Last Call Review of draft-ietf-tls-external-psk-guidance-03
review-ietf-tls-external-psk-guidance-03-artart-lc-thomson-2021-11-03-00

This document addresses some of the less obvious aspects of how pre-shared keys
can be used in TLS.  A lot of this advice isn't specific to TLS, but it is a
helpful document.  For someone who might be deploying a protocol that relies on
TLS - or might rely on it - the document is a useful resource.

My only concern overall, and it is a vague concern, so I don't think action is
needed, is that the document could probably use a little trimming.  There are
some parts of the document that are less useful than other parts.  For example,
the bit about who has the PSKs is great (one server, one client, don't swap
roles); but it is repeated a little across multiple sections.  The same applies
to a few of the other points.  It is probably not worth trying to edit the
document down so that each point is made just once, because it isn't that bad,
but a shorter document would be more impactful.

A specific concern is the somewhat offhand way that early data is treated.  The
only mention is in a throwaway: "primarily for the purposes of supporting TLS
connections with early data" buried in a bullet in Section 6.  This is a pretty
big topic and having absolutely no mention seems odd.  I do think that it needs
some treatment in the document.  When early data is used with an external PSK,
the only additional source of entropy that provides key diversity is the
client's random value, which puts a lot of weight on that value containing
sufficient entropy.  In this case, even if the PSK is good enough, the entropy
in the random is significant as it is what ensures traffic key diversity if the
PSK is reused.  Reusing a PSK for early data also likely leads to poor
anti-replay performance if the random is not good enough.

I have to apologize to the authors for missing this when it went through the
working group.  Fresh eyes and all that.