On 17.08.21 15:55, Mimi Zohar wrote:

On Tue, 2021-08-17 at 15:04 +0200, Ahmad Fatoum wrote:

quoted

Hi,

On 12.08.21 02:54, Mimi Zohar wrote:

quoted

On Wed, 2021-08-11 at 10:16 -0700, Eric Biggers wrote:

quoted

Neither of you actually answered my question, which is whether the support for
trusted keys in dm-crypt is a mistake.  I think you're saying that it is?  That
would imply that fscrypt shouldn't support trusted keys, but rather encrypted
keys -- which conflicts with Ahmad's patch which is adding support for trusted
keys.  Note that your reasoning for this is not documented at all in the
trusted-encrypted keys documentation; it needs to be (email threads don't really
matter), otherwise how would anyone know when/how to use this feature?

True, but all of the trusted-encrypted key examples in the
documentation are "encrypted" type keys, encrypted/decrypted based on a
"trusted" type key.  There are no examples of using the "trusted" key
type directly.  Before claiming that adding "trusted" key support in
dm-crypt was a mistake, we should ask Ahmad why he felt dm-crypt needed
to directly support "trusted" type keys.

I wanted to persist the dm-crypt key as a sealed blob. With encrypted keys,
I would have to persist and unseal two blobs (load trusted key blob, load
encrypted key blob rooted to trusted key) with no extra benefit.

I thus added direct support for trusted keys. Jarkko even commented on the
thread, but didn't voice objection to the approach (or agreement for that
matter), so I assumed the approach is fine.

I can see the utility of using a single trusted key for TPMs, but for CAAM,
I see none and having an encrypted key for every trusted key just makes
it more cumbersome.

In v1 here, I added encrypted key support as well, but dropped it for v2,
because I am not in a position to justify its use. Now that you and Eric
discussed it, should I send v3 with support for both encrypted and trusted
keys like with dm-crypt or how should we proceed?

With some applications, the indirection is important.   It allows the
"encrypted" key type to be updated/re-encypted based on a new "trusted"
key, without affecting the on disk encrypted key usage.

Those applications were already able to use the encrypted key support
in dm-crypt. For those where re-encryption/PCR-sealing isn't required,
direct trusted key support offers a simpler way to integrate.

As much as I expected, directly using "trusted" keys is a result of the
new trusted key sources.

More users = more use cases. You make it sound like a negative
thing.

I have no opinion as to whether this is/isn't a valid usecase.

So you'd be fine with merging trusted key support as is and leave encrypted
key support to someone who has a valid use case and wants to argue
in its favor?

Cheers,
Ahmad

thanks,

Mimi

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |