/**
* Computes the norm of an element
* <code>λ</code>
* of
* <code><b>Z</b>[τ]</code>
* .
* @param mu The parameter
* <code>μ</code>
* of the elliptic curve.
* @param lambda The element
* <code>λ</code>
* of
* <code><b>Z</b>[τ]</code>
* .
* @return The norm of
* <code>λ</code>
* .
*/
public static BigInteger Norm(sbyte mu, ZTauElement lambda)
{
BigInteger norm;
// s1 = u^2
var s1 = lambda.u.Multiply(lambda.u);
// s2 = u * v
var s2 = lambda.u.Multiply(lambda.v);
// s3 = 2 * v^2
var s3 = lambda.v.Multiply(lambda.v).ShiftLeft(1);
if (mu == 1)
{
norm = s1.Add(s2).Add(s3);
}
else if (mu == -1)
{
norm = s1.Subtract(s2).Add(s3);
}
else
{
throw new ArgumentException("mu must be 1 or -1");
}
return(norm);
}
/**
* Multiplies a {@link NBitcoin.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>
* .
* @return
* <code>λ * p</code>
*/
public static AbstractF2mPoint MultiplyTnaf(AbstractF2mPoint p, ZTauElement lambda)
{
var curve = (AbstractF2mCurve)p.Curve;
var mu = GetMu(curve.A);
var u = TauAdicNaf(mu, lambda);
var q = MultiplyFromTnaf(p, u);
return(q);
}
/**
* Computes the
* <code>[τ]</code>
* -adic window NAF of an element
* <code>λ</code>
* of
* <code><b>Z</b>[τ]</code>
* .
* @param mu The parameter μ of the elliptic curve.
* @param lambda The element
* <code>λ</code>
* of
* <code><b>Z</b>[τ]</code>
* of which to compute the
* <code>[τ]</code>
* -adic NAF.
* @param width The window width of the resulting WNAF.
* @param pow2w 2
* <sup>width</sup>
* .
* @param tw The auxiliary value
* <code>t<sub>w</sub></code>
* .
* @param alpha The
* <code>α<sub>u</sub></code>
* 's for the window width.
* @return The
* <code>[τ]</code>
* -adic window NAF of
* <code>λ</code>
* .
*/
public static sbyte[] TauAdicWNaf(sbyte mu, ZTauElement lambda,
sbyte width, BigInteger pow2w, BigInteger tw, ZTauElement[] alpha)
{
if (!(mu == 1 || mu == -1))
{
throw new ArgumentException("mu must be 1 or -1");
}
var norm = Norm(mu, lambda);
// Ceiling of log2 of the norm
var log2Norm = norm.BitLength;
// If length(TNAF) > 30, then length(TNAF) < log2Norm + 3.52
var maxLength = log2Norm > 30 ? log2Norm + 4 + width : 34 + width;
// The array holding the TNAF
var u = new sbyte[maxLength];
// 2^(width - 1)
var pow2wMin1 = pow2w.ShiftRight(1);
// Split lambda into two BigIntegers to simplify calculations
var r0 = lambda.u;
var r1 = lambda.v;
var i = 0;
// while lambda <> (0, 0)
while (!(r0.Equals(BigInteger.Zero) && r1.Equals(BigInteger.Zero)))
{
// if r0 is odd
if (r0.TestBit(0))
{
// uUnMod = r0 + r1*tw Mod 2^width
var uUnMod
= r0.Add(r1.Multiply(tw)).Mod(pow2w);
sbyte uLocal;
// if uUnMod >= 2^(width - 1)
if (uUnMod.CompareTo(pow2wMin1) >= 0)
{
uLocal = (sbyte)uUnMod.Subtract(pow2w).IntValue;
}
else
{
uLocal = (sbyte)uUnMod.IntValue;
}
// uLocal is now in [-2^(width-1), 2^(width-1)-1]
u[i] = uLocal;
var s = true;
if (uLocal < 0)
{
s = false;
uLocal = (sbyte)-uLocal;
}
// uLocal is now >= 0
if (s)
{
r0 = r0.Subtract(alpha[uLocal].u);
r1 = r1.Subtract(alpha[uLocal].v);
}
else
{
r0 = r0.Add(alpha[uLocal].u);
r1 = r1.Add(alpha[uLocal].v);
}
}
else
{
u[i] = 0;
}
var t = r0;
if (mu == 1)
{
r0 = r1.Add(r0.ShiftRight(1));
}
else
{
// mu == -1
r0 = r1.Subtract(r0.ShiftRight(1));
}
r1 = t.ShiftRight(1).Negate();
i++;
}
return(u);
}
/**
* Computes the
* <code>τ</code>
* -adic NAF (non-adjacent form) of an
* element
* <code>λ</code>
* of
* <code><b>Z</b>[τ]</code>
* .
* @param mu The parameter
* <code>μ</code>
* of the elliptic curve.
* @param lambda The element
* <code>λ</code>
* of
* <code><b>Z</b>[τ]</code>
* .
* @return The
* <code>τ</code>
* -adic NAF of
* <code>λ</code>
* .
*/
public static sbyte[] TauAdicNaf(sbyte mu, ZTauElement lambda)
{
if (!(mu == 1 || mu == -1))
{
throw new ArgumentException("mu must be 1 or -1");
}
var norm = Norm(mu, lambda);
// Ceiling of log2 of the norm
var log2Norm = norm.BitLength;
// If length(TNAF) > 30, then length(TNAF) < log2Norm + 3.52
var maxLength = log2Norm > 30 ? log2Norm + 4 : 34;
// The array holding the TNAF
var u = new sbyte[maxLength];
var i = 0;
// The actual length of the TNAF
var length = 0;
var r0 = lambda.u;
var r1 = lambda.v;
while (!(r0.Equals(BigInteger.Zero) && r1.Equals(BigInteger.Zero)))
{
// If r0 is odd
if (r0.TestBit(0))
{
u[i] = (sbyte)BigInteger.Two.Subtract(r0.Subtract(r1.ShiftLeft(1)).Mod(Four)).IntValue;
// r0 = r0 - u[i]
if (u[i] == 1)
{
r0 = r0.ClearBit(0);
}
else
{
// u[i] == -1
r0 = r0.Add(BigInteger.One);
}
length = i;
}
else
{
u[i] = 0;
}
var t = r0;
var s = r0.ShiftRight(1);
if (mu == 1)
{
r0 = r1.Add(s);
}
else
{
// mu == -1
r0 = r1.Subtract(s);
}
r1 = t.ShiftRight(1).Negate();
i++;
}
length++;
// Reduce the TNAF array to its actual length
var tnaf = new sbyte[length];
Array.Copy(u, 0, tnaf, 0, length);
return(tnaf);
}
