1 files changed, 64 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/ECDHCBasicAgreement.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/ECDHCBasicAgreement.java
new file mode 100644
index 000000000..49677805d
--- /dev/null
+++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/crypto/agreement/ECDHCBasicAgreement.java
@@ -0,0 +1,64 @@
+package org.spongycastle.crypto.agreement;
+
+import java.math.BigInteger;
+
+import org.spongycastle.crypto.BasicAgreement;
+import org.spongycastle.crypto.CipherParameters;
+import org.spongycastle.crypto.params.ECDomainParameters;
+import org.spongycastle.crypto.params.ECPrivateKeyParameters;
+import org.spongycastle.crypto.params.ECPublicKeyParameters;
+import org.spongycastle.math.ec.ECPoint;
+
+/**
+ * P1363 7.2.2 ECSVDP-DHC
+ *
+ * ECSVDP-DHC is Elliptic Curve Secret Value Derivation Primitive,
+ * Diffie-Hellman version with cofactor multiplication. It is based on
+ * the work of [DH76], [Mil86], [Kob87], [LMQ98] and [Kal98a]. This
+ * primitive derives a shared secret value from one party's private key
+ * and another party's public key, where both have the same set of EC
+ * domain parameters. If two parties correctly execute this primitive,
+ * they will produce the same output. This primitive can be invoked by a
+ * scheme to derive a shared secret key; specifically, it may be used
+ * with the schemes ECKAS-DH1 and DL/ECKAS-DH2. It does not assume the
+ * validity of the input public key (see also Section 7.2.1).
+ * <p>
+ * Note: As stated P1363 compatibility mode with ECDH can be preset, and
+ * in this case the implementation doesn't have a ECDH compatibility mode
+ * (if you want that just use ECDHBasicAgreement and note they both implement
+ * BasicAgreement!).
+ */
+public class ECDHCBasicAgreement
+    implements BasicAgreement
+{
+    ECPrivateKeyParameters key;
+
+    public void init(
+        CipherParameters key)
+    {
+        this.key = (ECPrivateKeyParameters)key;
+    }
+
+    public int getFieldSize()
+    {
+        return (key.getParameters().getCurve().getFieldSize() + 7) / 8;
+    }
+
+    public BigInteger calculateAgreement(
+        CipherParameters pubKey)
+    {
+        ECPublicKeyParameters pub = (ECPublicKeyParameters)pubKey;
+        ECDomainParameters params = pub.getParameters();
+
+        BigInteger hd = params.getH().multiply(key.getD()).mod(params.getN());
+
+        ECPoint P = pub.getQ().multiply(hd).normalize();
+
+        if (P.isInfinity())
+        {
+            throw new IllegalStateException("Infinity is not a valid agreement value for ECDHC");
+        }
+
+        return P.getAffineXCoord().toBigInteger();
+    }
+}