http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html

Rails 4.0 encrypts the contents of cookie-based sessions if secret_key_base has been set. Rails 3.x signed, but did not encrypt, the contents of cookie-based session. Signed cookies are "secure" in that they are verified to have been generated by your app and are tamper-proof. However, the contents can be viewed by end users, and encrypting the contents eliminates this caveat/concern without a significant performance penalty.