.out.println("Enter Message to Encrypt:");
msg = keyin.readLine();
size = msg.length();
code = "";
rmsg = "";
}
void encrypt()
{
for(int i = 0; i < size; i++)
{
m = BigInteger.valueOf((int)msg.charAt(i)); //Copy each character of msg into m
c = m.modPow(e, n); // Calculate c = (m^e) mod n
code += (char)c.intValue(); // Add encrypted character into code
}
}
void decrypt()
{
for(int i = 0; i < size; i++)
{
c = BigInteger.valueOf((int)code.charAt(i)); //Copy each character of code into c
m = c.modPow(d, n); // Calculate m = (c^d) mod n
rmsg += (char) m.intValue(); // Add decrypted character into message
}
}
void show()
{
System.out.println("nThe Message Entered at Sender's end is "" + msg + """);
System.out.println("The Encrypted Message sent to the Receiver is "" + code + """);
System.out.println("The Decrypted Message at Receiver's end is "" + rmsg + """);
}
public static void main(String args[])throws IOException
{
RSA obj = new RSA();
obj.read();
obj.encrypt();
obj.decrypt();
obj.show();
}
}