crypto/src/math/ec/multiplier/FixedPointUtilities.cs


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using System;

namespace Org.BouncyCastle.Math.EC.Multiplier
{
    public class FixedPointUtilities
    {
        public static readonly string PRECOMP_NAME = "bc_fixed_point";

        public static int GetCombSize(ECCurve c)
        {
            BigInteger order = c.Order;
            return order == null ? c.FieldSize + 1 : order.BitLength;
        }

        public static FixedPointPreCompInfo GetFixedPointPreCompInfo(PreCompInfo preCompInfo)
        {
            if ((preCompInfo != null) && (preCompInfo is FixedPointPreCompInfo))
            {
                return (FixedPointPreCompInfo)preCompInfo;
            }

            return new FixedPointPreCompInfo();
        }

        [Obsolete("Use 'Precompute(ECPoint)' instead, as minWidth parameter is now ignored")]
        public static FixedPointPreCompInfo Precompute(ECPoint p, int minWidth)
        {
            return Precompute(p);
        }

        public static FixedPointPreCompInfo Precompute(ECPoint p)
        {
            ECCurve c = p.Curve;
            int minWidth = GetCombSize(c) > 257 ? 6 : 5;

            int n = 1 << minWidth;
            FixedPointPreCompInfo info = GetFixedPointPreCompInfo(c.GetPreCompInfo(p, PRECOMP_NAME));
            ECPoint[] lookupTable = info.PreComp;

            if (lookupTable == null || lookupTable.Length < n)
            {
                int bits = GetCombSize(c);
                int d = (bits + minWidth - 1) / minWidth;

                ECPoint[] pow2Table = new ECPoint[minWidth + 1];
                pow2Table[0] = p;
                for (int i = 1; i < minWidth; ++i)
                {
                    pow2Table[i] = pow2Table[i - 1].TimesPow2(d);
                }

                // This will be the 'offset' value 
                pow2Table[minWidth] = pow2Table[0].Subtract(pow2Table[1]);

                c.NormalizeAll(pow2Table);
    
                lookupTable = new ECPoint[n];
                lookupTable[0] = pow2Table[0];

                for (int bit = minWidth - 1; bit >= 0; --bit)
                {
                    ECPoint pow2 = pow2Table[bit];

                    int step = 1 << bit;
                    for (int i = step; i < n; i += (step << 1))
                    {
                        lookupTable[i] = lookupTable[i - step].Add(pow2);
                    }
                }

                c.NormalizeAll(lookupTable);

                info.LookupTable = c.CreateCacheSafeLookupTable(lookupTable, 0, lookupTable.Length);
                info.Offset = pow2Table[minWidth];
                info.PreComp = lookupTable;
                info.Width = minWidth;

                c.SetPreCompInfo(p, PRECOMP_NAME, info);
            }

            return info;
        }
    }
}