 Secure Store server protects this information using state-of-the art cryptogra- phy and the Advanced Encryption Standard (AES). To retrieve key files for Factotum, from a local device like a laptop or PDA, the user only needs to provide a password to prove his or her identity, thanks to PAK, an advanced Bell Labs security protocol for doing password-authenticated key exchange. This approach thwarts the most common security threats, like so-called "dic- tionary attacks" on the password, by making it impossible for someone to eavesdrop in on the challenge-and-response approach used in most password schemes. When Factotum accesses a user's keys, it stores the information in protected memory and keeps it there for a short period of time. This is an improvement over today's common method of storing passwords on a user's hard drive, which is insecure. Factotum only holds user information in memory when the machine is running. When the machine is off, the secrets are only kept in Secure Store. The final security precaution is that Secure Store is located on ENCRYPTION the network, not on the user's PC; thus, even if a user's machine is hacked or stolen, the information stored in Secure Store is safe. "The new security features in Plan 9 integrate organically into the system, making it unique among security options in the marketplace today," says David Nicol, professor of computer science at Dartmouth College and asso- ciate director of research and development at the school's Institute for Security Technology Studies. "Bell Labs' design recognizes rightly that iden- tity and the authentication of identity are the heart and soul of security." 7.7.2 Data Hiding Data hiding, the practice of secretly embedding data in images and other file types, promises to be a significant security concern in the years ahead. An elec- trical engineer at Washington University in St. Louis has recently devised a theory that sets the limits for the amount of data that can be hidden in a system and then provides guidelines for how to store d