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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 to JDK 1.8. Note: this package includes the NTRU encryption algorithms.

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package org.bouncycastle.math.ec;

import java.math.BigInteger;

public class MontgomeryLadderMultiplier extends AbstractECMultiplier
{
    /**
     * Montgomery ladder.
     */
    protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
    {
        ECPoint[] R = new ECPoint[]{ p.getCurve().getInfinity(), p };

        int n = k.bitLength();
        int i = n;
        while (--i >= 0)
        {
            int b = k.testBit(i) ? 1 : 0;
            int bp = 1 - b;
            R[bp] = R[bp].add(R[b]);
            R[b] = R[b].twice();
        }
        return R[0];
    }
}