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On 06/16/2017 05:36 PM, Matt Caswell wrote:<br>
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cite="mid:cc28bc66-9f9b-5b4d-5957-d80080e176a7@openssl.org">
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<pre wrap="">The security properties of such "external" PSKs are substantially
different than the "ephemeral" PSKs used in resumption flows.
</pre>
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<pre wrap="">Ben - Even external PSKs incorporate an ephemeral, per connection, ECDHE
based secret (assuming a suitable kex_mode is used). What do you see as
the concern?</pre>
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<br>
The risk of accidentally using psk_ke instead of psk_dhe_ke is
noticeable, and in terms of concrete differences, there are
additional requirements on external PSKs that the KDF and PSK
identity must remain fixed across uses. That, combined with the
potential for insufficient entropy during key generation (mentioned
in section 2.2 of draft-20) seem to provide more openings for
cryptographic attacks than for the full resumption flow. It is
probably fine for uses where the other properties of external PSKs
are needed, but I'm not sure that the risk/reward balance favors
using it just to get a speedup -- TLS 1.3 resumption should already
be pretty fast.<br>
<br>
-Ben<br>
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