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package org.spongycastle.crypto.generators;
import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.params.ParametersWithRandom;
import org.spongycastle.crypto.params.RSAKeyParameters;
import org.spongycastle.crypto.params.RSAPrivateCrtKeyParameters;
import java.math.BigInteger;
import java.security.SecureRandom;
/**
* 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 static BigInteger ZERO = BigInteger.valueOf(0);
private static BigInteger ONE = BigInteger.valueOf(1);
private RSAKeyParameters key;
private SecureRandom random;
/**
* Initialise the factor generator
*
* @param param the necessary RSA key parameters.
*/
public void init(
CipherParameters param)
{
if (param instanceof ParametersWithRandom)
{
ParametersWithRandom rParam = (ParametersWithRandom)param;
key = (RSAKeyParameters)rParam.getParameters();
random = rParam.getRandom();
}
else
{
key = (RSAKeyParameters)param;
random = new SecureRandom();
}
if (key instanceof RSAPrivateCrtKeyParameters)
{
throw new IllegalArgumentException("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 IllegalStateException("generator not initialised");
}
BigInteger m = key.getModulus();
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.equals(ZERO) || factor.equals(ONE) || !gcd.equals(ONE));
return factor;
}
}
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