Question First, Photo Later

Authentication security have traditionally been achieved with passwords, whitelists and blacklists. This kind of security often comes at the price being hard to use and manage, making it unsuitable for content-sharing social communities like Facebook and QQ. So what do you do when you just want to share some private photos with a few friends but don't want to go to the trouble of password generation and distribution?

A usability research team at the DUB group, University of Washington, have come up with an interesting answer which they presented at the usability conference CHI 2008.
Shared Knowledge Access Control
They propose that users protect semi-private personal content behind questions of shared knowledge. For instance, "what is cousin lilly's favorite phrase?" can allow access from a hundred extended family members, without giving them accounts, passwords, and tediously adding them to access control lists.

The beauty of the system is that it allows just enough security for protecting this kind of social content to still keep the system easy to use and manage. It might not be perfectly secure (some types of questions tend to be too easy to guess), but it might just be perfectly acceptable and appropriate for the task.