1 files changed, 77 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/generators/RSABlindingFactorGenerator.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/generators/RSABlindingFactorGenerator.java
new file mode 100644
index 000000000..50b7db5e3
--- /dev/null
+++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/generators/RSABlindingFactorGenerator.java
@@ -0,0 +1,77 @@
+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;
+    }
+}