ECCryptoSystem.java

//package ecc.elliptic;

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

public class ECCryptoSystem implements CryptoSystem {
	MessageDigest hash;

	private EllipticCurve ec;    

	public ECCryptoSystem(EllipticCurve ec) {
		this.ec = ec;
		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) {
		ECKey ek = (ECKey) key;							//ek is an Elliptic key (sk=secret, beta=public)
		byte[] res=new byte[ek.mother.getPCS()+numbytes];			// PCS is compressed point size.
		hash.reset();

		BigInteger rk = new BigInteger(ek.mother.getp().bitLength() + 17, Rand.om);	//Get a random integer rk of ek.mother.getp()...+17 bits
		if (ek.mother.getOrder() != null) 
		{
			rk = rk.mod(ek.mother.getOrder());					//rk=rk%order
		}
		ECPoint gamma = ek.mother.getGenerator().multiply(rk);			//ECPoint gamma=generator(pre-defined) *rk
		ECPoint sec = ek.beta.multiply(rk);					//ECPoint sec =beta(public key) *rk
		System.arraycopy(gamma.compress(),0,res,0,ek.mother.getPCS());		//arraycopy(src,srcpos,dest,destpos,length)
		hash.update(sec.getx().toByteArray());					//Update hash accordingly, will be xored with its digest, bitwise
		hash.update(sec.gety().toByteArray());
		byte[] digest = hash.digest();
		for(int j = 0; j < numbytes; j++) {
			res[j+ek.mother.getPCS()]=(byte) (input[j]^digest[j]);
		}
		return res;
	}

	public byte[] decrypt(byte[] input, Key key) {
		ECKey dk = (ECKey) key;
		byte[] res=new byte[input.length-dk.mother.getPCS()];
		byte[] gammacom=new byte[dk.mother.getPCS()];				//gammacom is gamma in compressed fmt.
		hash.reset();

		System.arraycopy(input,0,gammacom,0,dk.mother.getPCS());		//copy the first (gammacom.size()) bytes from input, to decompress.
		ECPoint gamma = new ECPoint(gammacom,dk.mother);			//gamma is gammacom, when decompressed.
		ECPoint sec = gamma.multiply(dk.sk);					//sec is when gamma is multiplied with sk.(secret key of the supplied key)
		if(sec.isZero()) {
			hash.update(BigInteger.ZERO.toByteArray());
			hash.update(BigInteger.ZERO.toByteArray());
		} else {
			hash.update(sec.getx().toByteArray());
			hash.update(sec.gety().toByteArray());
		}
		byte[] digest = hash.digest();
		for(int j = 0; j < input.length-dk.mother.getPCS(); j++) {
			res[j]=(byte) (input[j+dk.mother.getPCS()]^digest[j]);
		}
		return res;
	}



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

	public String toString(){
		return "ECC - " + ec.toString();
	}

}