Re: [Webpush] ietf-webpush-encryption - untrusted push services

Martin Thomson <martin.thomson@gmail.com> Tue, 03 November 2015 01:21 UTC

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Date: Tue, 3 Nov 2015 10:21:54 +0900
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From: Martin Thomson <martin.thomson@gmail.com>
To: Peter Beverloo <beverloo@google.com>
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Subject: Re: [Webpush] ietf-webpush-encryption - untrusted push services
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On 2 November 2015 at 16:12, Peter Beverloo <beverloo@google.com> wrote:
> We don't trust the intermediary. What if they are the attacker?

I agree, this is the most interesting attack.  Requiring https:// as
we do removes most opportunities for all other attackers.

>     (1) There's a possibility of contributory behaviour with other DH
> groups.

I've done my reading on this now and realize that this is (probably) a
bit of a minefield.

See http://cr.yp.to/ecdh.html#validate
...and for the opposing view:
http://vnhacker.blogspot.jp/2015/09/why-not-validating-curve25519-public.html

The curve25519 draft doesn't reflect any of this nuance, which I find
interesting. I haven't followed the CFRG discussion, but I did find
this thread: http://www.ietf.org/mail-archive/web/cfrg/current/msg06501.html

The concern here is that this protocol depends on contributory
behaviour.  That's a valid concern.  At a minimum, we should consider
having some considerations on this.

A perhaps better approach would be to change the key derivation to
remove the dependency on contributory behaviour.  I need to think
about that a little.  This is a subject that needs to be addressed in
the http working group soon.

> Since P-256 is a prime-ordered group this doesn't apply today, but it would
> be a subtle issue if we were to change curves.

Yes, 25519.

>     (2) The client is vulnerable to DoS attacks.

If the push service is the attacker, then they have much better DoS
options available than what you describe, especially for a
battery-powered device.