Shostack + Friends Blog Archive


More Bad News for SSL

I haven’t read the paper yet, but Schneier has a post up which points to a paper “Side-Channel Leaks in Web Applications: a Reality Today, a Challenge Tomorrow,” by Shuo Chen, Rui Wang, XiaoFeng Wang, and Kehuan Zhang.about a new side-channel attack which allows an eavesdropper to infer information about the contents of an SSL connection in certain contexts, some of them fairly common.  For example (from Schneir’s link to Ed Felten’s commentary on the paper):

The new paper shows that this inference-from-size problem gets much, much worse when pages are using the now-standard AJAX programming methods, in which a web “page” is really a computer program that makes frequent requests to the server for information. With more requests to the server, there are many more opportunities for an eavesdropper to make inferences about what you’re doing — to the point that common applications leak a great deal of private information.

Consider a search engine that autocompletes search queries: when you start to type a query, the search engine gives you a list of suggested queries that start with whatever characters you have typed so far. When you type the first letter of your search query, the search engine page will send that character to the server, and the server will send back a list of suggested completions. Unfortunately, the size of that suggested completion list will depend on which character you typed, so an eavesdropper can use the size of the encrypted response to deduce which letter you typed. When you type the second letter of your query, another request will go to the server, and another encrypted reply will come back, which will again have a distinctive size, allowing the eavesdropper (who already knows the first character you typed) to deduce the second character; and so on. In the end the eavesdropper will know exactly which search query you typed. This attack worked against the Google, Yahoo, and Microsoft Bing search engines.

SSL has been touted as a Web security panacea for years, but the harsh reality is that its weaknesses are growing rapidly, made worse by the changing ways that HTTP is used–when the expected SSL-protected transaction was a page request followed by the return of a full page of content, it was extremely difficult to infer the contents of the connection.  Now that the requests and responses are relatively atomic, even down to the characte, this is no longer the case.

And as old assumptions fail, so does security built on top of those assumptions.

One comment on "More Bad News for SSL"

  • Not to be too much of a pedant but this isn’t really a problem with SSL. This is a problem with HTTP’s use of SSL for HTTPS.

    Recall that the SSH guys had a similar crypto problem a few years ago that allowed password prediction via inter-keystroke timing of the single-keystroke packets.

    None of these are really new types of attacks either, we’re just finding them in more places.

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