Re: [tor-talk] Facebook brute forcing hidden services

[Nick Mathewson <nickm@xxxxxxxxxxxx>]

On Fri, Oct 31, 2014 at 8:54 AM, Roger Dingledine <arma@xxxxxxx> wrote:
> On Fri, Oct 31, 2014 at 12:23:02PM +0000, Mike Cardwell wrote:
>> https://www.facebook.com/notes/protect-the-graph/making-connections-to-facebook-more-secure/1526085754298237
>>
>> So Facebook have managed to brute force a hidden service key for:
>>
>> http://facebookcorewwwi.onion/
>>
>> If they have the resources to do that, what's to stop them brute
>> forcing a key for any other existing hidden service?
>
> I talked to them about this. The short answer is that they did the vanity
> name thing for the first half of it ("facebook"), which is only 40 bits
> so it's possible to generate keys over and over until you get some keys
> whose first 40 bits of the hash match the string you want.
>
> Then they had some keys whose name started with "facebook", and they
> looked at the second half of each of them to decide which one they thought
> would be most memorable for the second half of the name as well. This
> one looked best to them -- meaning they could come up with a story about
> why that's a reasonable name for Facebook to use -- so they went with it.
>
> So to be clear, they would not be able to produce exactly this name
> again if they wanted to. They could produce other hashes that started
> with "facebook", but that's not brute forcing all of the hidden
> service name (all 80 bits).
>
> For those who want to explore the math more, read about the "birthday
> attack":
> https://en.wikipedia.org/wiki/Birthday_attack
>
> And for those who want to learn more (please help!) about the improvements
> we'd like to make for hidden services, including stronger keys and
> stronger names see,
> https://blog.torproject.org/blog/hidden-services-need-some-love

Also, if you're feeling technical, you might want to jump in on
reviewing and improving proposal 224 [prop224] , which include a brand
new, even less usable, but far more secure, name format. :)

[prop224] https://gitweb.torproject.org/torspec.git/blob_plain/HEAD:/proposals/224-rend-spec-ng.txt

cheers,
-- 
Nick
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