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

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.

Thanks,
-- 
Email: Herbert Xu [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
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