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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 and up. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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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);
	}
}