1 files changed, 71 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/ec/ECFixedTransform.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/ec/ECFixedTransform.java
new file mode 100644
index 000000000..7b35f1a10
--- /dev/null
+++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/ec/ECFixedTransform.java
@@ -0,0 +1,71 @@
+package org.spongycastle.crypto.ec;
+
+import java.math.BigInteger;
+
+import org.spongycastle.crypto.CipherParameters;
+import org.spongycastle.crypto.params.ECPublicKeyParameters;
+import org.spongycastle.math.ec.ECPoint;
+
+/**
+ * this transforms the original randomness used for an ElGamal encryption by a fixed value.
+ */
+public class ECFixedTransform
+    implements ECPairFactorTransform
+{
+    private ECPublicKeyParameters key;
+
+    private BigInteger k;
+
+    public ECFixedTransform(BigInteger k)
+    {
+        this.k = k;
+    }
+
+    /**
+     * initialise the underlying EC ElGamal engine.
+     *
+     * @param param the necessary EC key parameters.
+     */
+    public void init(
+        CipherParameters    param)
+    {
+        if (!(param instanceof ECPublicKeyParameters))
+        {
+            throw new IllegalArgumentException("ECPublicKeyParameters are required for fixed transform.");
+        }
+
+        this.key = (ECPublicKeyParameters)param;
+    }
+
+    /**
+     * 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("ECFixedTransform not initialised");
+        }
+
+        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).normalize(), phi.normalize());
+    }
+
+    /**
+     * Return the last transform value used by the transform
+     *
+     * @return a BigInteger representing k value.
+     */
+    public BigInteger getTransformValue()
+    {
+        return k;
+    }
+}