may also collect tactical communications intelligence in the context of a criminal investigation
by wiretapping the phones of particular suspects or by instructing their ISPs to copy IP traffic
to an analysis facility.
Similarly, censorship can be strategic or tactical. Strategic censorship may include permanent blocking of porn sites, or of news sites such as the BBC and CNN; this may be done at
the DNS level or by blocking a range of IP addresses. An example of tactical censorship might
be interference during an election with the Web servers of an opposition group; this might be
done by a service-denial attack or some other relatively deniable technique.
Censorship systems interact in various ways with other types of filtering. Where communications are decentralized, for example, through many blogs and bulletin boards, the censor
may use classic communications-intelligence techniques such as traffic analysis and snowball
sampling in order to trace sites that are candidates for suppression. (Snowball sampling refers
to tracking a suspect’s contacts and then their contacts recursively, adding suspects as a
snowball adds snow when rolling downhill.) Countersurveillance techniques may therefore become part of many censorship resistance strategies.
The interaction between censorship and surveillance is not new. During the early 1980s, the
resistance in Poland used radios that operated in bands also used by the BBC and Voice of
America; the idea was that the Russians would have to turn off their jammers in order to use
radio-direction finding to locate the dissidents. Today, many news sites have blogs or other
facilities that third parties can use to communicate with each other; so if a censor is reluctant
to jam The Guardian newspaper, then its dissidents could use blog posts on The Guardian
site to talk to each other, using pseudonyms. But many of the novel and interesting interactions have to do with applications.
Discussion
Communication is now a part of more and more applications. Some of these are designed
for communication, such as Skype, but bring new capabilities; in Skype’s case, it provides
encrypted communications and is also widely used. Previously, users of cryptography would
be likely to draw attention to themselves, especially in authoritarian countries. Today, Skype
and other voice-over IP (VoIP) products are used to save money on telephone bills, and provide voice privacy as a side effect.
Another example is given by Google Docs & Spreadsheets. Google purchased an online
word-processor product (Writely) and now makes it available to Internet users in many countries as Google Docs & Spreadsheets. People keep their private documents on Google’s
servers and edit them online via a Web-based interface. Such a document can be shared
instantly with other users; this provides a convenient channel for communications. In this
case, the communications are the side effect; the reason people use Google Docs & Spreadsheets is to avoid spending money on Microsoft Office.
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Steven J. Murdoch and Ross Anderson
by wiretapping the phones of particular suspects or by instructing their ISPs to copy IP traffic
to an analysis facility.
Similarly, censorship can be strategic or tactical. Strategic censorship may include permanent blocking of porn sites, or of news sites such as the BBC and CNN; this may be done at
the DNS level or by blocking a range of IP addresses. An example of tactical censorship might
be interference during an election with the Web servers of an opposition group; this might be
done by a service-denial attack or some other relatively deniable technique.
Censorship systems interact in various ways with other types of filtering. Where communications are decentralized, for example, through many blogs and bulletin boards, the censor
may use classic communications-intelligence techniques such as traffic analysis and snowball
sampling in order to trace sites that are candidates for suppression. (Snowball sampling refers
to tracking a suspect’s contacts and then their contacts recursively, adding suspects as a
snowball adds snow when rolling downhill.) Countersurveillance techniques may therefore become part of many censorship resistance strategies.
The interaction between censorship and surveillance is not new. During the early 1980s, the
resistance in Poland used radios that operated in bands also used by the BBC and Voice of
America; the idea was that the Russians would have to turn off their jammers in order to use
radio-direction finding to locate the dissidents. Today, many news sites have blogs or other
facilities that third parties can use to communicate with each other; so if a censor is reluctant
to jam The Guardian newspaper, then its dissidents could use blog posts on The Guardian
site to talk to each other, using pseudonyms. But many of the novel and interesting interactions have to do with applications.
Discussion
Communication is now a part of more and more applications. Some of these are designed
for communication, such as Skype, but bring new capabilities; in Skype’s case, it provides
encrypted communications and is also widely used. Previously, users of cryptography would
be likely to draw attention to themselves, especially in authoritarian countries. Today, Skype
and other voice-over IP (VoIP) products are used to save money on telephone bills, and provide voice privacy as a side effect.
Another example is given by Google Docs & Spreadsheets. Google purchased an online
word-processor product (Writely) and now makes it available to Internet users in many countries as Google Docs & Spreadsheets. People keep their private documents on Google’s
servers and edit them online via a Web-based interface. Such a document can be shared
instantly with other users; this provides a convenient channel for communications. In this
case, the communications are the side effect; the reason people use Google Docs & Spreadsheets is to avoid spending money on Microsoft Office.
70
Steven J. Murdoch and Ross Anderson