Black Box Version 1.0 Manual de usuario descargar pdf (Pagina 28)

Keybags

• Literally a container of encryption keys

• Used to decrypt protected data

• (Also used for syncing and for remote unlock)

• Lock: delete in-memory bag

• Unlock: decrypt copy with passcode and load

into memory

New Bug

• MDM sends “Clear Passcode”

• Device loads unlocked keybag

• But doesn’t set “keybag in memory” flag

–(Note: Inference. I don’t really know how it works)

• User sets new passcode and locks

• Device looks locked...but....

• ...device never clears in memory keybag

• SO DEVICE NOT COMPLETELY LOCKED

The keybag contains keys used to decrypt data on the device stored

with the various protection classes, for both ﬁles and keychain entries.

For example, data marked as "ProtectionComplete" is encrypted

whenever the device is locked. The key to decrypt this data is stored in

the keybag.

When you lock the device, the unencrypted keybag in memory is

deleted, which means the device no longer has access to keys for the

protected data. They simply don't exist in memory. So the device can no

longer access those protected ﬁles.

When you unlock the device, an encrypted copy of the keybag is

decrypted using the passcode, and loaded into memory, restoring the

keys for current use.

I don't know exactly how this bug works, but my supposition is this:

There's some ﬂag that indicates that an unlocked keybag is currently

loaded into memory. When you unlock a device, and it decrypts the

keybag, this ﬂag is set. Then, when you lock it again, the operating

system looks at that ﬂag. If it's set, then it knows there's a keybag in

memory that needs to be deleted, and so it clears it.

It appears that this ﬂag isn't set when the keybag in memory was loaded

through the MDM "ClearPasscode" command. So the device doesn't

even realize that the keybag is in memory, so it never takes steps to

clear it. Which means the keys remain available, and operations which

don't require direct user access (the screen is still locked) will succeed

even for what should be protected data.

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Black Box Version 1.0 Manual de usuario Pagina 28