teor teor2345@gmail.com writes:
On 20 Sep 2017, at 00:44, George Kadianakis desnacked@riseup.net wrote:
Legacy RENDEZVOUS1 cells are bigger than the prop224 ones. The prop224 spec suggests we pad the new cells so that they look similar in size to the legacy ones.
...
The suggestion is to pad the prop224 cells to 168 bytes using random data.
Would that work? My main question is whether the g^y part of the legacy cell has any distinguishers that could distinguish it from random data. It's encoded using OpenSSL's BN_bn2bin() and it's a 1024 bit DH public key. Are there any algebraic or openssl structure distinguishers we should be worrying about, or is random data sufficient to mask it out?
What's the threat model here?
I ask because regardless of whether the RENDEZVOUS1 cell plaintext is distinguishable between v2 and v3, the rend point can distinguish v2 and v3 using this one neat trick:
- if the service extends using TAP, the protocol is v2
- if the service extends using ntor, the protocol is v3
Thanks for the discussion and research, Ian and teor!
I summarized the findings here: https://trac.torproject.org/projects/tor/ticket/23420#comment:5
Not sure what's the right approach here.
Perhaps I'm fine with doing nothing at this point, and figuring this out in the future if v4 ever comes.
Cheers!