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

Stephan Mueller
2019-01-09T06:28:18Z

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 “this key may
> *only* be used to derive keys using such-and-such algorithm”, 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 key’s purpose? “Encrypt and authenticate a hibernation image”
> 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