1 files changed, 48 insertions, 0 deletions
diff --git a/core/src/main/java/org/bouncycastle/crypto/ec/ECElGamalDecryptor.java b/core/src/main/java/org/bouncycastle/crypto/ec/ECElGamalDecryptor.java
new file mode 100644
index 00000000..c8c548ec
--- /dev/null
+++ b/core/src/main/java/org/bouncycastle/crypto/ec/ECElGamalDecryptor.java
@@ -0,0 +1,48 @@
+package org.bouncycastle.crypto.ec;
+
+import org.bouncycastle.crypto.CipherParameters;
+import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
+import org.bouncycastle.math.ec.ECPoint;
+
+/**
+ * this does your basic decryption ElGamal style using EC
+ */
+public class ECElGamalDecryptor
+    implements ECDecryptor
+{
+    private ECPrivateKeyParameters key;
+
+    /**
+     * initialise the decryptor.
+     *
+     * @param param the necessary EC key parameters.
+     */
+    public void init(
+        CipherParameters param)
+    {
+        if (!(param instanceof ECPrivateKeyParameters))
+        {
+            throw new IllegalArgumentException("ECPrivateKeyParameters are required for decryption.");
+        }
+
+        this.key = (ECPrivateKeyParameters)param;
+    }
+
+    /**
+     * Decrypt an EC pair producing the original EC point.
+     *
+     * @param pair the EC point pair to process.
+     * @return the result of the Elgamal process.
+     */
+    public ECPoint decrypt(ECPair pair)
+    {
+        if (key == null)
+        {
+            throw new IllegalStateException("ECElGamalDecryptor not initialised");
+        }
+
+        ECPoint tmp = pair.getX().multiply(key.getD());
+
+        return pair.getY().add(tmp.negate());
+    }
+}