Quick, simple implementation of RSA encryption algorithm without external libraries

rsa.java

import java.awt.EventQueue;
import java.io.*;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Random;
import java.util.Scanner;


/**
 * Quick and dirty implementation of the RSA algorithm
 * Read through main() for a breakdown on the RSA workings
 */
public class RSA {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// 1. Find large primes p and q
		BigInteger p = largePrime(512);
		BigInteger q = largePrime(512);

		// 2. Compute n from p and q
		// n is mod for private & public keys, n bit length is key length
		BigInteger n = n(p, q);

		// 3. Compute Phi(n) (Euler's totient function)
		// Phi(n) = (p-1)(q-1)
		// BigIntegers are objects and must use methods for algebraic operations
		BigInteger phi = getPhi(p, q);

		// 4. Find an int e such that 1 < e < Phi(n) 	and gcd(e,Phi) = 1
		BigInteger e = genE(phi);

		// 5. Calculate d where  d ≡ e^(-1) (mod Phi(n))
		BigInteger d = extEuclid(e, phi)[1];

		// Print generated values for reference
		System.out.println("p: " + p);
		System.out.println("q: " + q);
		System.out.println("n: " + n);
		System.out.println("Phi: " + phi);
		System.out.println("e: " + e);
		System.out.println("d: " + d);

		// encryption / decryption example
		String message = "Encryption test example";
		// Convert string to numbers using a cipher
		BigInteger cipherMessage = stringCipher(message);
		// Encrypt the ciphered message
		BigInteger encrypted = encrypt(cipherMessage, e, n);
		// Decrypt the encrypted message
		BigInteger decrypted = decrypt(encrypted, d, n);
		// Uncipher the decrypted message to text
		String restoredMessage = cipherToString(decrypted);

		System.out.println("Original message: " + message);
		System.out.println("Ciphered: " + cipherMessage);
		System.out.println("Encrypted: " + encrypted);
		System.out.println("Decrypted: " + decrypted);
		System.out.println("Restored: " + restoredMessage);
	}


	/**
	 * Takes a string and converts each character to an ASCII decimal value
	 * Returns BigInteger
	 */
	public static BigInteger stringCipher(String message) {
		message = message.toUpperCase();
		String cipherString = "";
		int i = 0;
		while (i < message.length()) {
			int ch = (int) message.charAt(i);
			cipherString = cipherString + ch;
			i++;
		}
		BigInteger cipherBig = new BigInteger(String.valueOf(cipherString));
		return cipherBig;
	}


	/**
	 * Takes a BigInteger that is ciphered and converts it back to plain text
	 *	returns a String
	 */
	public static String cipherToString(BigInteger message) {
		String cipherString = message.toString();
		String output = "";
		int i = 0;
		while (i < cipherString.length()) {
			int temp = Integer.parseInt(cipherString.substring(i, i + 2));
			char ch = (char) temp;
			output = output + ch;
			i = i + 2;
		}
		return output;
	}


	/** 3. Compute Phi(n) (Euler's totient function)
	 *  Phi(n) = (p-1)(q-1)
	 *	BigIntegers are objects and must use methods for algebraic operations
	 */
	public static BigInteger getPhi(BigInteger p, BigInteger q) {
		BigInteger phi = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
		return phi;
	}

	/**
	 * Generates a random large prime number of specified bitlength
	 *
	 */
	public static BigInteger largePrime(int bits) {
		Random randomInteger = new Random();
		BigInteger largePrime = BigInteger.probablePrime(bits, randomInteger);
		return largePrime;
	}


	/**
	 * Recursive implementation of Euclidian algorithm to find greatest common denominator
	 * Note: Uses BigInteger operations
	 */
	public static BigInteger gcd(BigInteger a, BigInteger b) {
		if (b.equals(BigInteger.ZERO)) {
			return a;
		} else {
			return gcd(b, a.mod(b));
		}
	}


	/** Recursive EXTENDED Euclidean algorithm, solves Bezout's identity (ax + by = gcd(a,b))
	 * and finds the multiplicative inverse which is the solution to ax ≡ 1 (mod m)
	 * returns [d, p, q] where d = gcd(a,b) and ap + bq = d
	 * Note: Uses BigInteger operations
	 */
	public static BigInteger[] extEuclid(BigInteger a, BigInteger b) {
		if (b.equals(BigInteger.ZERO)) return new BigInteger[] {
			a, BigInteger.ONE, BigInteger.ZERO
		}; // { a, 1, 0 }
		BigInteger[] vals = extEuclid(b, a.mod(b));
		BigInteger d = vals[0];
		BigInteger p = vals[2];
		BigInteger q = vals[1].subtract(a.divide(b).multiply(vals[2]));
		return new BigInteger[] {
			d, p, q
		};
	}


	/**
	 * generate e by finding a Phi such that they are coprimes (gcd = 1)
	 *
	 */
	public static BigInteger genE(BigInteger phi) {
		Random rand = new Random();
		BigInteger e = new BigInteger(1024, rand);
		do {
			e = new BigInteger(1024, rand);
			while (e.min(phi).equals(phi)) { // while phi is smaller than e, look for a new e
				e = new BigInteger(1024, rand);
			}
		} while (!gcd(e, phi).equals(BigInteger.ONE)); // if gcd(e,phi) isnt 1 then stay in loop
		return e;
	}

	public static BigInteger encrypt(BigInteger message, BigInteger e, BigInteger n) {
		return message.modPow(e, n);
	}

	public static BigInteger decrypt(BigInteger message, BigInteger d, BigInteger n) {
		return message.modPow(d, n);
	}

	public static BigInteger n(BigInteger p, BigInteger q) {
		return p.multiply(q);

	}
}

Why you add

message = message.toUpperCase();

remove that is not working. that make my string text to Upper case at all.

@tfkproject The reason for this is because the decrypting algorithm only decrypts characters that are represented by a 2 digit ASCII decimal value. When it reads in values that should actually be represented by a 3 digit ASCII value, it outputs gibberish. @jbrown17 has a workaround by just converting all text to uppercase which therefore means it will always be represented by a 2 digit ASCII value. With a bit of critical thinking you can definitely come up with a solution so that all keyboard characters of any case can be successfully decrypted. thanks for the code! @jbrown17

Hey, if you don't want to convert message to UpperCase you should use sth like this:
BigInteger cipherMessage = new BigInteger(message.getBytes());
String restoredMessage = new String(decrypted.toByteArray());