WTauNafMultiplier.cs :  » PDF » iTextSharp » Org » BouncyCastle » Math » EC » Multiplier » C# / CSharp Open Source

using System;

using Org.BouncyCastle.Math.EC.Abc;

namespace Org.BouncyCastle.Math.EC.Multiplier{
  /**
  * Class implementing the WTNAF (Window
  * <code>&#964;</code>-adic Non-Adjacent Form) algorithm.
  */
  internal class WTauNafMultiplier
    : ECMultiplier
  {
    /**
    * Multiplies a {@link org.bouncycastle.math.ec.F2mPoint F2mPoint}
    * by <code>k</code> using the reduced <code>&#964;</code>-adic NAF (RTNAF)
    * method.
    * @param p The F2mPoint to multiply.
    * @param k The integer by which to multiply <code>k</code>.
    * @return <code>p</code> multiplied by <code>k</code>.
    */
    public ECPoint Multiply(ECPoint point, BigInteger k, PreCompInfo preCompInfo)
    {
      if (!(point is F2mPoint))
        throw new ArgumentException("Only F2mPoint can be used in WTauNafMultiplier");

      F2mPoint p = (F2mPoint)point;

      F2mCurve curve = (F2mCurve) p.Curve;
      int m = curve.M;
      sbyte a = (sbyte) curve.A.ToBigInteger().IntValue;
      sbyte mu = curve.GetMu();
      BigInteger[] s = curve.GetSi();

      ZTauElement rho = Tnaf.PartModReduction(k, m, a, s, mu, (sbyte)10);

      return MultiplyWTnaf(p, rho, preCompInfo, a, mu);
    }

    /**
    * Multiplies a {@link org.bouncycastle.math.ec.F2mPoint F2mPoint}
    * by an element <code>&#955;</code> of <code><b>Z</b>[&#964;]</code> using
    * the <code>&#964;</code>-adic NAF (TNAF) method.
    * @param p The F2mPoint to multiply.
    * @param lambda The element <code>&#955;</code> of
    * <code><b>Z</b>[&#964;]</code> of which to compute the
    * <code>[&#964;]</code>-adic NAF.
    * @return <code>p</code> multiplied by <code>&#955;</code>.
    */
    private F2mPoint MultiplyWTnaf(F2mPoint p, ZTauElement lambda,
      PreCompInfo preCompInfo, sbyte a, sbyte mu)
    {
      ZTauElement[] alpha;
      if (a == 0)
      {
        alpha = Tnaf.Alpha0;
      }
      else
      {
        // a == 1
        alpha = 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.F2mPoint F2mPoint}
    * by an element <code>&#955;</code> of <code><b>Z</b>[&#964;]</code>
    * using the window <code>&#964;</code>-adic NAF (TNAF) method, given the
    * WTNAF of <code>&#955;</code>.
    * @param p The F2mPoint to multiply.
    * @param u The the WTNAF of <code>&#955;</code>..
    * @return <code>&#955; * p</code>
    */
    private static F2mPoint MultiplyFromWTnaf(F2mPoint p, sbyte[] u,
      PreCompInfo preCompInfo)
    {
      F2mCurve curve = (F2mCurve)p.Curve;
      sbyte a = (sbyte) curve.A.ToBigInteger().IntValue;

      F2mPoint[] pu;
      if ((preCompInfo == null) || !(preCompInfo is WTauNafPreCompInfo))
      {
        pu = Tnaf.GetPreComp(p, a);
        p.SetPreCompInfo(new WTauNafPreCompInfo(pu));
      }
      else
      {
        pu = ((WTauNafPreCompInfo)preCompInfo).GetPreComp();
      }

      // q = infinity
      F2mPoint q = (F2mPoint) p.Curve.Infinity;
      for (int i = u.Length - 1; i >= 0; i--)
      {
        q = Tnaf.Tau(q);
        if (u[i] != 0)
        {
          if (u[i] > 0)
          {
            q = q.AddSimple(pu[u[i]]);
          }
          else
          {
            // u[i] < 0
            q = q.SubtractSimple(pu[-u[i]]);
          }
        }
      }

      return q;
    }
  }
}