1 files changed, 88 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/crypto/ec/ECFixedTransform.java b/core/src/main/java/org/spongycastle/crypto/ec/ECFixedTransform.java
new file mode 100644
index 00000000..9bd5162b
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/crypto/ec/ECFixedTransform.java
@@ -0,0 +1,88 @@
+package org.spongycastle.crypto.ec;
+
+import java.math.BigInteger;
+
+import org.spongycastle.crypto.CipherParameters;
+import org.spongycastle.crypto.params.ECDomainParameters;
+import org.spongycastle.crypto.params.ECPublicKeyParameters;
+import org.spongycastle.math.ec.ECMultiplier;
+import org.spongycastle.math.ec.ECPoint;
+import org.spongycastle.math.ec.FixedPointCombMultiplier;
+
+/**
+ * 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 text 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");
+        }
+
+        ECDomainParameters ec = key.getParameters();
+        BigInteger n = ec.getN();
+
+        ECMultiplier basePointMultiplier = createBasePointMultiplier();
+        BigInteger k = this.k.mod(n);
+
+        ECPoint[] gamma_phi = new ECPoint[]{
+            basePointMultiplier.multiply(ec.getG(), k).add(cipherText.getX()),
+            key.getQ().multiply(k).add(cipherText.getY())
+        };
+
+        ec.getCurve().normalizeAll(gamma_phi);
+
+        return new ECPair(gamma_phi[0], gamma_phi[1]);
+    }
+
+    /**
+     * Return the last transform value used by the transform
+     *
+     * @return a BigInteger representing k value.
+     */
+    public BigInteger getTransformValue()
+    {
+        return k;
+    }
+
+    protected ECMultiplier createBasePointMultiplier()
+    {
+        return new FixedPointCombMultiplier();
+    }
+}