import java.math.*;
import java.util.*;
public class Encryption {

	public static void main(String[] args) {
		//numbers
		BigInteger one, firstPrime, secondPrime,  key, D, n, P, Q;
		
		//inputs
		Scanner s = new Scanner(System.in);
		Scanner t = new Scanner(System.in);
		
		//prompt to fill a numbers
		System.out.println("Enter a Prime number for A:");
		firstPrime = s.nextBigInteger();
		System.out.println("Enter a prime number for B");
		secondPrime = s.nextBigInteger();
		
		//creates values for n, firstPrime, and Q to use
		n = firstPrime.multiply(secondPrime);
		P = firstPrime.subtract(BigInteger.ONE);
		Q = secondPrime.subtract(BigInteger.ONE);
		
		int x = 0;
		do {
			System.out.println("Enter another prime number for the public key");
			key = s.nextBigInteger();
			
			//checks to make sure that the numbers you entered were prime
			if(((firstPrime.gcd(key)).equals(BigInteger.ONE))&& ((Q.gcd(key)).equals(BigInteger.ONE))){
				x++;}
			}while(x==0);
		
		//goes until if statement is true, when (i x key).mod((first prime-1) x (second prime - 1)) = 1, the assigns the string rep. of i
		for(int i = 1;;i++) {
			D = new BigInteger(String.valueOf(i));
			if(((D.multiply(key)).mod(P.multiply(Q))).equals(BigInteger.ONE)) 
			break;
		}
		
		String in = "", out = "", text;
		System.out.println("Enter plain text to be encrypted");
		text = t.nextLine();
		
		for(int i = 0; i < text.length();i++) {
      
         //these BIs will serve for our chars being encrypted and decrypted
			BigInteger encrypt, decrypt;
			BigInteger transformer = new BigInteger(String.valueOf((int)(text.charAt(i))));
			encrypt = transformer.modPow(key, n);
			out += (char)encrypt.intValue();
			decrypt = encrypt.modPow(D, n);
			in += (char)decrypt.intValue();
			System.out.println(" Original letter: " + (char)decrypt.intValue() + "\tEncrypted Letter: " + (char)encrypt.intValue());
		}
		System.out.println("Full text encrypted: " + out);
		System.out.println("Full text decrypted: " + in);
				
			
		}
		
	}