[TLS] Heartbleed / protocol complexity

Hanno Böck <hanno@hboeck.de> Wed, 09 April 2014 21:25 UTC

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Date: Wed, 9 Apr 2014 23:25:05 +0200
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Subject: [TLS] Heartbleed / protocol complexity
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It's kinda surprising that nobody yet started a thread on the biggest
issue in TLS these days on the TLS WG list. So I make a start.

First thought might be "no protocol issue, software bug, not an issue
for the TLS standardization process". But I think there is an issue at
hand here: We have a severe bug in a rarely used TLS extension.

And: This is the second time in just a few days a TLS extension gives
headache. We just had the Dual EC paper exposing possible security
issues with the Extended Random extension (which luckily never came to

I see a number of issues here:
* Heartbeat extension is enabled in cases where it most likely will
  never be needed or used (HTTPS), but it still causes problems. That
  shouldn't be.
* Extensions make the protocol more complex. Complexity adds attack
  surface. Someone recently said "TLS 1.3 sounds like TLS 1.2 minus
  some features". Probably that's a good way to go forward. IMHO the
  lesson learned should be: TLS extensions shouldn't be added
  carelessly, need good justification and shouldn't be overly complex.
  Same goes for extra ciphers and other things that blow up the
  protocol variations.
* Heartbeat adds some completely unneccessary complexity by having a
  payload with an arbitrary length. There's no point in that. Fefe
  wrote something about it (german only):
  (I don't like his name blaming but he has a point on heartbeat and
  Lesson to learn: If it is decided that a new extension is needed it
  should be as simple as possible.


Hanno Böck

mail/jabber: hanno@hboeck.de