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Date:   Tue, 8 Jan 2019 15:54:22 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Stephan Mueller <smueller@...onox.de>
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


> On Jan 7, 2019, at 11:09 PM, Stephan Mueller <smueller@...onox.de> wrote:
> 
> Am Dienstag, 8. Januar 2019, 06:03:58 CET schrieb Herbert Xu:
> 
> Hi Herbert,
> 
>> Are we going to have multiple implementations for the same KDF?
>> If not then the crypto API is not a good fit.  To consolidate
>> multiple implementations of the same KDF, simply provide helpers
>> for them.
> 
> It is unlikely to have multiple implementations of a KDF. However, KDFs relate 
> to hashes like block chaining modes to raw block ciphers. Thus a KDF can be 
> applied with different hashes.
> 
> My idea was to add template support to RNGs (because KDFs are effectively a 
> type of RNG since they produce an arbitrary output from a fixed input). The 
> KDFs would be a template wrapping hashes. For example, the CTR-KDF from 
> SP800-108 could be instantiated like kdf-ctr(sha256).
> 
> 

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.

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