Debunking Google's log anonymization propaganda

Company says it will be reduce the amount of time that it will keep sensitive, identifying log data on its search engine customers. This is little more than snake oil.

Chris Soghoian
Chris Soghoian
Christopher Soghoian delves into the areas of security, privacy, technology policy and cyber-law. He is a student fellow at Harvard University's Berkman Center for Internet and Society , and is a PhD candidate at Indiana University's School of Informatics. His academic work and contact information can be found by visiting www.dubfire.net/chris/.
4 min read

Google announced on Monday that the company will be reducing the amount of time that it will keep sensitive, identifying log data on its search engine customers. To the naive reader, the announcement seems like a clear win for privacy. However, with a bit of careful analysis, it's possible to see that this is little more than snake oil, designed to look good for the newspapers, without delivering real benefits to end users.

In a post to the company blog on Monday, the company announced that it will be significantly reducing the amount of time that it hangs onto identifying user data in its Web server logs:

Today, we're announcing a new logs retention policy: we'll anonymize IP addresses on our server logs after 9 months. We're significantly shortening our previous 18-month retention policy to address regulatory concerns and to take another step to improve privacy for our users.

Hidden further down in the blog post, were a few more details:

We haven't sorted out all of the implementation details, and we may not be able to use precisely the same methods for anonymizing as we do after 18 months, but we are committed to making it work.

Google's announcement was extremely light on details, specifically, how the company planned to anonymize the records after 9 months. I contacted Google to find out more, and received an extremely interesting reply:

After nine months, we will change some of the bits in the IP address in the logs; after 18 months we remove the last eight bits in the IP address and change the cookie information. We're still developing the precise technical methods and approach to this, but we believe these changes will be a significant addition to protecting user privacy.... It is difficult to guarantee complete anonymization, but we believe these changes will make it very unlikely users could be identified.... We hope to be able to add the 9-month anonymization process to our existing 18-month process by early 2009, or even earlier.

To understand what this means (and how useless the new privacy "enhancements" are), consider the following:

When a user conducts a search using Google's search engine, the company stores three main types of information in a log file: the user's IP address (which is a unique network address given to her computer by her Internet service provider), the words that she searched for, and her cookie identifier (a unique value given to every Web-browser that visits a Google Web-property).

As per Google's existing policy, after 18 months Google "anonymizes" the IP address and cookie information from its logfiles. While the company hasn't said how it de-identifies the cookies, it has revealed in public statements that its IP anonymization technique consists of chopping off the last 8 bits of a user's IP address.

As an example, an IP address of a home user could be After 18 months, Google chops this down to 173.192.103.XXX.

Since each octet (the numbers between each period of an IP) can contain values from 1-255, Google's anonymization technique allows a user, at most, to hide among 254 other computers. In comparison, Microsoft deletes the cookies, the full IP address and any other identifiable user information from its search logs after 18 months.

Google has now revealed that it will change "some" of the bits of the IP address after 9 months, but less than the eight bits that it masks after the full 18 months. Thus, instead of Google's customers being able to hide among 254 other Internet users, perhaps they'll be able to hide among 64, or 127 other possible IP addresses.

By itself, this is a laughable level of anonymity. However, it gets worse.

First, remember that Google will not delete or anonymize user cookies from the logs when it slightly smudges IP addresses after nine months. Second, remember that as long as you use a Google Web property at least once every two years, the company will maintain a unique identifiable cookie value within your Web browser.

Thus, consider the following scenario:

In June 2008, a user from with cookie value 12345 conducts a search for "breast cancer risks." Nine months later, in March 2009, the company scrubs some portion of the IP address, perhaps to However, the cookie remains in the log.

In April 2009, that same user returns to Google, and conducts a search for "stephen colbert youtube videos," again from the same IP and the same cookie value 12345.

Even though the 9-month-old search logs have been "anonymized", because the cookie values remain, it is trivial to match the newer search results to the older searches, and thus completely reverse the anonymization process.

The simple truth is that any IP anonymization technique, no matter how strong or weak, is simply a waste of time, if cookie values are not also anonymized.

Unfortunately, Google is relying on the fact that the mainstream media (I'm looking at you New York Times and Washington Post) are clueless on these issues, as well as seemingly most of the technology press. Google's new anonymization policy is totally worthless, and the company deserves to be called out for its deception.

Disclaimer: I interned at Google during the summer of 2006 and received a $5,000 Google fellowship in both 2006 and 2007. I have also interned or worked for both the Electronic Privacy Information Center (EPIC) and the American Civil Liberties Union (ACLU) of Northern California, public interest groups that have been extremely critical of Google's privacy policies.