Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler

[Stephan Mueller]

Am Mittwoch, 9. Januar 2019, 00:54:22 CET schrieb Andy Lutomirski:

Hi Andy,
> 
> I think that, if the crypto API is going to grow a KDF facility, it should
> be done right. Have a key type or flag or whatever that says âthis key may
> *only* be used to derive keys using such-and-such algorithmâ, and have a
> helper to derive a key.  That helper should take some useful parameters and
> mix them in:
> 
> - What type of key is being derived?  ECDSA signing key?  HMAC key?  AES
> key?
> 
> - Can user code access the derived key?
> 
> - What is the keyâs purpose?  âEncrypt and authenticate a hibernation imageâ
> would be a purpose.
> 
> - Number of bytes.
> 
> All of these parameters should be mixed in to the key derivation.
> 
> Also, an AE key, even for AES+HMAC, should be just one derived key.  If you
> need 512 bits, ask for a 512-bit key, not two 256-bit keys.

I concur with your requirements. However, is the kernel crypto API the right 
place to enforce such policies? To me, the kernel crypto API is a tinker-toy 
set of ciphers.

The real policy enforcer would or should be the keyring facility. Thus, may I 
propose to:

- implement the cryptographic primitive of the KDF in the kernel crypto API

- implement the policy system how to use the KDF in the keyring facility

Ciao
Stephan