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Date:   Wed, 09 Jan 2019 07:27:28 +0100
From:   Stephan Mueller <smueller@...onox.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Lee, Chun-Yi" <joeyli.kernel@...il.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, keyrings@...r.kernel.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Oliver Neukum <oneukum@...e.com>,
        Ryan Chen <yu.chen.surf@...il.com>,
        David Howells <dhowells@...hat.com>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler

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


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