1 files changed, 73 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/ec/ECElGamalEncryptor.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/ec/ECElGamalEncryptor.java
new file mode 100644
index 000000000..c31a4308b
--- /dev/null
+++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/ec/ECElGamalEncryptor.java
@@ -0,0 +1,73 @@
+package org.spongycastle.crypto.ec;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+import org.spongycastle.crypto.CipherParameters;
+import org.spongycastle.crypto.params.ECPublicKeyParameters;
+import org.spongycastle.crypto.params.ParametersWithRandom;
+import org.spongycastle.math.ec.ECPoint;
+
+/**
+ * this does your basic ElGamal encryption algorithm using EC
+ */
+public class ECElGamalEncryptor
+    implements ECEncryptor
+{
+    private ECPublicKeyParameters key;
+    private SecureRandom          random;
+
+    /**
+     * initialise the encryptor.
+     *
+     * @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 encryption.");
+            }
+            this.key = (ECPublicKeyParameters)p.getParameters();
+            this.random = p.getRandom();
+        }
+        else
+        {
+            if (!(param instanceof ECPublicKeyParameters))
+            {
+                throw new IllegalArgumentException("ECPublicKeyParameters are required for encryption.");
+            }
+
+            this.key = (ECPublicKeyParameters)param;
+            this.random = new SecureRandom();
+        }
+    }
+
+    /**
+     * Process a single EC point using the basic ElGamal algorithm.
+     *
+     * @param point the EC point to process.
+     * @return the result of the Elgamal process.
+     */
+    public ECPair encrypt(ECPoint point)
+    {
+        if (key == null)
+        {
+            throw new IllegalStateException("ECElGamalEncryptor 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(point);
+
+        return new ECPair(gamma.normalize(), phi.normalize());
+    }
+}