decrypt_rails_session_cookie.rb

# Works with Rails 4.2.6
# Run it in rails' environment ('rails console')

def decrypt_session_cookie(cookie)
  unescaped_cookie = CGI.unescape(cookie)
  
  # Secret key base
  secret_key_base = Rails.application.secrets[:secret_key_base]
  
  # Values from rails app' config
  config      = Rails.application.config
  salt        = config.action_dispatch.encrypted_cookie_salt
  signed_salt = config.action_dispatch.encrypted_signed_cookie_salt
  
  # Default values for Rails 4 apps
  key_iter_num = 1000
  key_size     = 64
  cookies_serializer = JSON  # see: config.action_dispatch.cookies_serializer
  
  key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: key_iter_num)
  secret = key_generator.generate_key(salt)
  sign_secret = key_generator.generate_key(signed_salt)

  encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: cookies_serializer)
  encryptor.decrypt_and_verify(unescaped_cookie)
end