[tor-talk] Is Taking Checksum of Packet Payloads a Vulnerability?

Daniel Cohen danielc192 at gmail.com
Sat Dec 17 16:49:21 UTC 2011


I am new to Tor, but after reading about its design, and reading a few
research papers on its vulnerabilities (specifically timing attacks),
I had the following thought:

Suppose Alice is connecting to Bob via Tor, using HTTPS encryption.
She sends a packet to the Tor entry node (call it En). The packet
travels through the network, emerges from an exit node (call it Ex),
and arrives at Bob.

Alice => En => Tor Network => Ex => Bob

Now suppose that Alice's connection is being monitored, as well as a
group of the exit nodes (which are either hostile or having their
packets sniffed). When the encrypted packet leaves Alice on its way to
En, it is sniffed, and a checksum is made of its encrypted payload.
The packet then continues through the network as usual, and emerges
from an exit node.

It appears to me that the attacker need only check packets coming out
of exit nodes to see if their payload checksums match that of the
packet observed leaving Alice. Unlike timing attacks, which require a
reasonable number of packets to confirm Alice's identity, this attack
would require only one, since checksums have an almost 0% chance of
collision. If a packet with the same payload checksum as Alice's is
discovered, it almost certainly originated from her.

Is this a problem with Tor's architecture? If so, has this issue
already been addressed?


Daniel Cohen

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