[TLS] Review of draft-ietf-tls-external-psk-guidance

Watson Ladd <watsonbladd@gmail.com> Mon, 06 July 2020 19:47 UTC

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From: Watson Ladd <watsonbladd@gmail.com>
Date: Mon, 06 Jul 2020 15:47:29 -0400
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Subject: [TLS] Review of draft-ietf-tls-external-psk-guidance
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Dear WG,

I've taken a look at the draft and I think while its discussion of the
properties and limitations of the external PSKs are good, I think the
recommendations in section 7 could use some minor editorial work.

In particular  "SHOULD be combined with a DH exchange for forward
secrecy." I would like to see rephrased to make clear that this is
about the TLS PSK Key Exchange Mode. It wasn't immediately clear to me
on first read, especially given the next sentence is (maybe) about key
establishment outside of TLS.

"If only low-entropy keys are available, then key establishment
mechanisms such as Password Authenticated Key Exchange (PAKE) that
mitigate the risk of offline dictionary attacks SHOULD be employed".
I have some questions about the meaning of this sentence. If it's
about potential future additions to TLS ciphersuites, then it should
be more clear that this doesn't currently exist and will in the
future.  If it's about designing an ad-hoc key distribution mechanism
to be run one time ahead of PSK TLS, then I think we should say so
more clearly and provide guidance on how to do this and think through
the implications.

Section 7.1.1. While it's a good idea to compare byte by byte, humans
entering PSK identifiers may run into trouble due to all the ways
visually identical strings may not actually be identical. It might be
worth calling this out as a consideration.

Watson Ladd