October 5, 2013
An NSA presentation released by Edward Snowden contains mixed news for Tor users. The anonymizing service itself appears to have foxed US and UK government snoops, but instead they are using a zero-day flaw in the Firefox browser bundled with Tor to track users.
“These documents give Tor a huge pat on the back,” security guru Bruce Schneier told The Register. “If I was a Tor developer, I’d be really smiling after reading this stuff.”
The PowerPoint slide deck, prepared in June last year and entitled “Tor stinks”, details how the NSA and the UK’s Government Communications Headquarters (GCHQ) have been stymied by trying to track Tor users, thanks to the strength of the open source system.
“We will never be able to de-anonymize all Tor users all the time,” the presentation states. “With manual analysis we can de-anonymize a very small fraction of Tor users, however, no success de-anonymizing a user.”
The presentation says that both the NSA and GCHQ run Tor nodes themselves (the Brits use Amazon Web Services for this under a project entitled Newton’s Cradle), but these are only a very small number in comparison to the whole system. This makes tracking users using traditional signals-intelligence methods impossible.
There’s also a case of diminishing returns as Tor becomes more popular. With each user acting as a transport node, the sheer scale of the system means it becomes steadily more difficult for the intelligence community to run enough nodes to be useful for tracking.
The agencies have also tried to use “quantum” cookies to track targets who are using Tor. Some cookies appear to persist after Tor sessions, the presentation notes, and the agencies are investigating if this can be developed into a working tracking system.
A separate leaked document from GCHQ, published in the Washington Post, gives an indication of how this could be done. Operation Mullenize is a technique for “staining” individual user’s computers with trackable code, and is now being rolled out after a year of development. Over 200 stains were injected onto systems in two months last year, the report notes.
There are also indications that the NSA had been trying to influence the design of Tor to make it more crackable, a somewhat Kafkaesque approach given that Tor is primarily funded by the US government itself to provide anonymity to internet users operating under repressive governments.
The NSA has been accused of this before, having been said to be deliberately weakening NIST encryption standards. But Schneier said in the case of Tor, the agency appears to have had little luck.