Financial Cryptography

Hide & seek in the terrorist battle

Court cases often give us glimpses of security issues. A court in Britain has just convicted three from the liquid explosives gang, and now that it is over, there are press reports of the evidence. It looks now like the intelligence services achieved one of two possible victories by stopping the plot. Wired reports that NSA intercepts of emails have been entered in as evidence.

According to Channel 4, the NSA had previously shown the e-mails to their British counterparts, but refused to let prosecutors use the evidence in the first trial, because the agency didn’t want to tip off an alleged accomplice in Pakistan named Rashid Rauf that his e-mail was being monitored. U.S. intelligence agents said Rauf was al Qaeda’s director of European operations at the time and that the bomb plot was being directed by Rauf and others in Pakistan.

The NSA later changed its mind and allowed the evidence to be introduced in the second trial, which was crucial to getting the jury conviction. Channel 4 suggests the NSA’s change of mind occurred after Rauf, a Briton born of Pakistani parents, was reportedly killed last year by a U.S. drone missile that struck a house where he was staying in northern Pakistan.

Although British prosecutors were eager to use the e-mails in their second trial against the three plotters, British courts prohibit the use of evidence obtained through interception. So last January, a U.S. court issued warrants directly to Yahoo to hand over the same correspondence.

So there are some barriers between intercept and use in trial. The reason they came from the NSA is probably that old trick of avoiding prohibitions on domestic surveillance: if the trial had been in the USA, GCHQ might have provided the intercepts.

What however was more interesting is the content of the alleged messages. This BBC article includes 7 of them, here's one:

4 July 2006: Abdulla Ahmed Ali to Pakistan Accused plotter Abdulla Ahmed Ali

Listen dude, when is your mate gonna bring the projectors and the taxis to me? I got all my bits and bobs. Tell your mate to make sure the projectors and taxis are fully ready and proper I don't want my presentation messing up.

WHAT PROSECUTORS SAID IT MEANT Prosecutors said that projectors and taxis were code for knowledge and equipment because Ahmed Ali still needed some guidance. The word "presentation" could mean attack.

The others also have interesting use of code words, such as Calvin Klein aftershave for hydrogen peroxide (hair bleach). The use of such codes (as opposed to ciphers) is not new; historically they were well known. Code words tend not to be used now because ciphers cover more of the problem space, and once you know something of the activity, the listener can guess at the meanings.

In theory at least, and code words clearly didn't work to protect the liquid bombers. Worse for them, it probably made their conviction easier, because Muslims discussing the purchase of 4 litres of aftershave with other Mulsims in Pakistan seems very odd.

One remaining question was whether the plot would actually work. We all know that the airlines banned liquids because of this event. Many amateurs have opined that it is simply too hard to do liquid explosives. However, the BBC employed an expert to try it, and using what amounts to between half a litre to a liter of finished product, they got this result:

Certainly a dramatic explosion, enough to kill people within a few metres, and enough to blow a 2m hole in the fuselage. (The BBC video is only a minute long, well worth watching.)

Would this have brought down the aircraft? Not necessarily as there are many examples of airlines with such damage that have survived. Perhaps if the bomb was in a strategic spot (over wing? or near the fuel lines?) or the aircraft was stuck over the Atlantic with no easy vector. Either way, a bad day to fly, and as the explosives guy said, pity the passengers that didn't have their seat belt on.

Score one for the intel agencies. But the terrorists still achieved their second victory out of two: passengers are still terrorised in their millions when they forget to dispose of their innocent drinking water. What is somewhat of a surprise is that the terrorists have not as yet seized on the disruptive path that is clearly available, a la John Robb. I read somewhere that it only takes a 7% "security tax" on a city to destroy it over time, and we already know that the airport security tax has to be in that ballpark.