private static BigInteger ModPowMonty(BigInteger b, BigInteger e, BigInteger m, bool convert)
{
int n = m.magnitude.Length;
int powR = 32 * n;
bool smallMontyModulus = m.BitLength + 2 <= powR;
uint mDash = (uint)m.GetMQuote();
// tmp = this * R mod m
if (convert)
{
b = b.ShiftLeft(powR).Remainder(m);
}
int[] yAccum = new int[n + 1];
int[] zVal = b.magnitude;
Debug.Assert(zVal.Length <= n);
if (zVal.Length < n)
{
int[] tmp = new int[n];
zVal.CopyTo(tmp, n - zVal.Length);
zVal = tmp;
}
// Sliding window from MSW to LSW
int extraBits = 0;
// Filter the common case of small RSA exponents with few bits set
if (e.magnitude.Length > 1 || e.BitCount > 2)
{
int expLength = e.BitLength;
while (expLength > ExpWindowThresholds[extraBits])
{
++extraBits;
}
}
int numPowers = 1 << extraBits;
int[][] oddPowers = new int[numPowers][];
oddPowers[0] = zVal;
int[] zSquared = Arrays.Clone(zVal);
SquareMonty(yAccum, zSquared, m.magnitude, mDash, smallMontyModulus);
for (int i = 1; i < numPowers; ++i)
{
oddPowers[i] = Arrays.Clone(oddPowers[i - 1]);
MultiplyMonty(yAccum, oddPowers[i], zSquared, m.magnitude, mDash, smallMontyModulus);
}
int[] windowList = GetWindowList(e.magnitude, extraBits);
Debug.Assert(windowList.Length > 1);
int window = windowList[0];
int mult = window & 0xFF, lastZeroes = window >> 8;
int[] yVal;
if (mult == 1)
{
yVal = zSquared;
--lastZeroes;
}
else
{
yVal = Arrays.Clone(oddPowers[mult >> 1]);
}
int windowPos = 1;
while ((window = windowList[windowPos++]) != -1)
{
mult = window & 0xFF;
int bits = lastZeroes + BitLengthTable[mult];
for (int j = 0; j < bits; ++j)
{
SquareMonty(yAccum, yVal, m.magnitude, mDash, smallMontyModulus);
}
MultiplyMonty(yAccum, yVal, oddPowers[mult >> 1], m.magnitude, mDash, smallMontyModulus);
lastZeroes = window >> 8;
}
for (int i = 0; i < lastZeroes; ++i)
{
SquareMonty(yAccum, yVal, m.magnitude, mDash, smallMontyModulus);
}
if (convert)
{
// Return y * R^(-1) mod m
MontgomeryReduce(yVal, m.magnitude, mDash);
}
else if (smallMontyModulus && CompareTo(0, yVal, 0, m.magnitude) >= 0)
{
Subtract(0, yVal, 0, m.magnitude);
}
return new BigInteger(1, yVal, true);
}