Decrypt a Rails 5 session cookie

decrypt_cookie.rb

# rails 5.1.6
# usage: rails runner decrypt_cookie.rb <cookie value>

require 'cgi'
require 'json'
require 'active_support'

def verify_and_decrypt_session_cookie(cookie, secret_key_base = Rails.application.secrets.secret_key_base)
  cookie = CGI::unescape(cookie)
  salt = Rails.configuration.action_dispatch.encrypted_cookie_salt
  signed_salt = Rails.configuration.action_dispatch.encrypted_signed_cookie_salt
  key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000)
  secret = key_generator.generate_key(salt)[0, 32]
  sign_secret = key_generator.generate_key(signed_salt)
  encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)

  encryptor.decrypt_and_verify(cookie)
end

if $0 == __FILE__
  ARGV.each do |arg|
    puts verify_and_decrypt_session_cookie(arg)
  end
end