A wide range of people use Tor for daily Internet access. Individuals who don't want their ISP to collect information on the websites they visit, perhaps because they are seeking information on sensitive topics, use Tor. Government agents and the military use Tor. After all, it's difficult to go undercover online if the IP address you are connecting FROM resolves to fbi.gov. Tor is used by whistleblowers, such as Edward Snowden, to disclose information to the press. It is also used by normal citizens when a government blocks access to the Internet. In late March 2014, when the Turkish government attempted to ban access to Twitter, the number of Tor users in Turkey jumped from around 25,000 to just under 70,000 in about two weeks.

Tor relays are the individual nodes in the Tor network. A node is some type of computer that is running the Tor software. These relays route messages throughout the network. Exit nodes, which are the last nodes in the circuit, send the clients' messages to the destination service. One of the interesting aspects of Tor is that most of the nodes are run by volunteers. These relay operators volunteer their bandwidth, time, and energy bill to increase the capacity of the Tor network. The Tor Project encourages individuals to run relays in order to add to the diversity of the network. The motivation behind this recommendation is that it is easier to hide in a crowd—it's difficult to hide in a crowd of one person.

In 2006, in response to an increased Internet censorship, the Tor project added an anticensorship feature called a bridge relay or simply, a bridge (Dingledine, 2006). A bridge is a special form of a relay—it's not listed in the public relay directory. Strong Internet censors were able to block access to Tor through several methods. One method that was effective was to look up all the public relays, which were only a thousand or so at the time, and deny access to those IP addresses. Therefore, bridge relays were created and distributed by less public means in order to allow users to access the Tor network. If a client could access the bridge, which was already connected to Tor, then the client could access the Internet. In this context, less public means a distribution mechanism that does not work via the public relay list.

Censors were still able to block access to Tor; the technique was to identify the traffic patterns generated by Tor and block the connection. In 2010, Jacob Appelbaum and Nick Mathewson of the Tor Project suggested a method to obfuscate the Tor traffic between the client and the bridge in order to thwart deep packet inspection. As the obfuscation mechanism might need to change, and has changed in fact, the Tor Project wanted a generic protocol to allow different obfuscated proxies to run. This abstraction was known as pluggable transport.

In this chapter, we will set up your BBB to run as a Tor bridge, running an obfuscated proxy, using the obfs3 pluggable transport.

Tor is one of the best tools currently available to protect anonymity. However, like all the security tools discussed in this book, there are limits to its protection. First of all, Tor's threat model does not hold up to a global passive adversary. Such an adversary can passively monitor the entire Internet. If an adversary can monitor the entire Internet, then it can correlate the traffic entering the Tor network with the traffic leaving the network and can possibly deanonymize Tor clients. One of the trade-offs of this design is that Tor is a low-latency system, meaning that you can access the Internet using normal protocols such as HTTP without experiencing much delay. This is one of the reasons why diversity in the Tor network is important. If all the relays were in a particular country, it might be easier for that country to monitor the traffic. However, currently it is thought to be difficult, in spite of the recent leaks provided by Edward Snowden, for such an adversary to monitor the entire Internet.

Also, Tor will not automatically encrypt all your traffic. If you request information over unencrypted HTTP, the Tor exit node you use will relay this unencrypted information to its destination. A malicious exit node can monitor or manipulate your traffic; therefore, it's always best to use encrypted sessions, such as HTTPS, over Tor. Tor does not protect all your traffic just because you are running Tor. Applications generally need to be configured to use Tor. Even if you set up a transparent proxy on your home network and route all of your traffic through that proxy, the malware or exploits in your browser may leak your identity. This is why the Tor Project recommends that you use the Tor Browser, which essentially is a forked version of Mozilla Firefox that has been patched especially to not leak your identity. Lastly, Tor can't protect your identity if you choose to reveal it. If you decide to log in to Facebook over Tor, you've just told Facebook who you are and that you are using Tor since all the Tor exit nodes are public.

So, why run a Tor bridge on BBB? The impact and benefits of Tor is in the network. The more the Tor servers, the more the resources in the network. Many users in developed nations have high-speed Internet connections that are orders of magnitude faster than in countries where access to censor-free Internet is restricted. A bridge is likely to receive less traffic than a relay, as there are fewer bridge users than normal Tor users. Most likely, the limiting performance factor for your bridge will be your home network's upload speed. This can be a constraining factor if you are running a relay, but as a bridge, you are most likely helping those in precarious Internet situations and any donated bandwidth is appreciated. Lastly, running a bridge on BBB has the extra advantage of a low impact on your electric bill, as BBB will only draw about 460mA when loading a web page according to the BeagleBone Black System Reference Manual.