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Subject: Re: [RFC PATCH 0/2] TPM derived keys
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Ignat Korchagin <ignat@cloudflare.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>, Mimi Zohar <zohar@linux.ibm.com>, 
 David Howells <dhowells@redhat.com>, Paul Moore <paul@paul-moore.com>,
 James Morris <jmorris@namei.org>,  serge@hallyn.com,
 linux-integrity@vger.kernel.org, keyrings@vger.kernel.org, 
 linux-kernel@vger.kernel.org, kernel-team@cloudflare.com
Date: Tue, 14 May 2024 09:54:26 -0600
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On Tue, 2024-05-14 at 16:38 +0100, Ignat Korchagin wrote:
> On Tue, May 14, 2024 at 4:30 PM James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > 
> > On Tue, 2024-05-14 at 14:11 +0100, Ignat Korchagin wrote:
> > >   * if someone steals one of the disks - we don't want them to
> > > see it has encrypted data (no LUKS header)
> > 
> > What is the use case that makes this important?  In usual operation
> > over the network, the fact that we're setting up encryption is
> > easily identifiable to any packet sniffer (DHE key exchanges are
> > fairly easy to fingerprint), but security relies on the fact that
> > even knowing that we're setting up encryption, the attacker can't
> > gain access to it.  The fact that we are setting up encryption
> > isn't seen as a useful thing to conceal, so why is it important for
> > your encrypted disk use case?
> 
> In some "jurisdictions" authorities can demand that you decrypt the
> data for them for "reasons". On the other hand if they can't prove
> there is a ciphertext in the first place - it makes their case
> harder.

Well, this isn't necessarily a good assumption: the way to detect an
encrypted disk is to look at the entropy of the device blocks.  If the
disk is encrypted, the entropy will be pretty much maximal unlike every
other use case.  The other thing is that if the authorities have your
TPM, they already have access to the disk in this derived key scenario.
If *you* still have access to your TPM, you can update the storage seed
to shred the data.

James