Regarding FileVault 2, Part One, In Da Club

January 28th, 2013 by Allister Banks


IT needs to have a way to access FileVault 2(just called FV2 from here on) encrypted volumes in the case of a forgotten password or just getting control over a machine we’re asked to support. Usually an institution will employ a key escrow system to manage FDE(Full Disk Encryption) when working at scale. One technique, employed by Google’s previously mentioned Cauliflower Vest, is based on the ‘personal’ recovery key(a format I’ll refer to as the ‘license plate’, since it looks like this: RZ89-A79X-PZ6M-LTW5-EEHL-45BY.) The other involves putting a certificate in place, and is documented in Apple’s white paper on the topic. That paper only goes into the technical details later in the appendix, and I thought I’d review some of the salient points briefly.

There are three layers to the FV2 cake, divided by the keys interacted with when unlocking the drive:
Derived Encryption Keys(plural), the Key Encrypting Key(from the department of redundancy department) and the Volume Encrypting Key. Let’s use a (well-worn) abstraction so your eyes don’t glaze over. There’s the guest list and party promoter(DEKs), the bouncer(KEK), and the key to the FV2 VIP lounge(VEK). User accounts on the system can get on the (DEK) guest list for eventual entry to the VIP, and the promoter may remove those folks with skinny jeans, ironic nerd glasses without lenses, or Ugg boots with those silly salt-stained, crumpled-looking heels from the guest list, since they have that authority.

The club owner has his name on the lease(the ‘license plate’ key or cert-based recovery), and the bouncer’s paycheck. Until drama pop off, and the cops raid the joint, and they call the ambulance and they burn the club down… and there’s a new lease and ownership and staff, the bouncer knows which side of his bread is buttered.

The bouncer is a simple lad. He gets the message when folks are removed from the guest list, but if you tell him there’s a new owner(cert or license plate), he’s still going to allow the old owner to sneak anybody into the VIP for bottle service like it’s your birthday, shorty. Sorry about the strained analogy, but I hope you get the spirit of the issue at hand.

The moral of the story is, there’s an expiration method(re-wrapping the KEK based on added/modified/removed DEKs) for the(in this case, user…) passphrase-based unlock. ONLY. The FilevaultMaster.keychain cert has a password you can change, but if access has been granted to a previous version with a known password, that combination will continue to work until the drive is decrypted and re-encrypted. And the license plate version can’t be regenerated or invalidated after initial encryption.

So the two institutional-scale methods previously mentioned still get through the bouncer unlock the drive until you tear the roof of the mofo tear the club up de- and re-encrypt the volume.

But here’s an interesting point, there’s another type of DEK/passphrase-based unlock that can be expired/rotated besides per-user: a disk-based passphrase. I’ll get to describing that in Part Deux…

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