/*
* Finds a pair of prime BigInteger's {p, q: p = 2q + 1}
*
* (see: Handbook of Applied Cryptography 4.86)
*/
internal static BigInteger[] GenerateSafePrimes(int size, int certainty, SecureRandom random)
{
BigInteger p, q;
int qLength = size - 1;
int minWeight = size >> 2;
if (size <= 32)
{
for (;;)
{
q = new BigInteger(qLength, 2, random);
p = q.ShiftLeft(1).Add(BigInteger.One);
if (!p.IsProbablePrime(certainty))
continue;
if (certainty > 2 && !q.IsProbablePrime(certainty - 2))
continue;
break;
}
}
else
{
// Note: Modified from Java version for speed
for (;;)
{
q = new BigInteger(qLength, 0, random);
retry:
for (int i = 0; i < primeLists.Length; ++i)
{
int test = q.Remainder(BigPrimeProducts[i]).IntValue;
if (i == 0)
{
int rem3 = test % 3;
if (rem3 != 2)
{
int diff = 2 * rem3 + 2;
q = q.Add(BigInteger.ValueOf(diff));
test = (test + diff) % primeProducts[i];
}
}
int[] primeList = primeLists[i];
for (int j = 0; j < primeList.Length; ++j)
{
int prime = primeList[j];
int qRem = test % prime;
if (qRem == 0 || qRem == (prime >> 1))
{
q = q.Add(Six);
goto retry;
}
}
}
if (q.BitLength != qLength)
continue;
if (!q.RabinMillerTest(2, random))
continue;
p = q.ShiftLeft(1).Add(BigInteger.One);
if (!p.RabinMillerTest(certainty, random))
continue;
if (certainty > 2 && !q.RabinMillerTest(certainty - 2, random))
continue;
/*
* Require a minimum weight of the NAF representation, since low-weight primes may be
* weak against a version of the number-field-sieve for the discrete-logarithm-problem.
*
* See "The number field sieve for integers of low weight", Oliver Schirokauer.
*/
if (WNafUtilities.GetNafWeight(p) < minWeight)
continue;
break;
}
}
return new BigInteger[] { p, q };
}
