Blown To Bits

Millimeter wave scans = privacy infringement?

Tuesday, December 29th, 2009 by Harry Lewis

The recent attempt by a Nigerian man to blow up a plane flying into Detroit has brought the subject of millimeter wave scans back into public discussion. These scans use very short-wave radio signals to peek through people’s clothing and see what they may have underneath. Some privacy advocates resist the use of these devices, because they show genitalia, as well as revealing breast implants and so on.

Maybe I am missing something, but I can’t get excited about the fact that a security screener might get a glimpse of an X-ray like image of my private parts in the course of verifying that I wasn’t hiding some explosives there (as the alleged terrorist apparently was). It may not be useful or effective to screen everyone–maybe you’d do some obvious profiling (bought the ticket with cash, etc.) to reduce the workload on the screeners and keep them sharper. But if the image isn’t stored, I don’t see any privacy problem in principle here. In enlightened societies at least, we have mostly gotten past prudery in medical care–not many hospital patients would today insist on having their bedpans emptied only by same-sex attendants. If you want to use the technology of air travel, you need to accept the technology of security (provided, once again, that it really is security-enhancing and not just in place to create a phony sense of security).

By the way, the TSA hasn’t yet fixed the huge security hole, pointed out by Chris Soghoian several years ago, that they check the boarding pass against your ID at the security perimeter and the boarding pass against the electronic ticket record at the gate, but never verify that the ticket matches your ID, unless you check a bag. If you are not checking luggage, the two boarding passes could be different.

Privacy bonus: Canada’s Daily Post has an article about privacy loss, which quotes Blown to Bits and ends with a Christmas-spirit thought that sprung into my head when I was interviewed last week:

Harry Lewis, a professor of computer science at Harvard and co-author of Blown to Bits, said the book was written to get people thinking about how much of their personal information they surrender every day. He worries that the less privacy we enjoy, the more it will discourage social advances.

“The loss of privacy is a socially conforming force,” he said in an interview. “So many social experiments over the course of human history — religious innovations, political dissent — started among small groups of mutually trusted friends who gradually gained acceptance for their beliefs and their behaviours.”

If Jesus’s early followers had a Facebook group, he joked, “they would have been stamped out very quickly.”

5 Responses to “Millimeter wave scans = privacy infringement?”

  1. elad Says:

    “Maybe I am missing something, but I can‚Äôt get excited about the fact that a security screener might get a glimpse of an X-ray like image of my private parts in the course of verifying that I wasn‚Äôt hiding some explosives there”

    Well, it’s very nice that you’re not excited about it, and if Millimeter wave scanning was a voluntary procedure, that would even be relevant. Some people are opposed to it, and consider it an infringement of their privacy. I think we can all understand why that’s a reasonable position, even if you in particular are not bothered by it.

    Furthermore, the comparison to hospitals is a non-sequitor. Hospitals are not governmental organizations (or whatever TSA is) that have authority to use weapons and to incarcerate people. TSA has also already shown blatant disregard for privacy, rights, and the law. Giving them access to even more force than they have now is irresponsible and potentially dangerous. “Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither”. The American government and the TSA have shown so far that they cannot be trusted to act in the people’s best interests. The only democracies worse than the US in these regards seem to be the UK and possibly Australia. In such a situation, Millimeter wave scans do more harm then good. It’s not long from now until various authorities will find a way to misuse them.

  2. Harry Lewis Says:


    I’respect the right of those who wish to protect privacy that I am willing to sacrifice — up to the point where they want to endanger my life by getting on the same plane with me and bringing their like-minded friends with them. If we could get special planes where all the no-mm-wave-scan folks could fly together, that would solve everybody’s problem. That doesn’t seem likely, alas.

    Of course, my attitude is premised on the hypothesis that these scans would actually make flying safer. There is surely room for skepticism about that, especially if there is such fear of giving offense that information will be ignored that could usefully identify high-risk subpopulations.

  3. Jeff Collier Says:

    Professor Lewis,

    I have two problems with your proposal,

    First, wrt your condition that the images not be stored, there are two problems. One of the premises that Bits underscores again and again is that once information is out there, it cannot be retrieved. Mostly that premise applies to the internet and digitized material. But I ask: What’s to keep an unscrupulous scanner operator from selling the information to the tabloids that so-and-so has implants? Even if the idiot cannot provide the images, once it is said, the burden shifts to so-and-so to prove that they’re real. So, the relative privacy of particular information is not dependent on the storage of images.

    And you presume that once the procedure is in place that images will never be stored. The first (or perhaps second) missed scan and downed plane will erode this protection. Screams for scanner accountability will force TSA and others to keep a scan for some minimal period to insure that they can be reviewed in case of mishap. At that point, with the investment in the technology and the screams for accountability, I doubt the privacy advocates will stand a chance.

    [As an aside, I really deplore the cynical and devious tactic used by many in U.S. politics of getting the general populace to swallow something in pieces that they never would whole by getting them onto the slippery slope and then giving a good shove. It is a tactic becoming far too common, but that is a rant for another day…]

    wrt prudery in hospitals and privacy, hospitals operate with much greater liability than the TSA seems to.

    But despite all of this, I’m with you. It would seem that millimeter wave scanning gets past the “security theatre” that carry-on restrictions don’t seem to be providing. And I’m completely befuddled about what in-flight movement restrictions are going to accomplish. What I would do is create federal penalties for operators who leak private information. They should lose their jobs, their pensions, probably their house and maybe their freedom.

    Not so amazingly, this new technology illuminates another premise of the book: that people often revisit past decisions in light of new technology. I’m sure that non of the glamourazzi from the last millennium gave two seconds of thoughts to that subway scene from Total Recall when they were getting done.

  4. Harry Lewis Says:


    I resist slippery-slope arguments. The scans are just not that valuable, either to peeping toms or to finger-pointers after a failure, to make me worry about your what-ifs. And my hospital point was not about the behavior of the employees but that of the patients. Probably in Saudi Arabia they do have same-sex bed-pan changers because of modesty standards. We in the US don’t, because we accept that professionals will not behave in a sexual way. We shouldn’t bow to the modesty extremists on the scans either, the people who worry about dirty old men peering at security scans.

    Having read more from Bruce Schneier today, I think I understand his point of view. I think he is trying to hammer home the point that Americans love technological silver bullets. They will happily accept a magic kryptonite shield around the airplane that supposedly no terrorist could pass through, no matter how improbable it is that such a thing could work. What they won’t accept is a human judgment, even one based on expert data analysis, that one person might be a terrorist but another not. To the extent he is fighting against American technological utopianism, I am with him. I just think he takes his argument too far if it prevents the use of scanners.

  5. Leo Reyzin Says:

    Well, this is about five months too late, but the ACLU has an interesting entry in its blog about the body scanners:

    Apparently, professionalism isn’t quite as high down at the security checkpoint as one might wish, and the grainy scans of famous people or people one knows may well be more exciting than stuff on the internet.