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Online biometrics flaw gives hackers a 'fake finger'
24 September 2007
NewScientist.com news service
Anil Ananthaswamy

Enlarge Biometric passwords

Imagine being free to forget all of your passwords and use your fingerprint to log in to your online bank, eBay and email accounts. This tantalising vision has suffered a major blow: the scheme that makes it possible could also allow hackers to steal fingerprints and impersonate their victims.

Biometric-secured laptops store an image of your fingerprint, only letting you log in if you produce the matching finger. Proving your identity over the internet is more difficult, however, because the fingerprint data must be transmitted, giving snoopers the chance to hijack it.

Encrypting the fingerprint using conventional cryptography and then transmitting it is not an option as it would require the fingerprint scanned with your PC to exactly match the one stored by the website you wish to access. That isn't possible because fluctuations in the way fingers roll over scanners makes the same print slightly different each time.

Instead, a cryptographic scheme known as the "fuzzy vault" was devised that does not require a print to look exactly the same each time it is scanned.

Under the scheme, a template of the user's fingerprint is stored on a secure database as a list of coordinates that pinpoint specific features, called minutiae. Every time a user tries to log in remotely, a scan of their fingerprint gets converted into coordinates. The computer also creates a random code key, which is then used to encrypt each fingerprint coordinate. The result is a list of number pairs made up of the genuine coordinates and their encrypted partners. The system then generates thousands of fake versions of these pairs, called "chaff", and adds them to the list of real and encrypted pairs. The chaff is a disguise, or "fuzzy vault", around both the key and the fingerprint (see Diagram).

This full list of chaff and real coordinates can be transmitted safely. An intruder who gets hold of it can't tell chaff pairs from real ones. The receiving server, however, compares the list to the secure template and is able to pick out the true pairs. If it finds them it knows the user is genuine and allows them to log in. The fuzzy vault scheme has an added bonus: the server can also use the true pairs to recover the key, which can be used to encrypt subsequent communications with that same user with no further need of the template.

Now Preda Mihailescu at the University of Göttingen in Germany has shown that the fuzzy vault is not secure (www.arxiv.org/abs/0708.2974v1). His analysis shows that if more than about 500 chaff pairs are used, too much computing power is required to separate the true pairs from the chaff for the server to cope. Yet he also found that a fuzzy vault with about 500 chaff pairs can be broken in a day using a powerful desktop computer.

Breaking the vault gives an intruder access to the minutiae, which theoretically could be used to create a fake finger, allowing someone to be impersonated "for a lifetime", says Mihailescu.

Ari Juels of RSA Laboratories in Bedford, Massachusetts, one of the inventors of the fuzzy vault, notes that such vaults are not yet in use. But the finding is "highly significant", says Joachim von zur Gathen at the Bonn-Aachen International Center for Information Technology in Germany: "We have to go back to the drawing board."

From issue 2622 of New Scientist magazine, 24 September 2007, page 30

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