publickey.java

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

package rsa;

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

/** class to represent a public key */
public class publicKey implements Serializable
{
	/** size of block of bytes to encrypt in a single go */
	final static int BLOCKSIZE = 8;
	
	public BigInteger N;
	public BigInteger E;
	
	
	/** constructor */
	public publicKey(BigInteger _N, BigInteger _E)
	{
		N = _N;
		E = _E;
	}


	/** encrypts an entire string */
	public ArrayList encrypt(String plaintext)
	{
		// declare array of BigIntegers that is the ciphertext
		ArrayList ciphertext = new ArrayList();
		
		// declare stringbuffer to hold the bits in each block of bytes being encrypted
		StringBuffer bits = new StringBuffer();
		
		// loop through each character
		for (int i = 1; i <= plaintext.length(); i++)
		{
			// convert the character to a string of bits
			String charAsBits = Integer.toBinaryString((int)plaintext.charAt(i-1));
			
			// pad to 16 bits (Unicode)
			while (charAsBits.length() < 16)
				charAsBits = "0" + charAsBits;
			
			// add to the main string of bits for this block
			bits.append(charAsBits);
			
			// if we have reached the blocksize, encrypt this block using RSA and reset the string of bits
			if (i % BLOCKSIZE == 0)
			{
				BigInteger binaryValueBigInt = new BigInteger(bits.toString(), 2);
				ciphertext.add(binaryValueBigInt.modPow(E, N));
				bits.setLength(0);
			}
		}
		
		// if there is a partially completed block of bits, encrypt these
		if (bits.length() > 0)
		{
			BigInteger binaryValueBigInt = new BigInteger(bits.toString(), 2);
			ciphertext.add(binaryValueBigInt.modPow(E, N));
		}
		
		return ciphertext;
	}
}