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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 1.5 and up.
package org.bouncycastle.math.ec.endo;
import java.math.BigInteger;
import org.bouncycastle.math.ec.ECConstants;
import org.bouncycastle.math.ec.ECCurve;
import org.bouncycastle.math.ec.ECPointMap;
import org.bouncycastle.math.ec.ScaleXPointMap;
public class GLVTypeBEndomorphism implements GLVEndomorphism
{
protected final ECCurve curve;
protected final GLVTypeBParameters parameters;
protected final ECPointMap pointMap;
public GLVTypeBEndomorphism(ECCurve curve, GLVTypeBParameters parameters)
{
this.curve = curve;
this.parameters = parameters;
this.pointMap = new ScaleXPointMap(curve.fromBigInteger(parameters.getBeta()));
}
public BigInteger[] decomposeScalar(BigInteger k)
{
int bits = parameters.getBits();
BigInteger b1 = calculateB(k, parameters.getG1(), bits);
BigInteger b2 = calculateB(k, parameters.getG2(), bits);
BigInteger[] v1 = parameters.getV1(), v2 = parameters.getV2();
BigInteger a = k.subtract((b1.multiply(v1[0])).add(b2.multiply(v2[0])));
BigInteger b = (b1.multiply(v1[1])).add(b2.multiply(v2[1])).negate();
return new BigInteger[]{ a, b };
}
public ECPointMap getPointMap()
{
return pointMap;
}
public boolean hasEfficientPointMap()
{
return true;
}
protected BigInteger calculateB(BigInteger k, BigInteger g, int t)
{
boolean negative = (g.signum() < 0);
BigInteger b = k.multiply(g.abs());
boolean extra = b.testBit(t - 1);
b = b.shiftRight(t);
if (extra)
{
b = b.add(ECConstants.ONE);
}
return negative ? b.negate() : b;
}
}