diff --git a/UpdateStream.cc b/UpdateStream.cc
index ddab026..e30b334 100644
--- a/UpdateStream.cc
+++ b/UpdateStream.cc
@@ -13,17 +13,6 @@ namespace pEp {
     namespace UpdateClient {
         using namespace CryptoPP;
 
-        PublicKey load_key(string filename)
-        {
-            ByteQueue queue;
-            FileSource file("public.der", true);
-            file.TransferTo(queue);
-            queue.MessageEnd();
-            CryptoPP::RSA::PublicKey key;
-            key.Load(queue);
-            return key;
-        }
-
         UpdateDevice::UpdateDevice(const product p)
             : _https({HTTPSDevice()}, ios::binary), _p(p),
                 _key(32), _src(nullptr), _str(nullptr)
diff --git a/UpdateStream.hh b/UpdateStream.hh
index f79ed8c..5399912 100644
--- a/UpdateStream.hh
+++ b/UpdateStream.hh
@@ -25,7 +25,6 @@ namespace pEp {
         };
 
         typedef CryptoPP::RSA::PublicKey PublicKey;
-        PublicKey load_key(string filename);
 
         class UpdateDevice : public io::source {
             HTTPSStream _https;
diff --git a/downloadclient.cc b/downloadclient.cc
index 97a3c31..208762e 100644
--- a/downloadclient.cc
+++ b/downloadclient.cc
@@ -8,6 +8,24 @@
 
 namespace pEp {
     namespace UpdateClient {
+        PublicKey load_key(string filename)
+        {
+            CryptoPP::RSA::PublicKey key;
+
+            try {
+                CryptoPP::ByteQueue queue;
+                CryptoPP::FileSource file("public.der", true);
+                file.TransferTo(queue);
+                queue.MessageEnd();
+                key.Load(queue);
+            }
+            catch (exception&) {
+                throw CannotLoadKey(filename);
+            }
+
+            return key;
+        }
+
         string update(product p, PublicKey update_key)
         {
             UpdateStream us { UpdateDevice(p) };
diff --git a/downloadclient.hh b/downloadclient.hh
index 2440b1c..8fd5646 100644
--- a/downloadclient.hh
+++ b/downloadclient.hh
@@ -14,7 +14,48 @@ namespace pEp {
             DownloadError() : runtime_error("download not possible") { }
         };
 
+        struct CannotLoadKey : runtime_error {
+            CannotLoadKey(string filename)
+                : runtime_error("cannot load key from " + filename)
+            {
+            }
+        };
+
+        // load_key() - load BER encoded RSA public key
+        //
+        //  params:
+        //      filename (in)       keyfile to load from
+        //
+        //  returns:
+        //      CryptoPP::RSA::PublicKey
+        //
+        //  throws:
+        //      CannotLoadKey if key cannot be loaded
+
+        PublicKey load_key(string filename);
+
+        // update() - download update for a product if available
+        //
+        //  params:
+        //      p (in)              product to update with:
+        //          name (in)           product name
+        //          url (in)            download URL with GET param:
+        //              hash                serial number
+        //      update_key (in)     Update Key as CryptoPP::RSA::PublicKey
+        //
+        //  returns:
+        //      filename of downloaded file
+        //
+        //  caveat:
+        //      this function is using the current directory to download
+        //      therefore, the current directory has to be writable
+        //
+        //      see sample in test_updater.cc
+
         string update(product p, PublicKey update_key);
+
+        // update() - convenience function doing load_key() and update()
+
         string update(product p, string keyfile);
     }
 }
diff --git a/test_updater.cc b/test_updater.cc
index 6f5f00a..7b80deb 100644
--- a/test_updater.cc
+++ b/test_updater.cc
@@ -10,28 +10,6 @@ using namespace pEp::UpdateClient;
 
 int main()
 {
-    /*
-    product p { "pEp for Something", "https://fdik.org/cgidownload?hash=23232323234242" };
-    UpdateStream us { UpdateDevice(p) };
-
-    cout << "downloading...\n";
-
-    PublicKey update_key = load_key("public.der");
-    try {
-        us->open(update_key);
-    }
-    catch (exception&) { }
-
-    if (us->filename() != "") {
-        cout << "downloaded " << us->filename() << "\n";
-    }
-    else {
-        stringstream str;
-        str << us.rdbuf();
-        cout << str.str() << "\n";
-    }
-    */
-
     cout << "downloading...\n";
 
     product p { "pEp for Something", "https://fdik.org/cgidownload?hash=23232323234242" };