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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.*;
import org.bouncycastle.crypto.constraints.ConstraintUtils;
import org.bouncycastle.crypto.constraints.DefaultServiceProperties;
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;

        CryptoServicesRegistrar.checkConstraints(new DefaultServiceProperties("CramerShoupKeyGen", ConstraintUtils.bitsOfSecurityFor(this.param.getParameters().getP()), this.param.getParameters(), CryptoServicePurpose.KEYGEN));
    }

    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);
    }
}