Fix segfault with multiple KeyExchange objects

Moves DH object to an instance var that dies with the object.  Prior to this
change, using multiple KeyExchange objects would cause node to segfault.

src/keyexchange.cc

@@ -56,8 +56,8 @@ class KeyExchange: ObjectWrap
   public:
 
     static Persistent<FunctionTemplate> s_ct;
-    static DH* dh;
-    
+    DH* dh;
+
     /**
      * Node module initialization and method exposure
      */     
@@ -79,13 +79,16 @@ class KeyExchange: ObjectWrap
 
         target->Set(String::NewSymbol("KeyExchange"), s_ct->GetFunction());
     }
-    
-    KeyExchange()
+
+    KeyExchange() : ObjectWrap()
     {
+      dh = NULL;
+    }
+
+    ~KeyExchange(){
+      if (dh) DH_free(dh);
     }
 
-    ~KeyExchange(){}
-
     /**
      * Creates a new KeyExchange instance as a JavaScript Object
      */     
@@ -103,24 +106,25 @@ class KeyExchange: ObjectWrap
      * @return {Array} [privateKey, publicKey]
      */
     static Handle<Value> generateKeyPair(const Arguments& args) {
+      KeyExchange *ke = ObjectWrap::Unwrap<KeyExchange>(args.This());
       HandleScope scope;
 
       char private_key[128];
       char public_key[128];
 
       // Create DH keypair
-      KeyExchange::dh = DH_new();
-      KeyExchange::dh->p = BN_new();
-      KeyExchange::dh->g = BN_new();
+      ke->dh = DH_new();
+      ke->dh->p = BN_new();
+      ke->dh->g = BN_new();
+
+      BN_set_word(ke->dh->g, 2);
+      BN_bin2bn(g_dh1024p, 128, ke->dh->p);
+      DH_generate_key(ke->dh);
 
-      BN_set_word(KeyExchange::dh->g, 2);
-      BN_bin2bn(g_dh1024p, 128, KeyExchange::dh->p);
-      DH_generate_key(KeyExchange::dh); 
-
       //Fill the servers public key
-      BN_bn2bin(KeyExchange::dh->pub_key, (uint8_t*)public_key);
-      BN_bn2bin(KeyExchange::dh->priv_key, (uint8_t*)private_key);
-      
+      BN_bn2bin(ke->dh->pub_key, (uint8_t*)public_key);
+      BN_bn2bin(ke->dh->priv_key, (uint8_t*)private_key);
+
       Local<Array> keys = Array::New();
 
       keys->Set(uint32_t(0), Buffer::New(private_key, 128)->handle_);
@@ -133,15 +137,16 @@ class KeyExchange: ObjectWrap
      * Computes the shared secret from a public key
      */
     static Handle<Value> computeSharedSecret(const Arguments& args) {
+      KeyExchange *ke = ObjectWrap::Unwrap<KeyExchange>(args.This());
       HandleScope scope;
 
       Local<Object> far_key_obj = args[0]->ToObject();
 
       if (args.Length() < 1 || !Buffer::HasInstance(far_key_obj)) {
         return ThrowException(Exception::TypeError(String::New("Bad argument")));
       }
-      
-      if(DH_size(KeyExchange::dh) <= 0){
+
+      if(DH_size(ke->dh) <= 0){
         return ThrowException(Exception::TypeError(String::New("DH empty. Generate Keypair first.")));
       }
 
@@ -150,11 +155,10 @@ class KeyExchange: ObjectWrap
 
       char shared_secret[128];
       BIGNUM *bn_far_key = BN_bin2bn((const uint8_t*)far_key, far_key_size, NULL);
-      DH_compute_key((unsigned char*)shared_secret, bn_far_key, KeyExchange::dh);
-        
+      DH_compute_key((unsigned char*)shared_secret, bn_far_key, ke->dh);
+
       BN_free(bn_far_key);
-      DH_free(KeyExchange::dh);
-
+
       return scope.Close(Buffer::New(shared_secret, 128)->handle_);
     }
 
@@ -206,7 +210,6 @@ class KeyExchange: ObjectWrap
 };
 
 Persistent<FunctionTemplate> KeyExchange::s_ct;
-DH* KeyExchange::dh;
 
 //Node module exposure
 extern "C" {