On Mon, Feb 15, 2010 at 12:00:27PM +0100, Runa Sandvik wrote: > On Mon, Feb 15, 2010 at 11:25 AM, Sameer Ali <[email protected]> wrote: > > hi all, > > Hello, > > > I am new and start research in the field of anonymous communication. Could > > someone tell me please, why TOR use fixed size cell (all cells have same > > length)? > > Your question was answered by Mauricio Pasquier in another thread that > you started. See > http://archives.seul.org/or/talk/Feb-2010/msg00064.html
Mauricio's answer is "fixed size cells helps against traffic analysis because your data is indistinguishable from anyone's else data." More generally, we use fixed-size cells because we haven't given up on the hope that somebody will hand us a cool trick that makes end-to-end correlation attacks meaningfully slower. Right now we assume they're instantaneous in all circumstances, but maybe that won't be true forever. (For more background on end-to-end correlation, see this blog post: https://blog.torproject.org/blog/one-cell-enough ) We could probably switch to variable-sized cells (we even have them in the spec and use them in a few cases). It's looking less likely that somebody will figure out how to secure low-latency anonymity systems against end-to-end correlation. But I haven't totally given up hope. :) Also, it looks like Tor's cell chunking assists us to some extent against website fingerprinting attacks: http://freehaven.net/anonbib/#ccsw09-fingerprinting So it may be useful to keep it even if we don't expect defense against end-to-end correlation. Or maybe the latest website fingerprinting attacks could become much better if they take into account more of Tor's architecture during the attack. Hard to say. --Roger *********************************************************************** To unsubscribe, send an e-mail to [email protected] with unsubscribe or-talk in the body. http://archives.seul.org/or/talk/
