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FOSS in, FOSS out: software, process and operations

FileVault2 Hacks

Mac OS X 10.7 introduced a whole disk encryption service called FileVault2. This allows you to use AES 128 encryption to protect your data. This is a great feature but it has a few small drawbacks. It uses the password of your primary user account to unlock the system. I’m a fan of strong passwords but for encryption I’d prefer to use a longer pass phrase for increased entropy. Second the EFI-boot screen that is used to get the password to decrypt the disk shows the display name of all usersthat can unlock the system rather than blank fields for both username and password. This leaks information that I would really rather not leak. Fortunately I’ve found a little hack to work around both of these issues.

The key to my fix lies in this statement from the documentation:

Users not enabled for FileVault unlock are only able to log into the computer after an unlock-enabled user has started or unlocked the drive. Once unlocked, the drive remains unlocked and available to all users, until the computer is restarted. “OS X: About FileVault 2”

My fix is to create a new local user account that will only be used to unlock the disk encryption key. This will provide a fix for both issues. Since this account won’t be my primary account I can give it a much longer password without risk of RSI every time that OS X prompts me for an administrator password to install or update software. I can also give the user an innocuous display name to be shown on the unlock screen.

  1. Create a new account from the Users & Groups control panel:
    • New Account: Standard
    • Full Name: ************
    • Account name: encrypt
    • Password: omg this is a really long passphrase for me to remember
  2. Follow the instructions for enabling FileVault 2 and chose the new user as the only user who can unlock the disk.

If you already have FileVault 2 enabled you will need to remove the decryption right from the existing users. The easiest way I’ve found to do this is using the fdesetup command line tool. sudo fdesetup list will show you the accounts that are enabled. sudo fdesetup remove -user bd808 will remove the certificate for the bd808 user.

One last step is to make the new encrypt user log out as soon as they log in. This will return control to the normal OS X login system where you can configure the login screen to display username and password prompts instead of a list of local user accounts. There are probably several ways to do this, but I chose to make a small application that executes this apple script command:

logout
1
2
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ignoring application responses
  tell application "loginwindow" to «event aevtlogo»
end ignoring

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