Re: [TLS] TLS 1.2 Long-term Support Profile draft posted

Peter Gutmann <pgut001@cs.auckland.ac.nz> Sun, 20 March 2016 11:49 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Ilari Liusvaara <ilariliusvaara@welho.com>
Thread-Topic: [TLS] TLS 1.2 Long-term Support Profile draft posted
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Date: Sun, 20 Mar 2016 11:49:23 +0000
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Subject: Re: [TLS] TLS 1.2 Long-term Support Profile draft posted
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ilariliusvaara@welho.com <ilariliusvaara@welho.com>; writes:

>Well, if you have suitable implementation for it, the hash in EMS is over
>prefix of what hash in Finished is over (so if you can finalize multiple
>times, you can get away with just one computation).

The problem with this is that the standard hash API is { Init, [ Update,
Update, ... ], Final }, so this only works if you've got access to a custom
API that allows you to fork the state so one branch goes to Final and the
other continues with Update.  It's good if you've got it, but a lot of
implementations don't.

>Also, TLS 1.2 ServerKeyExchange signature is not taken over ClientHello and
>ServerHello. This was famously exploited for FREAK and LOGJAM (and then there
>is the DHE vs. ECDHE issue). Sadly, this was found out too late for changing
>EMS extension to extend the signature.

Hmm, what do people feel about adding this as a fix?  Given that the attacks
were based on implementations supporting crippled crypto when they shouldn't
have and accepting an export cipher suite when they shouldn't have (i.e. they
were pretty broken to begin with), is it worth the compatibility-breaking
change to try and address this?

>Then there is the problem that DHE parameter sizes are not negotiated. This
>is severe enough problem that it renders DHE effecively unusable in some
>contexts

That's such a can of worms there that I don't really want to get into it in 
-LTS, unless lots of people really want to see it addressed...

Peter.