Re: [TLS] Thoughts on Version Intolerance

Peter Gutmann <pgut001@cs.auckland.ac.nz> Thu, 21 July 2016 12:25 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: "mrex@sap.com" <mrex@sap.com>, Hubert Kario <hkario@redhat.com>
Thread-Topic: [TLS] Thoughts on Version Intolerance
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Date: Thu, 21 Jul 2016 12:25:48 +0000
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Subject: Re: [TLS] Thoughts on Version Intolerance
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Martin Rex <mrex@sap.com>; writes:

>Limiting PDU sizes to reasonably sane sizes is perfectly valid behaviour.
>X.509v3 certificates can theoretically include CAT MPEGs and amount to
>megabytes.

We really need a TLS scanner that does this just to see what happens.  When I
created that cat-MPEG cert, I fed it to both MSIE and Netscape.  Both happily
accepted it, and then essentially become nonfunctional because although they
saw nothing wrong with accepting a cert of that size, they couldn't actually
deal with it.  In Netscape's case I had to delete the .db file before it could
be used again.  I wouldn't be surprised if you can quite effectively DoS
assorted TLS implementations with stuff like this.

(This is one of the design features of TLS-LTS, if both sides do -LTS then the
Hello only needs a single extension to specify everything.  Combine that with
PSK, so no certs, and you get quite an efficient protocol, the only thing of
any size is the keyex and server signature).

Peter.