Am Mittwoch, 27. Januar 2016, 15:35:41 schrieb Herbert Xu:

Hi Herbert,

On Wed, Jan 27, 2016 at 08:33:00AM +0100, Stephan Mueller wrote:

quoted

With the current development of EXT4 encryption we currently have the
logic that the files are either open (read/writable) or closed (not
accessible).

There is a scenario for a third option: a file is writable in a "device-
locked" state, but not readable. The logic that would implement such
mechanism is nicely described in [1] section D.3.3, especially figure 4.
To use such a mechanism, the generated shared secret should definitely go
through a KDF to ensure that the key has the right size for the underlying
symmetric cipher.

This approach would allow locking your device, but yet the system could
still write confidential data (like getting emails, generating logs, etc.)
but the data is not accessible unless you unlock the device.

So, my idea was to provide a building block for such encryption scenarios
which I would think will come.

Besides, if crypto network protocols are contemplated to be included into
the kernel (like TLS), I would think that the KDF should be handled by the
kernel crypto API as a central place for such logic. Thus, my patch would
provide the framework by providing the RNG template handling to have that
KDF here.

While this is all very nice until such a user is ready for submission
into the kernel I'd rather not add this.

Ok, I have no problems with that. I put the patches onto my web server if 
people are interested in it [1].

[1] http://www.chronox.de/kdf.html

Ciao
Stephan