Provide access to the application's KeyGenerator

Available both as an env entry for rack and an instance method on Rails::Application for other uses

railties/lib/rails/application.rb

@@ -101,6 +101,14 @@ def reload_routes!
       routes_reloader.reload!
     end
 
+
+    # Return the application's KeyGenerator
+    def key_generator
+      # number of iterations selected based on consultation with the google security
+      # team. Details at https://github.com/rails/rails/pull/6952#issuecomment-7661220
+      @key_generator ||= ActiveSupport::KeyGenerator.new(config.secret_token, :iterations=>1000)
+    end
+
     # Stores some of the Rails initial environment parameters which
     # will be used by middlewares and engines to configure themselves.
     # Currently stores:
@@ -121,7 +129,8 @@ def env_config
         "action_dispatch.show_exceptions" => config.action_dispatch.show_exceptions,
         "action_dispatch.show_detailed_exceptions" => config.consider_all_requests_local,
         "action_dispatch.logger" => Rails.logger,
-        "action_dispatch.backtrace_cleaner" => Rails.backtrace_cleaner
+        "action_dispatch.backtrace_cleaner" => Rails.backtrace_cleaner,
+        "action_dispatch.key_generator" => key_generator
       })
     end
 

railties/test/application/configuration_test.rb

@@ -634,6 +634,7 @@ def index
       assert_equal      app.env_config['action_dispatch.show_exceptions'],   app.config.action_dispatch.show_exceptions
       assert_equal      app.env_config['action_dispatch.logger'],            Rails.logger
       assert_equal      app.env_config['action_dispatch.backtrace_cleaner'], Rails.backtrace_cleaner
+      assert_equal      app.env_config['action_dispatch.key_generator'],     Rails.application.key_generator
     end
 
     test "config.colorize_logging default is true" do