Re: [integrity]: latency tradeoffs

On Wed, Jan 15, 2014 at 7:21 AM, Adam Langley <agl@google.com> wrote:

> On Wed, Jan 15, 2014 at 4:16 AM, Mike West <mkwst@google.com> wrote:
> > 1. Performance isn't the goal. Integrity is the goal.
>
> Not quite, right? If integrity were the only goal then HTTPS provides
> that. I'm assuming that the desires motivating this are things like
> minimising backbone traffic (i.e. ISP caching), improving response
> times (i.e. using CDNs without fully trusting them).
>
 At the very least, performance would be a wonderful side effect, which is
what I view the entire caching portion about.

I also think that the latency point is that if we do this wrong, the
integrity check could provide a high latency cost for large files, so we
should want to reduce the latency even if we don't care about improving
performance over the status quo.

>
> > 2. I think the performance benefits of integrity would be focused on
> cache.
> > That is, the second load of a resource, regardless of its URL, could
> avoid
> > hitting the network entirely if we already have a matching resource
> locally.
> > For this case, we have the whole resource already, by definition.
>
> HTTPS resource can be cached equally well. If you're talking about
> using the cache as a content-addressable storage then I think that's
> too dangerous to allow. (As explored a little in "Origin confusion
> attacks". Also, talk to abarth about this.)
>
> Even if we do assume CAS behaviour, I don't believe that the metrics
> support this optimism. You should check with willchan and rvargas
> about observed disk cache behaviours.
>
> > I think this is problematic in most (all?) cases, given the nature of the
> > threat we're attempting to address. Trusting the resource to authenticate
> > itself doesn't provide much benefit if we're not sure we can trust the
> > resource in the first place.
>
> We're not trusting the resource to authenticate itself, but rather we
> are spreading the authentication data throughout the download itself
> in order to minimise processing latency. It's still the case that
> everything is authenticated by the single hash in the (trusted) HTML
> document.
>
> It's unclear whether the administrative overhead of doing this
> outweighs the latency advantages at this point.
>
>
> Cheers
>
> AGL
>
>