6.857 Computer and Network Security November 19, 2002Lecture Notes 21 : TempestLecturer: Ron Rivest Scribe: Giffin/Greenstadt/Plitwack/Tibbetts[These notes come from Fall 2001. These notes are neither sound nor complete. There is morematerial than is covered in lecture, and some is missing. Check your own notes for new topicsbrought up in 2002.]1 TempestMost electronic devices produce electromagnetic radiation when they operate. Can we do anythingwith this radiation?Researchers at Cambridge University (Kuhn and Anderson) set out to answer this question. Theystarted by varying the image on a monitor, causing the electron gun to turn off and on at a specificfrequency and amplitude, and played tunes on a nearby AM radio. This was a standard trick withold computers, varying the processor load to vary the amplitude/frequency.Even this simple trick has a use. If one is able to install a virus or other malicious code into a securefacility, the code could communicate sensitive information to an outsider by using the monitors totransmit on the AM band at night for instance. An estimated 50 bits per second can be transmittedthis way. In England, this could also be used to have televisions broadcast their identities to assistlaw enforcement searching for televisions that have not been properly registered.A more surreptitious attack is to pick up information that is not being deliberately broadcast. Infact, Kuhn and Anderson can read text off of a screen. With most monitors of today, they can readthe screen one-half to one mile away. They have built a functioning apparatus to do this.How do you protect against this attack? There are countermeasures. Tempest fonts, also producedby Kuhn and Anderson, are one solution. These fonts look like one thing on the actual screen andanother thing to remote tempest hardware.Q: Do the fonts have a pattern?A: Yes. Here is the trick:We can make a grey image for human consumption by either having solid grey, or by havingvarying amounts of white and black regions (pixels) at a high frequency. Humans only perceivethe low frequency components of the image. Tempest is confounded by the high frequencycomponents when they are chosen properly.Q: Can’t you just buy an LCD display?0May be freely reproduced for educational or personal use.12 1 TEMPESTA: They claim that LCDs are easier, not harder. You can buy a Tempest-proof monitor, but theyare expensive and no one does.Q: Tempest fonts are just a substitution cipher though, right?A: Yeah, need to solve that problem, but certainly possible.Q: Were these the first people to come up with this idea?A: It’s an old idea. This is the first unclassified paper on the topic. The idea of Tempest fonts isnew.Q: So an O will always look like a C?A: Correct.Q: But shouldn’t you randomize them?A: I suppose.Q: Will more than one monitor cause interference?A: I haven’t tried these things. Maybe a good antenna would help. There is a lot to be done inthis area.Q: Did they publish a specification for the their receiving machine?A: They were hand-wavy about it. But it doesn’t look too hard. They talk a little bit aboutthat. It would be good term project to build one. You would probably need more than a coathanger and an oscilloscope
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