Re: [RFC PATCH 0/3] move WEP implementation to skcipher interface

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Marcel Holtmann <marcel@holtmann.org>
Cc: Eric Biggers <ebiggers@kernel.org>,
	"open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
	<linux-crypto@vger.kernel.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Johannes Berg <johannes@sipsolutions.net>,
	"open list:NFC SUBSYSTEM" <linux-wireless@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [RFC PATCH 0/3] move WEP implementation to skcipher interface
Date: Fri, 7 Jun 2019 22:27:22 +0200	[thread overview]
Message-ID: <CAKv+Gu-ek4nK+cACx5QZTbp=ciQq_Fvtn9y3g-wFWSOabyczZg@mail.gmail.com> (raw)
In-Reply-To: <97BB95F6-4A4C-4984-9EAB-6069E19B4A4F@holtmann.org>

On Fri, 7 Jun 2019 at 22:24, Marcel Holtmann <marcel@holtmann.org> wrote:
>
> Hi Eric,
>
> >> One of the issues that I would like to see addressed in the crypto API
> >> is they way the cipher abstraction is used. In general, a cipher should
> >> never be used directly, and so it would be much better to clean up the
> >> existing uses of ciphers outside of the crypto subsystem itself, so that
> >> we can make the cipher abstraction part of the internal API, only to
> >> be used by templates or crypto drivers that require them as a callback.
> >>
> >> As a first step, this series moves all users of the 'arc4' cipher to
> >> the ecb(arc4) skcipher, which happens to be implemented by the same
> >> driver, and is already a stream cipher, given that ARC4_BLOCK_SIZE
> >> actually evaluates to 1.
> >>
> >> Next step would be to switch the users of the 'des' and 'aes' ciphers
> >> to other interfaces that are more appropriate, either ecb(...) or a
> >> library interface, which may be more appropriate in some cases. In any
> >> case, the end result should be that ciphers are no longer used outside
> >> of crypto/ and drivers/crypto/
> >>
> >> This series is presented as an RFC, since I am mostly interested in
> >> discussing the above, but I prefer to do so in the context of actual
> >> patches rather than an abstract discussion.
> >>
> >> Ard Biesheuvel (3):
> >>  net/mac80211: switch to skcipher interface for arc4
> >>  lib80211/tkip: switch to skcipher interface for arc4
> >>  lib80211/wep: switch to skcipher interface for arc4
> >>
> >
> > The way the crypto API exposes ARC4 is definitely broken.  It treats it as a
> > block cipher (with a block size of 1 byte...), when it's actually a stream
> > cipher.  Also, it violates the API by modifying the key during each encryption.
> >
> > Since ARC4 is fast in software and is "legacy" crypto that people shouldn't be
> > using, and the users call it on virtual addresses, perhaps we should instead
> > remove it from the crypto API and provide a library function arc4_crypt()?  We'd
> > lose support for ARC4 in three hardware drivers, but are there real users who
> > really are using ARC4 and need those to get acceptable performance?  Note that
> > they aren't being used in the cases where the 'cipher' API is currently being
> > used, so it would only be the current 'skcipher' users that might matter.
> >
> > Someone could theoretically be using "ecb(arc4)" via AF_ALG or dm-crypt, but it
> > seems unlikely…
>
> that is not unlikely, we use ecb(arc4) via AF_ALG in iwd. It is what the WiFi standard defines to be used.
>

Ah ok, good to know. That does imply that the driver is not entirely
broken, which is good news I suppose.