Re: [TLS] analysis of wider impact of TLS1.3 replayabe data

On Sun, Mar 13, 2016 at 8:51 PM, Colm MacCárthaigh <colm@allcosts.net>
wrote:

> On Sun, Mar 13, 2016 at 4:14 AM, Stephen Farrell <
> stephen.farrell@cs.tcd.ie> wrote:
>
>> With 0rtt, I think it also becomes a dangerous
>> implement. So, that's my personal opinion, while not wearing an
>> AD hat.
>>
>
> +1 for this as 0RTT is outlined in the draft. But to expand a little:
>
> * Losing forward secrecy for "GET" requests and user cookies seems like a
> very bad privacy trade-off. Collection of wire data, combined with a bug or
> an attack that could disclose the ECDH parameters, does not seem so far
> fetched these days.
>
> * I imagine that 0RTT data may be intended for pre-fetching hints, e.g.
> "I'm request URL foo as user bar, so please get a start on fetching the
> resources I need". That suggests trivial application-level "is the cache
> warm" side channels, or "lets trash the whole cache" DOS attacks, would
> both be common.
>
> * A vast number of APIs build on top of TLS/HTTPS are not idempotent or
> replay safe. Even carefully designed APIs may have subtle side effects such
> as a user being charged for the request, or the request counting towards a
> throttle of some kind. Even ones as silly as there go your 10 free nytimes
> articles; or booting an opponent out of your less-than-friendly sudoku
> tournament.
>
> * The draft calls out that early data needs to be handled differently than
> regular TLS data. This reminds me of the problems with renegotiatable
> client authentication - similar to what Marsh Ray highlighted a long time
> ago. What if a request or action spans both the early and the regular data
> sections? what would a "safe" API look like, for an SSL/TLS library layer?
>
> It seems like the benefits of of 0-RTT boil down to two things;
>
> 1. Increased throughput
> 2. Give the recipient a chance to a get a head-start on servicing some
> kind of request.
>
> For 1. Raw data throughput could be improved by envelope encrypting the
> early data; and transferring the envelope key only once the session has
> been fully authenticated
>

Can you expand on this. Cryptographically, this is effectively how both the
DHE and PSK
modes work: you get a key in session N (which is authenticated) and you use
it
in session N+1.

-Ekr

. Not that interesting to most people; but works. I can't think of a good
> way to achieve 2 without all of the problems that have been mentioned.
>
> At a minimum I'd suggest that the draft could not call it "data", but
> maybe instead "hint"; and make it so that early_data and application_data
> are not supposed to appear sequentially in a stream. E.g. someone calling
> TLSRead() or whatever should never expect to see the early_data; it's a
> deliberate and separate stream.
>

> --
> Colm
>
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