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HTTP client fingerprinting using SSL handshake analysis

June 17, 2009

My first SSL Labs project is about HTTP client fingerprinting when SSL is used. A cipher suite, in SSL, is a collection of cryptographic techniques that defines a secure communication channel. There are hundreds of cipher suites, and they are all built out of a dozen or so basic building blocks: key exchange, encryption and integrity validation algorithms. Different programs often use different cipher suites. By observing the list of supported cipher suites one can determine the maximal communication strength, and often even guess the make of the SSL client on the other side.

Possible uses:

  • System administrators can make informed decisions about which cipher suites to enable in the SSL servers they maintain. The general goal is disable as many of the weak(er) cipher suites, while leaving enough to still make it possible for users to connect.
  • Cross-checking the supported cipher suites with the HTTP client identity offered in the User-Agent header may help uncover some automated attack tools that masquarade themselves as browsers. Although the cipher suites used by an SSL client is completely under its control, such evasive actions require a new layer of SSL expertise. (Whereas it is trivial to spoof the contents of the User-Agent header.)

The proof of concept is an Apache module, which monitors SSL handshakes to extract the supported cipher suites. This approach is quite handy, because you can add the fingerprinting facility to your existing installation and suffer negligible overhead.