1 files changed, 71 insertions, 0 deletions
diff --git a/core/src/main/java/org/bouncycastle/crypto/agreement/DHBasicAgreement.java b/core/src/main/java/org/bouncycastle/crypto/agreement/DHBasicAgreement.java
new file mode 100644
index 00000000..d2e2a09f
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+++ b/core/src/main/java/org/bouncycastle/crypto/agreement/DHBasicAgreement.java
@@ -0,0 +1,71 @@
+package org.bouncycastle.crypto.agreement;
+
+import java.math.BigInteger;
+
+import org.bouncycastle.crypto.BasicAgreement;
+import org.bouncycastle.crypto.CipherParameters;
+import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
+import org.bouncycastle.crypto.params.DHParameters;
+import org.bouncycastle.crypto.params.DHPrivateKeyParameters;
+import org.bouncycastle.crypto.params.DHPublicKeyParameters;
+import org.bouncycastle.crypto.params.ParametersWithRandom;
+
+/**
+ * a Diffie-Hellman key agreement class.
+ * <p>
+ * note: This is only the basic algorithm, it doesn't take advantage of
+ * long term public keys if they are available. See the DHAgreement class
+ * for a "better" implementation.
+ */
+public class DHBasicAgreement
+    implements BasicAgreement
+{
+    private DHPrivateKeyParameters  key;
+    private DHParameters            dhParams;
+
+    public void init(
+        CipherParameters    param)
+    {
+        AsymmetricKeyParameter  kParam;
+
+        if (param instanceof ParametersWithRandom)
+        {
+            ParametersWithRandom rParam = (ParametersWithRandom)param;
+            kParam = (AsymmetricKeyParameter)rParam.getParameters();
+        }
+        else
+        {
+            kParam = (AsymmetricKeyParameter)param;
+        }
+
+        if (!(kParam instanceof DHPrivateKeyParameters))
+        {
+            throw new IllegalArgumentException("DHEngine expects DHPrivateKeyParameters");
+        }
+
+        this.key = (DHPrivateKeyParameters)kParam;
+        this.dhParams = key.getParameters();
+    }
+
+    public int getFieldSize()
+    {
+        return (key.getParameters().getP().bitLength() + 7) / 8;
+    }
+
+    /**
+     * given a short term public key from a given party calculate the next
+     * message in the agreement sequence. 
+     */
+    public BigInteger calculateAgreement(
+        CipherParameters   pubKey)
+    {
+        DHPublicKeyParameters   pub = (DHPublicKeyParameters)pubKey;
+
+        if (!pub.getParameters().equals(dhParams))
+        {
+            throw new IllegalArgumentException("Diffie-Hellman public key has wrong parameters.");
+        }
+
+        return pub.getY().modPow(key.getX(), dhParams.getP());
+    }
+}