credits: http://big-elephants.com/2014-01/handling-rails-4-sessions-with-go

access_rails_cookie_session.rb

# decrypt Rails4 cookie simple algo

cookie = request.cookie_jar[Rails.application.config.session_options[:key]]
cookie = CGI.unescape(cookie)
config = Rails.application.config

encrypted_cookie_salt = config.action_dispatch.encrypted_cookie_salt               # "encrypted cookie" by default
encrypted_signed_cookie_salt = config.action_dispatch.encrypted_signed_cookie_salt # "signed encrypted cookie" by default

key_generator = ActiveSupport::KeyGenerator.new(config.secret_key_base, iterations: 1000)
secret        = key_generator.generate_key(encrypted_cookie_salt)
sign_secret   = key_generator.generate_key(encrypted_signed_cookie_salt)

encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret)
session   = encryptor.decrypt_and_verify(cookie).with_indifferent_access

# same but from “outside” Rails

session = request.cookie_jar.encrypted[Rails.application.config.session_options[:key]].try(:with_indifferent_access)