netdev - Re: [PATCH v3 1/2] mac80211: fils_aead: Use crypto api CMAC shash rather than bare cipher

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Date:   Wed, 08 Feb 2017 08:46:48 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "<linux-wireless@...r.kernel.org>" <linux-wireless@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        Jouni Malinen <jouni@....qualcomm.com>
Subject: Re: [PATCH v3 1/2] mac80211: fils_aead: Use crypto api CMAC shash
 rather than bare cipher

On Wed, 2017-02-08 at 07:45 +0000, Ard Biesheuvel wrote:
> On 8 February 2017 at 07:00, Johannes Berg <johannes@...solutions.net
> > wrote:
> > This looks strange to me:
> > 
> > > +static int aes_s2v(struct crypto_shash *tfm,
> > >                  size_t num_elem, const u8 *addr[], size_t len[],
> > > u8 *v)
> > >  {
> > > -     u8 d[AES_BLOCK_SIZE], tmp[AES_BLOCK_SIZE];
> > > +     u8 d[AES_BLOCK_SIZE], tmp[AES_BLOCK_SIZE] = {};
> > > +     SHASH_DESC_ON_STACK(desc, tfm);
> > 
> > desc declared
> > 
> > > 
> > > +     crypto_shash_digest(desc, tmp, AES_BLOCK_SIZE, d);
> > 
> > used here
> > 
> 
> Each digest() call combines a init()/update()/final() sequence
> 
> > > +     crypto_shash_init(desc);
> > 
> > but initialized now?
> > 
> 
> ... for the 6th time, or so. The final vector may require two
> update()s, so we cannot use digest() here. But we can use finup() for
> the last one, which combines update() and final().
> 
> Hence,
> 
> init()/finup()
> 
> or
> 
> init()/update()/finup()
> 
> depending on the length of the last vector.

Great, thanks for the explanation :)

johannes