Re: [tor-dev] Is Taking Checksum of Packet Payloads a Vulnerability?

Note that the data sent from Alice to En is encrypted with a key only they share, rendering this attack impossible.

On Dec 17, 2011 11:25 AM, "Daniel Cohen" <danielc192@xxxxxxxxx> wrote:

Hi,

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%Âchance ofÂcollision. 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?

Thanks,

Daniel Cohen

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