Re: [PATCH v2 1/2] security/keys/secure_key: Adds the secure key support based on CAAM.

[Mimi Zohar]

On Fri, 2018-08-03 at 07:23 -0700, James Bottomley wrote:
> On Fri, 2018-08-03 at 07:58 -0400, Mimi Zohar wrote:
> > On Thu, 2018-08-02 at 17:14 +0100, David Howells wrote:
> > > Udit Agarwal <udit.agarwal@xxxxxxx> wrote:
> > > 
> > > > +==========
> > > > +Secure Key
> > > > +==========
> > > > +
> > > > +Secure key is the new type added to kernel key ring service.
> > > > +Secure key is a symmetric type key of minimum length 32 bytes
> > > > +and with maximum possible length to be 128 bytes. It is produced
> > > > +in kernel using the CAAM crypto engine. Userspace can only see
> > > > +the blob for the corresponding key. All the blobs are displayed
> > > > +or loaded in hex ascii.
> > > 
> > > To echo Mimi, this sounds suspiciously like it should have a
> > > generic interface, not one that's specifically tied to one piece of
> > > hardware - particularly if it's named with generic "secure".
> > > 
> > > Can you convert this into a "symmetric" type and make the backend
> > > pluggable?
> > 
> > TPM 1.2 didn't support symmetric keys. ÂFor this reason, the TPM
> > "unseals" the random number, used as a symmetric key, and returns the
> > "unsealed" data to the kernel.
> > 
> > Does anyone know if CAAM or TPM 2.0 have support for symmetric keys?
> 
> It depends what you mean by "support".  The answer is technically yes,
> it's the TPM2_EncryptDecrypt primitive.  However, the practical answer
> is that symmetric keys are mostly used for bulk operations and the TPM
> and its bus are way too slow to support that, so the only real,
> practical use case is to have the TPM govern the release conditions for
> symmetric keys which are later used by a fast bulk encryptor/decryptor
> based in software.
> 
> > ÂIf they have symmetric key support, there would be no need for the
> > symmetric key ever to leave the device in the clear. ÂThe device
> > would unseal/decrypt data, such as an encrypted key.
> > 
> > The "symmetric" key type would be a generic interface for different
> > devices.
> 
> It's possible, but it would only work for a non-bulk use case; do we
> have one of those?

"trusted" keys are currently being used to decrypt other keys (eg.
encrypted, ecryptfs, ...).

Mimi