Dotnet/Ed25519
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Optimized ExpMod to operate without recursion

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This commit is contained in:
Thorsten Sommer 2020-01-07 00:21:29 +01:00
parent 5424f5b75c
commit 3c64879ccf
1 changed files with 24 additions and 9 deletions
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33
Ed25519/Extensions.cs
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@ -54,20 +54,35 @@ namespace Ed25519
return x; return x;
} }
internal static BigInteger ExpMod(this BigInteger number, BigInteger exponent, BigInteger modulo) public static BigInteger ExpMod(this BigInteger number, BigInteger exponent, BigInteger modulo)
{ {
if (exponent.Equals(BigInteger.Zero)) var numberOperations = (int)Math.Ceiling(BigInteger.Log(exponent, 2)) + 1;
var series = new bool[numberOperations];
var previousNumber = exponent;
for (var n = 0; n < numberOperations; n++)
{ {
return BigInteger.One; if (n == 0)
{
series[n] = !exponent.IsEven;
continue;
}
var next = BigInteger.Divide(previousNumber, Constants.TWO);
series[n] = !next.IsEven;
previousNumber = next;
} }
var result = BigInteger.Pow(number.ExpMod(exponent / Constants.TWO, modulo), 2).Mod(modulo); var result = BigInteger.One;
for (var n = numberOperations - 2; n >= 0; n--)
{
result = BigInteger.Pow(result, 2).Mod(modulo);
if (series[n])
{
result *= number;
result = result.Mod(modulo);
}
}
if (exponent.IsEven)
return result;
result *= number;
result = result.Mod(modulo);
return result; return result;
} }