class ActiveSupport::EncryptedFile

def key

environment variable is not set and the key file does not exist.
If +raise_if_missing_key+ is true, raises MissingKeyError if the
specified by +env_key+, then trying the key file specified by +key_path+.
Returns the encryption key, first trying the environment variable 
def key
  read_env_key || read_key_file || handle_missing_key
end