One such technique is based on trying to spot patterns in the signals entering and leaving the Tor network, to try to de-anonymise its users. The effort was based on a long-discussed theoretical weakness of the network: that if one agency controlled a large number of the "exits" from the Tor network, they could identify a large amount of the traffic passing through it.
The proof-of-concept attack demonstrated in the documents would rely on the NSA's cable-tapping operation, and the agency secretly operating computers, or 'nodes', in the Tor system. However, one presentation stated that the success of this technique was "negligible" because the NSA has "access to very few nodes" and that it is "difficult to combine meaningfully with passive Sigint".
While the documents confirm the NSA does indeed operate and collect traffic from some nodes in the Tor network, they contain no detail as to how many, and there are no indications that the proposed de-anonymization technique was ever implemented.
Other efforts mounted by the agencies include attempting to direct traffic toward NSA-operated servers, or attacking other software used by Tor users. One presentation, titled 'Tor: Overview of Existing Techniques', also refers to making efforts to "shape", or influence, the future development of Tor, in conjunction with GCHQ.
Another effort involves measuring the timings of messages going in and out of the network to try to identify users. A third attempts to degrade or disrupt the Tor service, forcing users to abandon the anonymity protection.
Such efforts to target or undermine Tor are likely to raise legal and policy concerns for the intelligence agencies.
Foremost among those concerns is whether the NSA has acted, deliberately or inadvertently, against internet users in the US when attacking Tor. One of the functions of the anonymity service is to hide the country of all of its users, meaning any attack could be hitting members of Tor's substantial US user base.
Several attacks result in implanting malicious code on the computer of Tor users who visit particular websites. The agencies say they are targeting terrorists or organized criminals visiting particular discussion boards, but these attacks could also hit journalists, researchers, or those who accidentally stumble upon a targeted site.
The efforts could also raise concerns in the
Material published online for a discussion event held by the
"[T]he technologies of internet repression, monitoring and control continue to advance and spread as the tools that oppressive governments use to restrict internet access and to track citizen online activities grow more sophisticated. Sophisticated, secure, and scalable technologies are needed to continue to advance internet freedom."
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