A First-Hop Traffic Analysis Attack Against Tor

We introduce an attack against encrypted web traffic passing over the first hop – between client and Tor gateway – of the Tor network. The attack makes use only of packet timing information on the uplink and so is impervious to packet padding defences. In addition, we show that the attack is robust against the randomised routing used in Tor. We demonstrate the effectiveness of the attack at identifying the web sites being visited by a Tor user, achieving mean success rates of 68%. As well as being of interest in its own right, this timing-only attack serves to highlight deficiencies in existing defences and so to areas where it would be beneficial for Tor and VPN designers to focus further attention.