Authentication error with apt-get

I am trying to install CppCMS (A C++ network library) on my Debian computer. I have added the repository sources to sources.list file. When I run:

$ sudo apt-get source cppcms

I get the warning that the package could not be authenticated. It does not ask me if I want to proceed anyway, it prints E: Some packages could not be authenticated and aborts. With some Google searches I found that I should add the authentication key with apt-key, however I cannot seem to find the key on CppCMS website. What should I do, then?

Thank you for your time