In my last post, I had promised to say more about my article on the limits of anonymization and the power of reidentification. Although I haven’t said anything for a few weeks, others have, and I especially appreciate posts by Susannah Fox, Seth Schoen, and Nate Anderson. Not only have these people summarized my article well, they have also added a lot of insightful commentary, and I commend these three posts to you.
Today brings news relating to one of the central examples in my paper: Netflix has announced plans to commit a privacy blunder that could cost it millions of dollars in fines and civil damages.
In my article, I focus on Netflix’s 2006 decision to release millions of records containing the movie rating preferences of “anonymized” users to the public, in order to fuel a crowd-sourcing competition called the Netflix Prize. The Netflix Prize has been a huge win for Netflix’s public relations, but it has also been a win for academics, who have used the data to improve the science of guessing human behavior from past preferences.
The Netflix Prize was also a watershed event for reidentification research because Arvind Narayanan and Vitaly Shmatikov of U. Texas revealed that they could reidentify some of the “anonymized” users with ease, proving that we are more uniquely tied to our movie rating preferences than intuition would suggest. In my paper, I argue that we should worry about this privacy breach even if we don’t think movie ratings are terribly sensitive, because it can be used to enable other, more terrifying privacy breaches.
I never argue, however, that Netflix deserves punishment or sanction for having released this data. In my opinion, Netflix acted pretty responsibly. It consulted with computer scientists in a (failed) attempt to anonymize successfully. It tried perturbing the data in order to make reidentification harder. And other experts seem to have been surprised by how easy it was for Narayanan and Shmatikov to reidentify. Even with the benefit of hindsight, I find nothing to blame in how Netflix handled the privacy implications of what it did.
Although I give Netflix a pass for its past privacy breach, I am astonished to learn from the New York Times that the company plans a second act:
The new contest is going to present the contestants with demographic and behavioral data, and they will be asked to model individuals’ “taste profiles,” the company said. The data set of more than 100 million entries will include information about renters’ ages, gender, ZIP codes, genre ratings and previously chosen movies. Unlike the first challenge, the contest will have no specific accuracy target. Instead, $500,000 will be awarded to the team in the lead after six months, and $500,000 to the leader after 18 months.
Netflix should cancel this new, irresponsible contest, which it has dubbed Netflix Prize 2. Researchers have known for more than a decade that gender plus ZIP code plus birthdate uniquely identifies a significant percentage of Americans (87% according to Latanya Sweeney’s famous study.) True, Netflix plans to release age not birthdate, but simple arithmetic shows that for many people in the country, gender plus ZIP code plus age will narrow their private movie preferences down to at most a few hundred people. Netflix needs to understand the concept of “information entropy”: even if it is not revealing information tied to a single person, it is revealing information tied to so few that we should consider this a privacy breach.
I have no doubt that researchers will be able to use the techniques of Narayanan and Shmatikov, together with databases revealing sex, zip code, and age, to tie many people directly to these supposedly anonymized new records.
Because of this, if it releases the data, Netflix might be breaking the law. The Video Privacy Protection Act (VPPA), 18 USC 2710 prohibits a “video tape service provider” (a broadly defined term) from revealing “personally identifiable information” about its customers. Aggrieved customers can sue providers under the VPPA and courts can order “not less than $2500” in damages for each violation. If somebody brings a class action lawsuit under this statute, Netflix might face millions of dollars in damages.
The good news is Netflix has time to avoid this multi-million dollar privacy blunder. As far as I can tell, the Netflix Prize 2 has not yet been launched.
Dear Netflix executives: Don’t do this to your customers, and don’t do this to your shareholders. Cancel the Netflix Prize 2, while you still have the chance.