Re: [RFC PATCH 0/2] TPM derived keys

Ignat Korchagin
2024-05-14T15:30:29Z

On Tue, May 14, 2024 at 4:26 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Tue May 14, 2024 at 6:21 PM EEST, Jarkko Sakkinen wrote:
> > On Tue May 14, 2024 at 5:30 PM EEST, Jarkko Sakkinen wrote:
> > > On Tue May 14, 2024 at 5:00 PM EEST, Jarkko Sakkinen wrote:
> > > > On Tue May 14, 2024 at 4:11 PM EEST, Ignat Korchagin wrote:
> > > > > For example, a cheap NAS box with no internal storage (disks connected
> > > > > externally via USB). We want:
> > > > > * disks to be encrypted and decryptable only by this NAS box
> > > >
> > > > So how this differs from LUKS2 style, which also systemd supports where
> > > > the encryption key is anchored to PCR's? If I took hard drive out of my
> > > > Linux box, I could not decrypt it in another machine because of this.
> > >
> > > Maybe you could replace the real LUKS2 header with a dummy LUKS2
> > > header, which would need to be able the describe "do not use this" and
> > > e.g. SHA256 of the actual header. And then treat the looked up header as
> > > the header when the drive is mounted.
> > >
> > > LUKS2 would also need to be able to have pre-defined (e.g. kernel
> > > command-line or bootconfig) small internal storage, which would be
> > > also encrypted with TPM's PRCs containing an array of LUKS2 header
> > > and then look up that with SHA256 as the key.
> > >
> > > Without knowing LUKS2 implementation to me these do not sound reaching
> > > the impossible engineer problems so maybe this would be worth of
> > > investigating...
> >
> > Or why you could not just encrypt the whole header with another key
> > that is only in that device? Then it would appear as random full
> > length.
> >
> > I.e. unsealing
> >
> > 1. Decrypt LUKS2 header with TPM2 key
> > 2. Use the new resulting header as it was in the place of encrypted
> > stored to the external drive.
> > 3. Decrypt key from the LUK2S header etc.
>
> Maybe something like:
>
> 1. Asymmetric for LUKS2 (just like it is)
> 2. Additional symmetric key, which is created as non-migratable and stored
> to the TPM2 chip. This deciphers the header, i.e. takes the random
> away.

This could work, but you still have the problem of - if the header
gets wiped, all the data is lost.
As for storing things on the TPM chip - that doesn't scale. Today you
only think about disk encryption, tomorrow there is a new application,
which wants to do the same thing and so on. One of the features of
derived keys - you don't store anything, just recreate/derive when
needed and it scales infinitely.

> BR, Jarkko