You can also use Livebook itself to validate the iframe signature. Add :req as a dependency and then:
resp = Req.get!("https://livebookusercontent.com/iframe/v4.html")
Base.encode64 :crypto.hash :sha256, req.body
Livebook expects it to be: vd7g1B8fLBFZH6C6KNpG4H8B0SQ/oIuqKaTW6jD053A=
But please check you are indeed downloading that iframe. 
