RSACryptoSystem.java

//package ecc.rsa;

//import ecc.*;
import java.math.BigInteger;
import java.io.*;
import java.util.*;
import java.security.MessageDigest;

public class RSACryptoSystem implements CryptoSystem {
    MessageDigest hash;

    public RSACryptoSystem() {
	try {
	    hash = MessageDigest.getInstance("SHA-1");
	} catch (java.security.NoSuchAlgorithmException e) {
	    System.out.println("RSACryptoSystem: THIS CANNOT HAPPEN\n"+e);
	    System.exit(0);
	}
    }

    public int blockSize() {
	return 20;
    }

    public byte[] encrypt(byte[] input, int numbytes, Key key) {
	RSAKey k = (RSAKey) key;
	hash.reset();
	BigInteger hashelem = new BigInteger(k.n.bitLength() + 17, Rand.om).mod(k.n);
	byte[] cryptelm = hashelem.modPow(k.e, k.n).toByteArray();
	byte[] res = new byte[cryptelm.length+numbytes+2];
	res[0] = (byte)((cryptelm.length)>>8);
	res[1] = (byte)cryptelm.length;
	System.arraycopy(cryptelm,0,res,2,cryptelm.length);
	hash.update(hashelem.toByteArray());
	byte[] digest = hash.digest();
	for(int j = 0; j < numbytes; j++) {
	    res[cryptelm.length+2+j] = (byte)(input[j]^digest[j]);
	}
	return res;
    }

    public byte[] decrypt(byte[] input, Key key) {
	RSAKey k = (RSAKey) key;
	byte[] cryptelm = new byte[((input[0]&255)<<8)+input[1]&255];
	byte[] res = new byte[input.length-2-cryptelm.length];
	System.arraycopy(input, 2, cryptelm, 0, cryptelm.length);
	hash.reset();
	hash.update(new BigInteger(cryptelm).modPow(k.d, k.n).toByteArray());
	byte[] digest = hash.digest();
	for(int j = 0; j < res.length; j++) {
	    res[j] = (byte)(input[cryptelm.length+2+j]^digest[j]);
	}
	return res;
    }

    /** This method generates a new key for the cryptosystem.
     *@return the new key generated*/
    public Key generateKey() {
	return new RSAKey(1024);
    }

    public String toString(){
	return "RSA-Cryptosystem";
    }
}