1 files changed, 94 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/crypto/agreement/DHAgreement.java b/core/src/main/java/org/spongycastle/crypto/agreement/DHAgreement.java
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+++ b/core/src/main/java/org/spongycastle/crypto/agreement/DHAgreement.java
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+package org.spongycastle.crypto.agreement;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+import org.spongycastle.crypto.AsymmetricCipherKeyPair;
+import org.spongycastle.crypto.CipherParameters;
+import org.spongycastle.crypto.generators.DHKeyPairGenerator;
+import org.spongycastle.crypto.params.DHKeyGenerationParameters;
+import org.spongycastle.crypto.params.DHParameters;
+import org.spongycastle.crypto.params.DHPublicKeyParameters;
+import org.spongycastle.crypto.params.DHPrivateKeyParameters;
+import org.spongycastle.crypto.params.AsymmetricKeyParameter;
+import org.spongycastle.crypto.params.ParametersWithRandom;
+
+/**
+ * a Diffie-Hellman key exchange engine.
+ * <p>
+ * note: This uses MTI/A0 key agreement in order to make the key agreement
+ * secure against passive attacks. If you're doing Diffie-Hellman and both
+ * parties have long term public keys you should look at using this. For
+ * further information have a look at RFC 2631.
+ * <p>
+ * It's possible to extend this to more than two parties as well, for the moment
+ * that is left as an exercise for the reader.
+ */
+public class DHAgreement
+{
+    private DHPrivateKeyParameters  key;
+    private DHParameters            dhParams;
+    private BigInteger              privateValue;
+    private SecureRandom            random;
+
+    public void init(
+        CipherParameters    param)
+    {
+        AsymmetricKeyParameter  kParam;
+
+        if (param instanceof ParametersWithRandom)
+        {
+            ParametersWithRandom    rParam = (ParametersWithRandom)param;
+
+            this.random = rParam.getRandom();
+            kParam = (AsymmetricKeyParameter)rParam.getParameters();
+        }
+        else
+        {
+            this.random = new SecureRandom();
+            kParam = (AsymmetricKeyParameter)param;
+        }
+
+        
+        if (!(kParam instanceof DHPrivateKeyParameters))
+        {
+            throw new IllegalArgumentException("DHEngine expects DHPrivateKeyParameters");
+        }
+
+        this.key = (DHPrivateKeyParameters)kParam;
+        this.dhParams = key.getParameters();
+    }
+
+    /**
+     * calculate our initial message.
+     */
+    public BigInteger calculateMessage()
+    {
+        DHKeyPairGenerator dhGen = new DHKeyPairGenerator();
+        dhGen.init(new DHKeyGenerationParameters(random, dhParams));
+        AsymmetricCipherKeyPair dhPair = dhGen.generateKeyPair();
+
+        this.privateValue = ((DHPrivateKeyParameters)dhPair.getPrivate()).getX();
+
+        return ((DHPublicKeyParameters)dhPair.getPublic()).getY();
+    }
+
+    /**
+     * given a message from a given party and the corresponding public key,
+     * calculate the next message in the agreement sequence. In this case
+     * this will represent the shared secret.
+     */
+    public BigInteger calculateAgreement(
+        DHPublicKeyParameters   pub,
+        BigInteger              message)
+    {
+        if (!pub.getParameters().equals(dhParams))
+        {
+            throw new IllegalArgumentException("Diffie-Hellman public key has wrong parameters.");
+        }
+
+        BigInteger p = dhParams.getP();
+
+        return message.modPow(key.getX(), p).multiply(pub.getY().modPow(privateValue, p)).mod(p);
+    }
+}