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package org.spongycastle.crypto.ec;
import java.math.BigInteger;
import java.security.SecureRandom;
import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.params.ECDomainParameters;
import org.spongycastle.crypto.params.ECPublicKeyParameters;
import org.spongycastle.crypto.params.ParametersWithRandom;
import org.spongycastle.math.ec.ECMultiplier;
import org.spongycastle.math.ec.ECPoint;
import org.spongycastle.math.ec.FixedPointCombMultiplier;
/**
* this transforms the original randomness used for an ElGamal encryption.
*/
public class ECNewRandomnessTransform
implements ECPairFactorTransform
{
private ECPublicKeyParameters key;
private SecureRandom random;
private BigInteger lastK;
/**
* initialise the underlying EC ElGamal engine.
*
* @param param the necessary EC key parameters.
*/
public void init(
CipherParameters param)
{
if (param instanceof ParametersWithRandom)
{
ParametersWithRandom p = (ParametersWithRandom)param;
if (!(p.getParameters() instanceof ECPublicKeyParameters))
{
throw new IllegalArgumentException("ECPublicKeyParameters are required for new randomness transform.");
}
this.key = (ECPublicKeyParameters)p.getParameters();
this.random = p.getRandom();
}
else
{
if (!(param instanceof ECPublicKeyParameters))
{
throw new IllegalArgumentException("ECPublicKeyParameters are required for new randomness transform.");
}
this.key = (ECPublicKeyParameters)param;
this.random = new SecureRandom();
}
}
/**
* Transform an existing cipher test pair using the ElGamal algorithm. Note: it is assumed this
* transform has been initialised with the same public key that was used to create the original
* cipher text.
*
* @param cipherText the EC point to process.
* @return returns a new ECPair representing the result of the process.
*/
public ECPair transform(ECPair cipherText)
{
if (key == null)
{
throw new IllegalStateException("ECNewRandomnessTransform not initialised");
}
ECDomainParameters ec = key.getParameters();
BigInteger n = ec.getN();
ECMultiplier basePointMultiplier = createBasePointMultiplier();
BigInteger k = ECUtil.generateK(n, random);
ECPoint[] gamma_phi = new ECPoint[]{
basePointMultiplier.multiply(ec.getG(), k).add(cipherText.getX()),
key.getQ().multiply(k).add(cipherText.getY())
};
ec.getCurve().normalizeAll(gamma_phi);
lastK = k;
return new ECPair(gamma_phi[0], gamma_phi[1]);
}
/**
* Return the last random value generated for a transform
*
* @return a BigInteger representing the last random value.
*/
public BigInteger getTransformValue()
{
return lastK;
}
protected ECMultiplier createBasePointMultiplier()
{
return new FixedPointCombMultiplier();
}
}
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