I have some old backups that are encrypted using windows EFS and I no longer have access to the certificate used to encrypt the FEK symmetric key included in the files but I do have the password, user name and as well large portion of the actual binaries before encryption. I do know that AES used by EFS is not vulnerable to known plain text attack but I was hoping that with the above information I can figure out how windows generated the FEK symmetric key that are used to encrypt the file. Any help would be appreciated. Thanks.

  • If you can do that, you break entire EFS system.
    – Ajay
    Dec 30 '15 at 11:56

FEK is random

Sorry. (But if it were any different, this would be terrible for EFS.)

Microsoft Corporation, TechNet, How EFS Works:

When you save a file to be encrypted, a random cryptographic generator supplies a unique file encryption key (FEK), which is a fast symmetric key designed for bulk encryption. The FEK encrypts the data in blocks.

  • Thanks, but is't unique per user certificate or per file?
    – Fady Anwar
    Dec 31 '15 at 13:00
  • It`s gonna be 128 random bits. Unique to the this universe most likely (no doubles in our lifetime, not unlike a UUID). And NO relation to anything. Not to the cert, not to the file. (And like I said: anything else would be weird/counter-intuitive/bad.) Dec 31 '15 at 13:51
  • So does this means that each file on the same folder encrypted by the same user would get a different FEK?!
    – Fady Anwar
    Jan 3 '16 at 10:55
  • Yes. This is the way it's supposed to be. Jan 3 '16 at 12:02

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