Decrypt Rails 4 session cookie

decrypt.rb

# `cookie` is the session cookie string
message = URI.unescape cookie
config = Rails.application.config

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

encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)

encryptor.decrypt_and_verify message