[tor-dev] A series of questions about Tor (m1 support, forward secrecy, v3 auth)

hackerncoder hackerncoder at encryptionin.space
Fri Jul 23 23:46:21 UTC 2021

Disclaimer: I do not know that much about Tor, but I can read (and have
read parts of) the specifications

And sorry to Holmes for sending this twice. I'm not used to mailing 
lists, "reply" is right next to (and before, so when running on 
auto-pilot that's the first thing I see) "reply list".

 > Hi everyone,
 > A few disjointed questions that have come up recently in our work 
with Tor:
 > We just got a report from a user that the tor binary for Mac was 
using much more CPU on Apple Silicon / M1 than it used on Intel. Has 
anyone scene anything like this? Is there an arm64 build of tor binary 
for Mac, existing or in the works?
 > (Related: do Tor developers have a few M1 Macs to test on? We could 
probably donate one if not!)
 > Is there a good source for documentation

In general I would point at https://spec.torproject.org, they are
technical and long when you just want info on one specific thing, but is
still good nonetheless. In particular you may find rend-spec-v3
(onion/hidden services, v3) and tor-spec good.

 > on how forward secrecy works in Tor, and on what security guarantees 
it provides? Googling finds things like this reddit post 
(https://www.reddit.com/r/TOR/comments/cryrjx/does_tor_use_pfs/) but I 
can’t find any detailed information about it, what threat models it 
fits, etc.

In tor-spec there point "2" for connections[1], they are made with TLS,
so if supported by both client and relay it may be it is possible that
FS happens here.

 > One specific question is, if two users are communicating by sending 
messages over a connection to an onion service (like ricochet) and an 
attacker surveils their internet traffic and compromises their devices 
at a later date, will the attacker be able to recover the clear text of 
their conversation? When are keys for a given connection destroyed? Does 
it happen continuously throughout the course of a Tor connection? Or on 
the creation of a new circuit? Or what?
 > Does v3 onion authentication protect against DOS attacks? That is, 
can someone who is not authorized to connect to an onion address with 
authentication enabled still cause problems for that onion address? Can 
they connect to it at all, in the sense of being able to send data to 
the tor client at that onion address? Or does the Tor network itself 
prevent this connection from even happening?

No. The rendevouz specifications (v3) [2] make it so only authorized
clients (if enabled) are able to figure out (e.g.) the introduction
points of the onion service, thereby being unable to contact it.

"The second layer of descriptor encryption is designed to protect
descriptor confidentiality against unauthorized clients. If client
authorization is enabled, it's encrypted using the descriptor_cookie,
and contains needed information for connecting to the hidden service,
like the list of its introduction points."

 > A related question is, if we’re looking to deny connections to an 
onion address to any unauthorized users, and we’re considering turning 
off onion authentication and implementing some standard authentication 
scheme that seems fairly well-supported at the web server layer, is 
there any security-related reason why we would be better off using 
Tor’s own authentication instead? Using our own authentication scheme 
will be a bit easier to control, rather than having to send commands to 
Tor (and possibly restart it for removing users?) but I’m wondering if 
there are security properties we lose by doing that >
 > Thanks!
 > Also, apologies if any of these questions aren’t clear or well-formed!
 > Holmes

[1] https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n196
[2] https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n1287

More information about the tor-dev mailing list