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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!