package rsa;

import java.io.*;
import java.lang.*;
import java.util.*;
import java.security.*;
import java.security.interfaces.*;
import java.math.*;

/**
 *
 * @author John D'Ambrosio
 */
public class Main {

    // this is a very primitive encoding scheme... not really for efficiency
    public static BigInteger encode(String inputString)
    {
        BigInteger addend, result = BigInteger.valueOf(0), multiplier = BigInteger.valueOf(256);
        
        System.out.print("Encoding...");
        for(int n=inputString.length()-1; n>=0; n--)
        {
            addend = BigInteger.valueOf(inputString.charAt(n));
            result = result.add(addend);
            //System.out.println("\nCharacter " + n + ": " + inputString.charAt(n));
            //System.out.println("Sum: " + result);
            if(n>0) result = result.multiply(multiplier);
            //System.out.println("Product: " + result);
        }
        
        System.out.println("done!");
        return result;
    }
    
    public static String decode(BigInteger inputBigInt)
    {
        String result = new String();
        BigInteger quotientAndRemainder[], divisor = BigInteger.valueOf(256);
 
        System.out.print("Decoding...");
        while(inputBigInt.compareTo(BigInteger.ZERO) != 0)
        {
            quotientAndRemainder = inputBigInt.divideAndRemainder(divisor);
            inputBigInt = quotientAndRemainder[0];
            result = result.concat(String.valueOf((char)(quotientAndRemainder[1].byteValue())));
            //System.out.println("\nQuotient: " + quotientAndRemainder[0]);
            //System.out.println("Remainder: " + quotientAndRemainder[1]);
            //System.out.println("Result: " + result);
        }
        
        System.out.println("done!");
        return result;
    }    
    
    /** Creates a new instance of Main */
    public Main() {
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        try
        {
            Random rand = new Random();
            BigInteger d, e, p, q, m, n, in, msg, out;
            
            p = BigInteger.probablePrime(24, rand);
            q = BigInteger.probablePrime(24, rand);
            m = p.multiply(q);
            n = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
            e = BigInteger.probablePrime(16, rand);
            d = e.modInverse(n);
            in = encode("RSA");
            msg = in.modPow(e,m);
            out = msg.modPow(d,m);
            
            System.out.println("\nInput: RSA");
            System.out.println("\nChoose two random primes of the same magnitude:" + 
                               "\np = " + p.toString(16) + "\nq = " + q.toString(16)); 
            System.out.println("\nNow determine the:" + 
                               "\nModulus of reduction, p * q, m = " + m.toString(16) +
                               "\nEuler-Phi of m, (p-1)*(q-1), n = " + n.toString(16) +
                               "\nGCD(m,n) = " + (m.gcd(n)).toString(16));            
            System.out.println("\nChoose an encryption exponent, e = " + e.toString(16) +
                               "\nGCD(e,n) = " + (e.gcd(n)).toString(16));
            System.out.println("\nNow, using the Extended Euclidean Algorithm to find inverse(e)," +
                               "\nCalculate the decryption exponent, d = " + d.toString(16) +
                               "\ne * d = " + ((e.multiply(d)).mod(n)).toString(16) + " mod(n)");            
            System.out.println("\nMake the following public:" +
                               "\nm = " + m.toString(16) + ", e = " + e.toString(16));
            System.out.println("\nForm the encrypted message, in^e mod(m) = " +
                               "\n" + in + "^" + e + " mod(" + m.toString(16) + ") = msg = " + msg.toString(16));            
            System.out.println("\nFinally, decrypt the message, msg^d mod(m) =" +
                               "\n" + msg.toString(16) + "^" + d.toString(16) + " mod(" + m.toString(16) + ") = out = " + out.toString(16));
            System.out.println("\nOutput: " + decode(out));
        }
        catch (Exception e)
        {
            System.err.println("Caught exception " + e.toString());
        }
    }
}