Decrypt Rails 4 session cookie

gistfile1.rb

cookie = CGI.unescape("cookie here")

secret_key_base = "secret key here"

encrypted_cookie_salt = "encrypted cookie"
encrypted_signed_cookie_salt = "signed encrypted cookie"

key_generator = ActiveSupport::KeyGenerator.new(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, serializer: ActiveSupport::MessageEncryptor::NullSerializer)
encryptor.decrypt_and_verify(cookie)