Subject: Re: [core] Benjamin Kaduk's No Objection on draft-ietf-core-echo-request-tag-12: (with COMMENT)

From: Christian M. Amsüss <[email-masked]>
Date: Thu, 18 Mar 2021 11:12:28 +0100

Hello Ben,

thanks for your input, we've now taken a first round on it. While we're
editing the results in (from which there'll be a point-to-point response
later), one point would profit from earlier feedback:

> While I recognize that there is going to be a spectrum of requirements
> for determining freshness, I would have expected the far extreme of that
> spectrum to include a strongly time-limited single-use cryptographic
> nonce (akin to what the ACME protocol of RFC 8555 uses but with time
> limit), as well as discussion of some points on the spectrum and which
> ones might be more or less appropriate in various cases.  I do see some
> discussion of different use cases, but not much about the tradeoffs
> along the spectrum, and no discussion at all about the strongest
> properties that it is possible to obtain with this mechanism.

We didn't find any purpose that a strict single-use Echo value would
serve. There are cases (which are to become part of the explored
spectrum) where an action that requires one very recent event-based Echo
value. And when taken, the action produces an event / server state
change that makes that Echo value not good for anything any more. Still,
there is nothing inherently single-use about the value: For example, it
doesn't get used up if included in a safe request performed between when
it is obtained and when the action is taken.

Do you see any applications where an even stricter usable-once behavior
would be useful?

Best regards
Christian

-- 
To use raw power is to make yourself infinitely vulnerable to greater powers.
  -- Bene Gesserit axiom