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using System;
using Org.BouncyCastle.Math.EC.Abc;
namespace Org.BouncyCastle.Math.EC.Multiplier
{
/**
* Class implementing the WTNAF (Window
* <code>τ</code>-adic Non-Adjacent Form) algorithm.
*/
public class WTauNafMultiplier
: AbstractECMultiplier
{
// TODO Create WTauNafUtilities class and move various functionality into it
internal static readonly string PRECOMP_NAME = "bc_wtnaf";
/**
* Multiplies a {@link org.bouncycastle.math.ec.AbstractF2mPoint AbstractF2mPoint}
* by <code>k</code> using the reduced <code>τ</code>-adic NAF (RTNAF)
* method.
* @param p The AbstractF2mPoint to multiply.
* @param k The integer by which to multiply <code>k</code>.
* @return <code>p</code> multiplied by <code>k</code>.
*/
protected override ECPoint MultiplyPositive(ECPoint point, BigInteger k)
{
if (!(point is AbstractF2mPoint))
throw new ArgumentException("Only AbstractF2mPoint can be used in WTauNafMultiplier");
AbstractF2mPoint p = (AbstractF2mPoint)point;
AbstractF2mCurve curve = (AbstractF2mCurve)p.Curve;
int m = curve.FieldSize;
sbyte a = (sbyte)curve.A.ToBigInteger().IntValue;
sbyte mu = Tnaf.GetMu(a);
BigInteger[] s = curve.GetSi();
ZTauElement rho = Tnaf.PartModReduction(k, m, a, s, mu, (sbyte)10);
return MultiplyWTnaf(p, rho, curve.GetPreCompInfo(p, PRECOMP_NAME), a, mu);
}
/**
* Multiplies a {@link org.bouncycastle.math.ec.AbstractF2mPoint AbstractF2mPoint}
* by an element <code>λ</code> of <code><b>Z</b>[τ]</code> using
* the <code>τ</code>-adic NAF (TNAF) method.
* @param p The AbstractF2mPoint to multiply.
* @param lambda The element <code>λ</code> of
* <code><b>Z</b>[τ]</code> of which to compute the
* <code>[τ]</code>-adic NAF.
* @return <code>p</code> multiplied by <code>λ</code>.
*/
private AbstractF2mPoint MultiplyWTnaf(AbstractF2mPoint p, ZTauElement lambda,
PreCompInfo preCompInfo, sbyte a, sbyte mu)
{
ZTauElement[] alpha = (a == 0) ? Tnaf.Alpha0 : Tnaf.Alpha1;
BigInteger tw = Tnaf.GetTw(mu, Tnaf.Width);
sbyte[]u = Tnaf.TauAdicWNaf(mu, lambda, Tnaf.Width,
BigInteger.ValueOf(Tnaf.Pow2Width), tw, alpha);
return MultiplyFromWTnaf(p, u, preCompInfo);
}
/**
* Multiplies a {@link org.bouncycastle.math.ec.AbstractF2mPoint AbstractF2mPoint}
* by an element <code>λ</code> of <code><b>Z</b>[τ]</code>
* using the window <code>τ</code>-adic NAF (TNAF) method, given the
* WTNAF of <code>λ</code>.
* @param p The AbstractF2mPoint to multiply.
* @param u The the WTNAF of <code>λ</code>..
* @return <code>λ * p</code>
*/
private static AbstractF2mPoint MultiplyFromWTnaf(AbstractF2mPoint p, sbyte[] u, PreCompInfo preCompInfo)
{
AbstractF2mCurve curve = (AbstractF2mCurve)p.Curve;
sbyte a = (sbyte)curve.A.ToBigInteger().IntValue;
AbstractF2mPoint[] pu;
if ((preCompInfo == null) || !(preCompInfo is WTauNafPreCompInfo))
{
pu = Tnaf.GetPreComp(p, a);
WTauNafPreCompInfo pre = new WTauNafPreCompInfo();
pre.PreComp = pu;
curve.SetPreCompInfo(p, PRECOMP_NAME, pre);
}
else
{
pu = ((WTauNafPreCompInfo)preCompInfo).PreComp;
}
// TODO Include negations in precomp (optionally) and use from here
AbstractF2mPoint[] puNeg = new AbstractF2mPoint[pu.Length];
for (int i = 0; i < pu.Length; ++i)
{
puNeg[i] = (AbstractF2mPoint)pu[i].Negate();
}
// q = infinity
AbstractF2mPoint q = (AbstractF2mPoint) p.Curve.Infinity;
int tauCount = 0;
for (int i = u.Length - 1; i >= 0; i--)
{
++tauCount;
int ui = u[i];
if (ui != 0)
{
q = q.TauPow(tauCount);
tauCount = 0;
ECPoint x = ui > 0 ? pu[ui >> 1] : puNeg[(-ui) >> 1];
q = (AbstractF2mPoint)q.Add(x);
}
}
if (tauCount > 0)
{
q = q.TauPow(tauCount);
}
return q;
}
}
}
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