| package org.bouncycastle.math.ec; |
| |
| import java.math.BigInteger; |
| |
| public class FixedPointUtil |
| { |
| public static final String PRECOMP_NAME = "bc_fixed_point"; |
| |
| public static int getCombSize(ECCurve c) |
| { |
| BigInteger order = c.getOrder(); |
| return order == null ? c.getFieldSize() + 1 : order.bitLength(); |
| } |
| |
| public static FixedPointPreCompInfo getFixedPointPreCompInfo(PreCompInfo preCompInfo) |
| { |
| if ((preCompInfo != null) && (preCompInfo instanceof FixedPointPreCompInfo)) |
| { |
| return (FixedPointPreCompInfo)preCompInfo; |
| } |
| |
| return new FixedPointPreCompInfo(); |
| } |
| |
| public static FixedPointPreCompInfo precompute(ECPoint p, int minWidth) |
| { |
| ECCurve c = p.getCurve(); |
| |
| int n = 1 << minWidth; |
| FixedPointPreCompInfo info = getFixedPointPreCompInfo(c.getPreCompInfo(p, PRECOMP_NAME)); |
| ECPoint[] lookupTable = info.getPreComp(); |
| |
| if (lookupTable == null || lookupTable.length < n) |
| { |
| int bits = getCombSize(c); |
| int d = (bits + minWidth - 1) / minWidth; |
| |
| ECPoint[] pow2Table = new ECPoint[minWidth]; |
| pow2Table[0] = p; |
| for (int i = 1; i < minWidth; ++i) |
| { |
| pow2Table[i] = pow2Table[i - 1].timesPow2(d); |
| } |
| |
| c.normalizeAll(pow2Table); |
| |
| lookupTable = new ECPoint[n]; |
| lookupTable[0] = c.getInfinity(); |
| |
| 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.setPreComp(lookupTable); |
| info.setWidth(minWidth); |
| |
| c.setPreCompInfo(p, PRECOMP_NAME, info); |
| } |
| |
| return info; |
| } |
| } |