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package org.spongycastle.crypto.ec;
import java.math.BigInteger;
import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.params.ECPublicKeyParameters;
import org.spongycastle.math.ec.ECPoint;
/**
* this transforms the original randomness used for an ElGamal encryption by a fixed value.
*/
public class ECFixedTransform
implements ECPairFactorTransform
{
private ECPublicKeyParameters key;
private BigInteger k;
public ECFixedTransform(BigInteger k)
{
this.k = k;
}
/**
* initialise the underlying EC ElGamal engine.
*
* @param param the necessary EC key parameters.
*/
public void init(
CipherParameters param)
{
if (!(param instanceof ECPublicKeyParameters))
{
throw new IllegalArgumentException("ECPublicKeyParameters are required for fixed transform.");
}
this.key = (ECPublicKeyParameters)param;
}
/**
* 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("ECFixedTransform not initialised");
}
ECPoint g = key.getParameters().getG();
ECPoint gamma = g.multiply(k);
ECPoint phi = key.getQ().multiply(k).add(cipherText.getY());
return new ECPair(cipherText.getX().add(gamma).normalize(), phi.normalize());
}
/**
* Return the last transform value used by the transform
*
* @return a BigInteger representing k value.
*/
public BigInteger getTransformValue()
{
return k;
}
}
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