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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.4.

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package org.bouncycastle.crypto.generators;

import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.AsymmetricCipherKeyPairGenerator;
import org.bouncycastle.crypto.KeyGenerationParameters;
import org.bouncycastle.crypto.params.CramerShoupKeyGenerationParameters;
import org.bouncycastle.crypto.params.CramerShoupParameters;
import org.bouncycastle.crypto.params.CramerShoupPrivateKeyParameters;
import org.bouncycastle.crypto.params.CramerShoupPublicKeyParameters;
import org.bouncycastle.util.BigIntegers;

/**
 * a Cramer Shoup key pair generator
 * 
 */
public class CramerShoupKeyPairGenerator implements AsymmetricCipherKeyPairGenerator {
	
	private static final BigInteger ONE = BigInteger.valueOf(1);

	private CramerShoupKeyGenerationParameters param;

	public void init(KeyGenerationParameters param) {
		this.param = (CramerShoupKeyGenerationParameters) param;
	}

	public AsymmetricCipherKeyPair generateKeyPair() {
		CramerShoupParameters csParams = param.getParameters();

		CramerShoupPrivateKeyParameters sk = generatePrivateKey(param.getRandom(), csParams);
		CramerShoupPublicKeyParameters pk = calculatePublicKey(csParams, sk);
		sk.setPk(pk);

		return new AsymmetricCipherKeyPair(pk, sk);
	}

	private BigInteger generateRandomElement(BigInteger p, SecureRandom random) {
		return BigIntegers.createRandomInRange(ONE, p.subtract(ONE), random);
	}
	
	private CramerShoupPrivateKeyParameters generatePrivateKey(SecureRandom random, CramerShoupParameters csParams){
		BigInteger p = csParams.getP();
		CramerShoupPrivateKeyParameters key = new CramerShoupPrivateKeyParameters(csParams,
				generateRandomElement(p, random), generateRandomElement(p, random),
				generateRandomElement(p, random), generateRandomElement(p, random),
				generateRandomElement(p, random));
		return key;
	}

	private CramerShoupPublicKeyParameters calculatePublicKey(CramerShoupParameters csParams, CramerShoupPrivateKeyParameters sk) {
		BigInteger g1 = csParams.getG1();
		BigInteger g2 = csParams.getG2();
		BigInteger p = csParams.getP();
		
		BigInteger c = g1.modPow(sk.getX1(), p).multiply(g2.modPow(sk.getX2(), p));
		BigInteger d = g1.modPow(sk.getY1(), p).multiply(g2.modPow(sk.getY2(), p));
		BigInteger h = g1.modPow(sk.getZ(), p);
		
		return new CramerShoupPublicKeyParameters(csParams, c, d, h);
	}
}