Rails 4.0 introduces ActiveSupport::KeyGenerator and uses this as a base from which to generate and verify signed cookies (among other things). Generate a secret with bundle exec rake secret and use this for the value of SECRET_KEY_BASE in config/general.yml.
As of 0.29.0.0
The COOKIE_STORE_SESSION_SECRET config item has been removed and replaced with SECRET_KEY_BASE. You should migrate the original value to the new config key.
Rails 4.0 introduces ActiveSupport::KeyGenerator and uses this as a base from which to generate and verify signed cookies (among other things). Generate a secret with
bundle exec rake secret
and use this for the value of SECRET_KEY_BASE in config/general.yml.