Automatic encryption of stored messages

[Mikhail T.]

Hello!

Is there a way to encrypt all of the Cyrus' user-specific files on the 
disk? So that somebody breaking in -- or stealing the server -- has no 
access to the messages (and other data) unless a user's password is also 
available?

This is how I'd envision it:

    * A somewhat secret "salt" and the location of a (memory-backed)
      cyrus-only directory, such as /tmp/cyruspw, are set in a config-file.
    * A user logs in using a pam-module, which creates a symlink such as
      /tmp/cyruspw//user/ -> somehash(/salt/+/password+user/).
    * Subsequent access to the user's files uses the above hash as the
      key for user's messages, attachments, etc.
    * New messages are stored unencrypted (or on a shared encrypted
      partition) until the next time this user logs in. When he does,
      his user-specific encryption begins in background.
    * When the user disconnects, and all unencrypted messages are
      encrypted, the user's entry is removed from /tmp/cyruspw until his
      next login.
    * A (rare) password-change will require all unencrypted messages to
      be re-encrypted with the new hash -- until the re-encryption is
      completed, the server will keep the old hash around (in a
      different location, that would survive a system restart, such as
      /var/cyrus/oldpw//user /->/ oldhash/) to provide access to the
      not-yet-re-encrypted messages.

A fancier system could also provide for a "master password" to allow 
message-recovery in case of the user forgetting their password (or a 
law-enforcement subpoena). Some people may choose to not use this 
feature in their setup...

I think, this ensures reasonable protection for the stored messages 
without any cooperation from the MUA -- a stolen server (or a 
decommissioned hard-drive) will not reveal user's e-mails. Also, each 
user is assured, that their old emails are not accessible to anyone, 
/when he isn't accessing them himself/.

This may be possible to do right now -- without changing the Cyrus' code 
at all -- by using encrypted filesystems. Unfortunately, it means, each 
user's files would have to reside on their own filesystem... The 
proposed mechanism would provide a better way without /too much/ 
trouble. What do you think?

    -mi

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