privatekey.java

Its about cryptography example. useful for chatting with some sort of security

package rsa;

import java.io.*;
import java.math.BigInteger;
import java.security.SecureRandom;
import java.util.*;

/** class to represent a private key */
public class privateKey
{
	/** probability when generating prime numbers for RSA key */
	public static int PRIME_PROBABILITY = 100;

	/** value of 1 */
	final static BigInteger ONE = BigInteger.valueOf(1);

	public BigInteger P;
	public BigInteger Q;
	public BigInteger PQ;
	public BigInteger E;
	public BigInteger N;
	public BigInteger D;



	/** constructor - pass in two prime numbers and a value for E */
	public privateKey(BigInteger _P, BigInteger _Q, BigInteger _E)
	{
		P = _P;
		Q = _Q;
		E = _E;

		N = P.multiply(Q);
		PQ = P.subtract(ONE).multiply(Q.subtract(ONE));
		D = E.modInverse(PQ);
	}


	/** constructor that generates two massive prime numbers and E automatically */
	public privateKey(int bitSize)
	{
		// create secure random number generator
		SecureRandom sr = new SecureRandom();

		// generate random prime numbers
		do
		{
			P = new BigInteger(bitSize / 2, PRIME_PROBABILITY, sr);
			do
			{
				Q = new BigInteger(bitSize / 2, PRIME_PROBABILITY, sr);
			} while (P.compareTo(Q) == 0);

			E = new BigInteger(bitSize / 4, PRIME_PROBABILITY, sr);
			PQ = P.subtract(ONE).multiply(Q.subtract(ONE));
			N = P.multiply(Q);
		}
		while (N.bitLength() != bitSize || E.gcd(PQ).compareTo(ONE) !=0); // ensure E is relatively prime to PQ

		// N = P.multiply(Q);
		//System.out.println("N bitlength=" + N.bitLength());
		D = E.modInverse(PQ);
	}


	/** decrypts an entire string */
	public String decrypt(ArrayList ciphertext)
	{
		// declare the returned plaintext
		StringBuffer plaintext = new StringBuffer();

		// loop through each BigInteger in the array
		for (int i = 1; i <= ciphertext.size(); i++)
		{
			// get the next BigInteger
			BigInteger c = (BigInteger)ciphertext.get(i-1);

			// decrypt it using RSA
			BigInteger plainTextNum = c.modPow(D, N);

			// get this number as a string of bits
			String bits = plainTextNum.toString(2);

			// pad to ensure the string contains a multiple of 16 bits (Unicode)
			while (bits.length() % 16 != 0)
				bits = "0" + bits;

			// convert each set of 16 bits to a character, add to the plaintext, then delete from the stringbuffer
			for (int j = 0; j <  bits.length(); j += 16)
				plaintext.append((char)Integer.parseInt(bits.substring(j, j+16), 2));
		}
		return plaintext.toString();
	}


	/** returns the public key for this private key */
	public publicKey getPublicKey()
	{
		return new publicKey(N, E);
	}
}