executive summary HTTP cache-control headers such as If-Modified-Since allow servers to track individual users in a manner similar to cookies, but with less constraints. This is a problem for user privacy against which browsers currently provide little protection. problem statement Alice is browsing the web; Bob runs a number of otherwise-unrelated web servers. Alice makes several requests to Bob's servers over time. Bob would like to tie together as many as possible of the requests made by Alice to learn more about Alice's usage patterns and identity: we call this identifying the request chain. Alice would like to access Bob's servers but not give away this information. existing approaches cookies The standard approach for associating user requests across several responses is the HTTP `Cookie' state-management extension. The Cookie response header allows a server to ask the client to store arbitrary short opaque data, which should be returned for future requests of that server matching particular criteria. Cookies are commonly used to store per-user form defaults, to manage web application sessions, and to associate requests between executions of the user agent. The user agent always has the option to just ignore the Set-Cookie response header, but most implementations default to obeying it to preserve functionality. Cookies can optionally specify an expiry time after which they should no longer be used, that they should persist on disk between client session, or that they should only be passed over transmission-level-secure connections. The privacy implications of cookies have been [1]extensively discussed, and several problems have been found and recitified in the past. One example of privacy compromise through cookies is the use of cookies attached to banner images downloaded from a central banner server: the same cookie is used within images linked from several servers, and so the user can be tracked as they move around. other approaches An obvious means to associate requests is by source IP address. Over the short term this will generally work quite well, as a client is likely to use a single IP address during a browsing session. Even then it is complicated by proxies acting for multiple clients, network address translation, or multiuser machines. Over a longer term, the information is convolved by dynamically-assigned IPs, mobile computers moving between networks, dialup pools and the like. Indeed, cookies were proposed in large part to allow legitimate stateful applications to cope with the impossibility of uniquely identifying users by IP address. the meantime exploit The fundament of the meantime exploit is that the server wishes to `tag' the client with some information that will later be reported back, allowing the server to identify a chain. Cookies are a good approach to this, but their privacy implications are well known and so Bob requires a more surreptitious approach. The HTTP cache-control headers are perfect for this: the data is provided by the server, stored but not verified by the client, and then provided verbatim back to the server on the next matching request. Two headers in particular are useful: Last-Modified and ETag. Both are designed to help the client and server negotiate whether to use a cached copy or fetch the resource again. The general approach of meantime is that rather than using the headers for their intended purpose, Bob's servers will instead send down a unique tag for the client. Last-Modified is constrained to be a date, and therefore is somewhat inflexible. Nevertheless, the server can reasonably choose any second since the Unix epoch, which allows it to tag on the order of one billion distinct clients. ETag allows an arbitrary short string to be stored and passed. It is not so commonly implemented in user agents at the moment, and so not such a good choice. In both cases the tag will be lost if the client discards the resource from its cache, or if it does not request the exact same resource in the future, or if the request is unconditional. (For example, Netscape sends an unconditional response when the user presses Shift+Reload.) Bob has less control over this than he has with cookies, which can be instructed to persist for an arbitrarily long period. The date is only sent back for the exact same URL, including any query parameters. By contrast, cookies can be returned for all resources in a site or section of a site. This makes Bob's job a little harder. Bob therefore should make sure that all pages link to a small common resource: perhaps a one-pixel image. This image is generated by a script that supplies and records a unique timestamp to each client, and records whatever is already present. For a demonstration, more explanation and details, please see http://www.linuxcare.com.au/mbp/meantime/ Cheers! -- Martin Pool, Linuxcare, Inc. +61 2 6262 8990 mbp@linuxcare.com, http://www.linuxcare.com/ Linuxcare. Support for the revolution.