[tor-dev] Sybil attack detection (was: Karsten's July 2014)

Andrea Shepard andrea at torproject.org
Wed Aug 6 01:59:41 UTC 2014

On Tue, Aug 05, 2014 at 11:24:32AM -0400, Philipp Winter wrote:
> On Tue, Aug 05, 2014 at 11:37:45AM +0200, Karsten Loesing wrote:
> > Started looking into better algorithms to detect Sybil attacks on the
> > Tor network.  Current thinking is that we should define relay similarity
> > metrics like common IP address prefix length or time between first seen
> > in a consensus, go throw the consensus archives, and see which relays
> > look similar but are not defined to be in the same family.
> Do you already have some code or more details on that?  I'm quite
> interested in this topic and I'm wondering if it wouldn't be best to
> start with something simple like cosine similarity [0].  We would have
> to transform a relay descriptor into a numerical vector and then
> calculate the cosine similarity to other relay vectors.  However, this
> might scale poorly as we would need (n^2)/2 comparisons.
> We might also want to weigh the vector's elements differently as some of
> them are easy to control for an attacker (advertised bandwidth, uptime,
> flags, exit policy) and others require more effort (IP address, ASN,
> observed bandwidth).  Like you mentioned, the key thing to look at might
> be time, i.e., uptime and derived features such as "total online time in
> last month" etc.
> [0] https://en.wikipedia.org/wiki/Cosine_similarity

You only need O(n*log(n)) if you can define any similarity metric that
respects the triangle inequality.  There's a lot of research on data
structures for this:


Andrea Shepard
<andrea at torproject.org>
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