Create base controller class

app/controllers/api/v1/registrant/base_controller.rb

@@ -0,0 +1,31 @@
+require 'rails5_api_controller_backport'
+require 'auth_token/auth_token_decryptor'
+
+module Api
+  module V1
+    module Registrant
+      class BaseController < ActionController::API
+        before_action :authenticate
+
+        private
+
+        def bearer_token
+          pattern = /^Bearer /
+          header  = request.headers['Authorization']
+          header.gsub(pattern, '') if header && header.match(pattern)
+        end
+
+        def authenticate
+          decryptor = AuthTokenDecryptor.create_with_defaults(bearer_token)
+          decryptor.decrypt_token
+
+          if decryptor.valid?
+            sign_in decryptor.user
+          else
+            render json: { error: 'Not authorized' }, status: 403
+          end
+        end
+      end
+    end
+  end
+end