public static BigInteger GetPrime(int bits, Ibasa.Numerics.Random.Generator random)
{
if (bits < 2)
throw new ArgumentException("bits is less than two.", "bits");
BigInteger pi = 2;
if (bits > 4)
pi = pi * 3;
if (bits > 6)
pi = pi * 5 * 7;
if (bits > 8)
pi = pi * 11 * 13;
if (bits > 10)
pi = pi * 17 * 19 * 23 * 29;
byte[] bytes = new byte[((bits + 7) / 8) + 1];
var bytemask = (byte)(uint.MaxValue >> (32 - (bits % 8)));
var intmask = BigInteger.Pow(2, bits) - 1;
BigInteger prime;
do
{
do
{
random.NextBytes(bytes);
bytes[bytes.Length - 1] = 0;
bytes[bytes.Length - 2] &= bytemask;
prime = new BigInteger(bytes);
} while (prime.IsEven);
} while (BigInteger.GreatestCommonDivisor(prime, pi) != 1);
while (!prime.IsProbablyPrime(100, random))
{
prime = (prime + pi) & intmask;
}
return prime;
}
