Tahoe-LAFS, the Least-Authority Filesystem, is a secure, decentralized filesystem. It is developed as a Free Software, Open Source project.
The Least-Authority Filesystem offers security and fault-tolerance properties far greater than those of other distributed filesystems -- in addition to being protected against external attackers, users of Tahoe-LAFS are protected from the servers themselves, even if some of the servers are malicious, and they are protected from other users, even though they can choose to share specific files or directories with specific users.
Security is nothing without usability, and to that end Tahoe-LAFS integrates cleanly with the World Wide Web using the principles of REST, and it provides a simple and flexible method of sharing access to your files (by sharing the URL of that file, using the principles of Capability Security).
We have created and deployed an implementation of the Least-Authority Filesystem -- Tahoe-LAFS v1.1 -- which we believe provides these strong security properties. However, we know that there is no substitute for peer review, and so we are challenging the hackers of the world to prove us wrong. If you find a major security flaw in the design of the Least-Authority Filesystem, or in the implementation of Tahoe-LAFS, then you win a customized t-shirt with your exploit and a big "Thank you" from us printed on the front. Also, you will be entered into the Hall of Fame on http://tahoe-lafs.org/hacktahoelafs .
Two people who discovered security flaws in earlier designs and helped us to fix them have been retroactively declared as the -2nd and -1st winners of the "Hack Tahoe-LAFS!" contest. Explanations of the security flaws that they discovered, how we fixed them, and pictures of them with their customized t-shirts are on the http://tahoe-lafs.org/hacktahoelafs web site.
If you want to be the 1st winner of the "Hack Tahoe-LAFS!" contest, you'll have to find a security design flaw that we overlooked, or an implementation mistake that you can exploit. The metric of success is that if you discover anything which compels us to change Tahoe-LAFS and to alert current users about the issue, then your discovery is worthy of a customized t-shirt.
Other than that anything goes, because one of the first rules of security is that you can win by breaking the rules. People are already relying on Tahoe-LAFS to store their files safely and privately, so if there is any way in which Tahoe-LAFS is endangering their data, we want to learn about it as soon as possible.
To get started, see the description on http://tahoe-lafs.org/hacktahoelafs of what security properties Tahoe-LAFS is supposed to provide. That web site has news, a live Tahoe-LAFS storage grid which you can play with, example targets you can attack, the Hall of Fame, detailed design notes, and full source code.
Thanks, and good luck!
Zooko O'Whielacronx, on behalf of the tahoe-lafs.org team