Where to store user specific data for .net test project?

I have NUnit test project and public/private keys for some user account.

Now I store those keys in code. But it's wrong way for user specific and security data.

So, where is better to store a such data?

Options: environment variables, *.resx, .runsettings, *.csproj.user (if it's possible?).

I'd like the *.csproj.user but is it possible to store user data in *.csproj.user?