103 lines
3.4 KiB
C#
103 lines
3.4 KiB
C#
#if !BESTHTTP_DISABLE_ALTERNATE_SSL && (!UNITY_WEBGL || UNITY_EDITOR)
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using System;
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namespace Org.BouncyCastle.Math.EC.Multiplier
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{
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/**
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* Class implementing the WNAF (Window Non-Adjacent Form) multiplication
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* algorithm.
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*/
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public class WNafL2RMultiplier
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: AbstractECMultiplier
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{
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/**
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* Multiplies <code>this</code> by an integer <code>k</code> using the
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* Window NAF method.
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* @param k The integer by which <code>this</code> is multiplied.
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* @return A new <code>ECPoint</code> which equals <code>this</code>
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* multiplied by <code>k</code>.
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*/
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protected override ECPoint MultiplyPositive(ECPoint p, BigInteger k)
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{
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// Clamp the window width in the range [2, 16]
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int width = System.Math.Max(2, System.Math.Min(16, GetWindowSize(k.BitLength)));
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WNafPreCompInfo wnafPreCompInfo = WNafUtilities.Precompute(p, width, true);
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ECPoint[] preComp = wnafPreCompInfo.PreComp;
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ECPoint[] preCompNeg = wnafPreCompInfo.PreCompNeg;
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int[] wnaf = WNafUtilities.GenerateCompactWindowNaf(width, k);
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ECPoint R = p.Curve.Infinity;
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int i = wnaf.Length;
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/*
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* NOTE: We try to optimize the first window using the precomputed points to substitute an
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* addition for 2 or more doublings.
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*/
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if (i > 1)
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{
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int wi = wnaf[--i];
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int digit = wi >> 16, zeroes = wi & 0xFFFF;
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int n = System.Math.Abs(digit);
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ECPoint[] table = digit < 0 ? preCompNeg : preComp;
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// Optimization can only be used for values in the lower half of the table
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if ((n << 2) < (1 << width))
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{
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int highest = LongArray.BitLengths[n];
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// TODO Get addition/doubling cost ratio from curve and compare to 'scale' to see if worth substituting?
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int scale = width - highest;
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int lowBits = n ^ (1 << (highest - 1));
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int i1 = ((1 << (width - 1)) - 1);
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int i2 = (lowBits << scale) + 1;
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R = table[i1 >> 1].Add(table[i2 >> 1]);
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zeroes -= scale;
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//Console.WriteLine("Optimized: 2^" + scale + " * " + n + " = " + i1 + " + " + i2);
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}
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else
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{
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R = table[n >> 1];
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}
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R = R.TimesPow2(zeroes);
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}
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while (i > 0)
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{
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int wi = wnaf[--i];
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int digit = wi >> 16, zeroes = wi & 0xFFFF;
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int n = System.Math.Abs(digit);
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ECPoint[] table = digit < 0 ? preCompNeg : preComp;
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ECPoint r = table[n >> 1];
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R = R.TwicePlus(r);
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R = R.TimesPow2(zeroes);
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}
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return R;
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}
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/**
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* Determine window width to use for a scalar multiplication of the given size.
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*
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* @param bits the bit-length of the scalar to multiply by
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* @return the window size to use
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*/
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protected virtual int GetWindowSize(int bits)
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{
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return WNafUtilities.GetWindowSize(bits);
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}
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}
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}
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#endif
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