Device FIDO U2F successfully installed to your application.

You need to:

1) Migrate database changes by running

'rails db:migrate'

2) Adapt your Devise models to include both the FIDO U2F registration and

authentication modules, e.g. change

'devise :database_authenticable ...'

to 

'devise :database_authenticable ..., 
 :fido_usf_registerable, :fido_usf_authenticatable'

3) Modify your application controller

(i.e. app/controllers/application_controller.rb) to always prepend the 
'protect_from_forgery' within the action chain, e.g. change

'protect_from_forgery with: :exception'

to

'protect_from_forgery with: :exception, prepend: true'

4) Add 'u2f-api.js' to your javascript assets, e.g. include in

'app/assets/javascript/application.js' the following statement:

'//= require u2f-api'

5) You need to run your server with SSL. Otherwise U2F refuses to work.

Probably you want to install the "thin" gem and use:

'thin start --ssl -p 3000'

to start your development server.

DISCLAIMER: IF YOU DO NOT APPLY ALL CHANGES, IT IS LIKELY THAT FIDO U2F WILL NOT WORK AS EXPECTED!