using System;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Math;
using Org.BouncyCastle.Security;
namespace Org.BouncyCastle.Crypto.Generators{
/**
* Generate a random factor suitable for use with RSA blind signatures
* as outlined in Chaum's blinding and unblinding as outlined in
* "Handbook of Applied Cryptography", page 475.
*/
public class RsaBlindingFactorGenerator
{
private RsaKeyParameters key;
private SecureRandom random;
/**
* Initialise the factor generator
*
* @param param the necessary RSA key parameters.
*/
public void Init(
ICipherParameters param)
{
if (param is ParametersWithRandom)
{
ParametersWithRandom rParam = (ParametersWithRandom)param;
key = (RsaKeyParameters)rParam.Parameters;
random = rParam.Random;
}
else
{
key = (RsaKeyParameters)param;
random = new SecureRandom();
}
if (key.IsPrivate)
throw new ArgumentException("generator requires RSA public key");
}
/**
* 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;
}
}
}
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