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Cryptographic researchers have outlined techniques to greatly reduce the time it takes to crack alphanumeric Windows passwords.

Brute force attacks on such passwords have always been possible but the techniques outlined in a paper from the Swiss Federal Institute of Technology in Lausanne (EPFL) show how such passwords could be broken up to eight or 10 times more quickly than previously possible.

The technique involves building a large lookup tables that matches the hashed (encoded) versions of passwords stored in Windows with text entered by a user.

By loading such pre-calculated data stored in memory its possible to reduce the time a particular piece of crypto analysis will take. Crypto analysts like Ron Rivest have understood this time-memory trade-off since the 1980s. The Swiss researchers have simply optimised the process, developing a demo programme than can crack Windows passwords more quickly. This demo is available online here.

A paper on this work, Making a Faster Cryptanalytic Time-Memory Trade-Off, is to be presented by Philippe Oechslin and his colleagues at the Swiss Federal Institute of Technology in Lausanne during the Crypto'03 conference next month.

An abstract for the paper explains: "Using 1.4GB of data (two CD-ROMs) we can crack 99.9% of all alphanumerical passwords hashes in 13.6 seconds whereas it takes 101 seconds with the current approach using distinguished points. We show that the gain could be even much higher depending on the parameters used."

By email, Oechslin told us the researchers have since refined the techniques, bringing down the average time it takes to crack passwords to five seconds.

The research re-invigorates calls for Microsoft, in particular, to improve the cryptographic security of its passwords. Although NTHash is more secure than LANManager (the password scheme used in Win 9x) it's still insufficiently random, as the Swiss researchers have shown.

Meanwhile users shouldn't be too concerned about the issue since lifting password hash files in only possible where an attacker has control of your machine. If that happens, weak NT passwords will be the least of your worries. ®

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