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;
+		}
+	}
+}