[tor-dev] Analysis of the Relative Severity of Tagging Attacks

Watson Ladd watsonbladd at gmail.com
Mon Mar 12 13:18:07 UTC 2012

On Sun, Mar 11, 2012 at 10:45 PM, Robert Ransom <rransom.8774 at gmail.com> wrote:
> On 2012-03-12, Watson Ladd <watsonbladd at gmail.com> wrote:
>> On Sun, Mar 11, 2012 at 8:32 PM, Robert Ransom <rransom.8774 at gmail.com>
>> wrote:
>>> But http://www.cl.cam.ac.uk/~rja14/Papers/bear-lion.pdf and an
>>> end-to-end MAC is more likely as a solution to the end-to-end tagging
>>> attack, because (a) per-hop MACs would take up much more space in each
>>> cell and disclose the length of a circuit to the exit node, and (b)
>>> with per-hop MACs, if you can get a forgery accepted (which happens
>>> with probability 2^(-n), where n is the number of bits in the MAC, for
>>> any MAC that Tor could use), you know with probability 2^(-n) that the
>>> next hop is the last one.
>> You are going to have to be careful and explain this to me. I get the
>> leaking the length of a circuit and position in the chain. But we use
>> length 3 circuits in the current client node all the time, and if you
>> weren't the start or the end, you are the middle. The forgery
>> acceptance probability for Poly1305 is 2^-128. Forgery is not going to
>> happen.
> Non-truncated Poly1305 takes 16 bytes per relay, so it would eat up at
> least 48 bytes per 512-byte cell, and more on 4-hop circuits (which
> Tor clients do build fairly often) and hidden-service rendezvous
> circuits.  Non-truncated Poly1305 is not going to happen.
>> I also don't see what Bear/Lionness gets us. It does solve problems
>> with losing sync. It does so at a cost of determining when identical
>> ORs are sent, which happens a lot: think multiple http requests.
> What do you mean by "ORs"?
I ment cells: brain glitch.
> (The BEAR/LION key would likely be different for each cell that a
> relay processes.)
Different how: if we simply increment the key we still remain open to
replay attacks.
>> Losing semantic security is a Bad Thing. I'll freely admit there are
>> issues with incorporating a leak of circuit length into the protocol,
>> as well as possibly (depending on details of TLS) leaking what lengths
>> end where to a global adversary.
> An end-to-end MAC inside the BEAR/LION wrapper should provide all the
> security properties we need (note that the MAC key would also need to
> be different for each cell).
So we need to include nonces with each cell, which we need to do anyway.
> Robert Ransom
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