# File lib/dragonfly/job.rb, line 154
    def sha
      unless app.secret
        raise CannotGenerateSha, "A secret is required to sign and verify Dragonfly job requests. "\
                                 "Use `secret '...'` or `verify_urls false` (not recommended!) in your config."
      end
      OpenSSL::HMAC.hexdigest('SHA256', app.secret, to_unique_s)[0,16]
    end