[Mac_crypto] MacOS X (Panther) FileVault

[Nicko van Someren]

On 13 Nov 2003, at 5:12, David Shayer wrote:

> I was told that FileVault replaces your home directory with an 
> encrypted disk image, much like PGP Disk, so its probably blockwise 
> underneath the file system layer. Files in your home directory are 
> copied into the disk image, and some file system links redirect calls 
> to the home directory to the disk image, and keep the user from seeing 
> it as another mounted disk.

This is basically correct.  FileVault uses an auto-mounting version of 
the encrypted disk image facility that was in 10.2, tweaked to allow 
the image to be opened even before your main key chain is available 
(since the key chain is stored inside your home directory).  The 
standard encrypted image format uses a random key stored on your key 
chain, which is itself encrypted with a salted and hashed copy of the 
keychain pass phrase, which defaults to your login password.  My 
suspicion is that for the FileVault there is some other key chain file 
in the system folder which stores the key for decrypting your home 
directory disk image and that the pass phrase for that is just your 
login password.

> File Vault will automatically expand or contract the disk image at 
> certain points. It creates a new image, copies everything over, and 
> deletes the old image.

Yup, it essentially does an "hdiutil compact" command when you log out.

> I don't know what mode of AES-128 it uses.

I believe that it uses counter mode, since it's efficient when doing 
random access to the encrypted data.

	Nicko