/**
* Generate a suitable blind factor for the public key the generator was initialised with.
*
* @return a random blind factor
*/
public BigInteger GenerateBlindingFactor()
{
if (key == null)
throw new InvalidOperationException("generator not initialised");
BigInteger m = key.Modulus;
int length = m.BitLength - 1; // must be less than m.BitLength
BigInteger factor;
BigInteger gcd;
do
{
factor = new BigInteger(length, random);
gcd = factor.Gcd(m);
}
while (factor.SignValue == 0 || factor.Equals(BigInteger.One) || !gcd.Equals(BigInteger.One));
return factor;
}
/// <summary>Choose a random prime value for use with RSA</summary>
/// <param name="bitlength">the bit-length of the returned prime</param>
/// <param name="e">the RSA public exponent</param>
/// <returns>a prime p, with (p-1) relatively prime to e</returns>
protected virtual BigInteger ChooseRandomPrime(int bitlength, BigInteger e)
{
for (;;)
{
BigInteger p = new BigInteger(bitlength, 1, param.Random);
if (p.Mod(e).Equals(BigInteger.One))
continue;
if (!p.IsProbablePrime(param.Certainty))
continue;
if (!e.Gcd(p.Subtract(BigInteger.One)).Equals(BigInteger.One))
continue;
return p;
}
}