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Re: [TLS] TLS client puzzles

Christian Huitema <huitema@microsoft.com> Thu, 30 June 2016 04:42 UTC

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From: Christian Huitema <huitema@microsoft.com>
To: Geoffrey Keating <geoffk@geoffk.org>, Kyle Rose <krose@krose.org>
Thread-Topic: TLS client puzzles
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Date: Thu, 30 Jun 2016 04:42:02 +0000
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Cc: "<tls@ietf.org>" <tls@ietf.org>, Dmitry Khovratovich <khovratovich@gmail.com>
Subject: Re: [TLS] TLS client puzzles
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On Wednesday, June 29, 2016 7:37 PM, Geoffrey Keating wrote:
> 
> Kyle Rose <krose@krose.org> writes:
> 
> > Let's finish that last sentence:
> >
> > I have to think a lot more about the IoT/resource-constrained client
> > problem, but I still don't think the existence of clients that would be
> > denied service by this scheme renders the concept completely inapplicable.
> 
> Perhaps for the resource-constrained/IoT situation, what you want is
> to mostly be using a pre-shared key, track clients, and only make the
> suspicious (too many connections too fast) or new ones solve the
> puzzle.

I am also a little worried that this can become a mechanism to DOS the clients. Imagine an adversary capable of redirecting a client request, either by hacking the DNS or hacking a proxy. The hacked server can send the client a puzzle, and force the client into spending some energy. And with the proposed extension, all that happens before the client has a chance to authenticate the server.

-- Christian Huitema

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