Comment 6 for bug 1857398

I put these questions to Tom Caputi, who wrote the ZFS encryption. The quoted text below is what I asked him, and the unquoted text is his response:

> 1. Does ZFS rewrite the wrapped/encrypted master key in place? If
> not, the old master key could be retrieved off disk, decrypted
> with the known passphrase, and used to decrypt at least
> _existing_ data.

1) No. This is definitely an attack vector (although a very minor
   one). At the time we had said that we would revisit the idea of
   overwriting old keys when TRIM was added. That was several years ago
   and TRIM is now in. I will talk to Brian about it after I am back
   from the holiday.

> 2. Does a "zfs change-key" create a new master key? If not, the old
> master key could be used to decrypt _new_ data as well, at least
> until the master key is rotated.

2) zfs change-key does not create a new master key. It simply re-wraps
   the existing master key. The master keys are never rotated. The key
   rotation is done by using the master keys to generate new keys.