1 files changed, 76 insertions, 0 deletions
diff --git a/core/src/main/java/org/bouncycastle/crypto/ec/ECNewRandomnessTransform.java b/core/src/main/java/org/bouncycastle/crypto/ec/ECNewRandomnessTransform.java
new file mode 100644
index 00000000..b0379841
--- /dev/null
+++ b/core/src/main/java/org/bouncycastle/crypto/ec/ECNewRandomnessTransform.java
@@ -0,0 +1,76 @@
+package org.bouncycastle.crypto.ec;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+import org.bouncycastle.crypto.CipherParameters;
+import org.bouncycastle.crypto.params.ECPublicKeyParameters;
+import org.bouncycastle.crypto.params.ParametersWithRandom;
+import org.bouncycastle.math.ec.ECPoint;
+
+/**
+ * this transforms the original randomness used for an ElGamal encryption.
+ */
+public class ECNewRandomnessTransform
+    implements ECPairTransform
+{
+    private ECPublicKeyParameters key;
+    private SecureRandom          random;
+
+    /**
+     * initialise the underlying EC ElGamal engine.
+     *
+     * @param param the necessary EC key parameters.
+     */
+    public void init(
+        CipherParameters    param)
+    {
+        if (param instanceof ParametersWithRandom)
+        {
+            ParametersWithRandom    p = (ParametersWithRandom)param;
+
+            if (!(p.getParameters() instanceof ECPublicKeyParameters))
+            {
+                throw new IllegalArgumentException("ECPublicKeyParameters are required for new randomness transform.");
+            }
+
+            this.key = (ECPublicKeyParameters)p.getParameters();
+            this.random = p.getRandom();
+        }
+        else
+        {
+            if (!(param instanceof ECPublicKeyParameters))
+            {
+                throw new IllegalArgumentException("ECPublicKeyParameters are required for new randomness transform.");
+            }
+
+            this.key = (ECPublicKeyParameters)param;
+            this.random = new SecureRandom();
+        }
+    }
+
+    /**
+     * Transform an existing cipher test pair using the ElGamal algorithm. Note: it is assumed this
+     * transform has been initialised with the same public key that was used to create the original
+     * cipher text.
+     *
+     * @param cipherText the EC point to process.
+     * @return returns a new ECPair representing the result of the process.
+     */
+    public ECPair transform(ECPair cipherText)
+    {
+        if (key == null)
+        {
+            throw new IllegalStateException("ECNewRandomnessTransform not initialised");
+        }
+
+        BigInteger             n = key.getParameters().getN();
+        BigInteger             k = ECUtil.generateK(n, random);
+
+        ECPoint  g = key.getParameters().getG();
+        ECPoint  gamma = g.multiply(k);
+        ECPoint  phi = key.getQ().multiply(k).add(cipherText.getY());
+
+        return new ECPair(cipherText.getX().add(gamma), phi);
+    }
+}