Marc Hedlund sez, "Wesabe just open sourced a project called Grendel that makes it easy for web apps to encrypt data using the user's login password, and only decrypt that data when the user is logged in. Let's say you're using a word processing web app and don't want your documents stored plaintext -- the web app could use Grendel to easily encrypt your docs for you, using OpenPGP. Log in and you can edit; log out and only you can get at the data again (since only you have your password). There are some hooks for encrypting with multiple keys if you want to share docs with selected other users on the system. Since people are throwing a ton of sensitive data in web apps these days I think having some tools to help make that safer would be a good thing."
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Of course, data on web sites is usually shared with at least some other people in some way. Sometimes a user might want to share their information with the web site support staff, so the staff can help solve a problem or fix a bug. Or, the user might want to share their sensitive data with selected other users on the site, such as coworkers or family members. Grendel allows this, letting you encrypt data with multiple keys so that more than one user's password can gain access.
It's very easy to screw up when building a cryptography system -- check out Nate Lawson's excellent Google Tech Talk on common crypto flaws, or Matasano's Socratic dialog on similar topics, for a map of the pitfalls available to you, and us.