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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.ECPublicKeyParameters;
import org.spongycastle.crypto.params.ParametersWithRandom;
import org.spongycastle.math.ec.ECPoint;
/**
* this does your basic Elgamal encryption algorithm using EC
*/
public class ECNewPublicKeyTransform
implements ECPairTransform
{
private ECPublicKeyParameters key;
private SecureRandom random;
/**
* initialise the 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 public key transform.");
}
this.key = (ECPublicKeyParameters)p.getParameters();
this.random = p.getRandom();
}
else
{
if (!(param instanceof ECPublicKeyParameters))
{
throw new IllegalArgumentException("ECPublicKeyParameters are required for new public key transform.");
}
this.key = (ECPublicKeyParameters)param;
this.random = new SecureRandom();
}
}
/**
* Transform an existing cipher test pair using the ElGamal algorithm. Note: the input cipherText will
* need to be preserved in order to complete the transformation to the new public key.
*
* @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("ECNewPublicKeyTransform not initialised");
}
BigInteger n = key.getParameters().getN();
BigInteger k = ECUtil.generateK(n, random);
ECPoint g = key.getParameters().getG();
ECPoint gamma = g.multiply(k);
ECPoint phi = key.getQ().multiply(k).add(cipherText.getY());
return new ECPair(gamma.normalize(), phi.normalize());
}
}
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