1 files changed, 69 insertions, 0 deletions
diff --git a/Crypto/src/crypto/generators/RSABlindingFactorGenerator.cs b/Crypto/src/crypto/generators/RSABlindingFactorGenerator.cs
new file mode 100644
index 000000000..e2f63face
--- /dev/null
+++ b/Crypto/src/crypto/generators/RSABlindingFactorGenerator.cs
@@ -0,0 +1,69 @@
+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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