public void MmodInverseBigInteger()
{
BigInteger a = zero, mod, inv;
for (int j = 3; j < 50; j++)
{
mod = BigInteger.FromInt64(j);
for (int i = -j + 1; i < j; i++)
{
try {
a = BigInteger.FromInt64(i);
inv = BigMath.ModInverse(a, mod);
Assert.True(one.Equals(((a * inv) % mod)), "bad inverse: " + a + " inv mod " + mod + " equals " + inv);
Assert.True(inv.CompareTo(mod) < 0, "inverse greater than modulo: " + a + " inv mod " + mod + " equals " + inv);
Assert.True(inv.CompareTo(BigInteger.Zero) >= 0, "inverse less than zero: " + a + " inv mod " + mod + " equals " + inv);
} catch (ArithmeticException) {
Assert.True(!one.Equals(BigMath.Gcd(a, mod)), "should have found inverse for " + a + " mod " + mod);
}
}
}
for (int j = 1; j < 10; j++)
{
mod = bi2 + BigInteger.FromInt64(j);
for (int i = 0; i < 20; i++)
{
try {
a = bi3 + (BigInteger.FromInt64(i));
inv = BigMath.ModInverse(a, mod);
Assert.True(one.Equals((a * inv) % mod), "bad inverse: " + a + " inv mod " + mod + " equals " + inv);
Assert.True(inv.CompareTo(mod) < 0, "inverse greater than modulo: " + a + " inv mod " + mod + " equals " + inv);
Assert.True(inv.CompareTo(BigInteger.Zero) >= 0, "inverse less than zero: " + a + " inv mod " + mod + " equals " + inv);
} catch (ArithmeticException) {
Assert.True(!one.Equals(BigMath.Gcd(a, mod)), "should have found inverse for " + a + " mod " + mod);
}
}
}
}
public static BigInteger ModPow(BigInteger value, BigInteger exponent, BigInteger m)
{
if (m.Sign <= 0)
{
// math.18=BigInteger: modulus not positive
throw new ArithmeticException(Messages.math18); //$NON-NLS-1$
}
BigInteger b = value;
if (m.IsOne | (exponent.Sign > 0 & b.Sign == 0))
{
return(BigInteger.Zero);
}
if (b.Sign == 0 && exponent.Sign == 0)
{
return(BigInteger.One);
}
if (exponent.Sign < 0)
{
b = BigMath.ModInverse(value, m);
exponent = -exponent;
}
// From now on: (m > 0) and (exponent >= 0)
BigInteger res = (BigInteger.TestBit(m, 0))
? Division.OddModPow(Abs(b),
exponent, m)
: Division.EvenModPow(Abs(b), exponent, m);
if ((b.Sign < 0) && BigInteger.TestBit(exponent, 0))
{
// -b^e mod m == ((-1 mod m) * (b^e mod m)) mod m
res = ((m - BigInteger.One) * res) % m;
}
// else exponent is even, so base^exp is positive
return(res);
}