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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Fri, 7 Jun 2019 at 19:59, Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Fri, Jun 07, 2019 at 04:49:41PM +0200, Ard Biesheuvel wrote:
> > 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.
>

In fact, this is what I started out doing, i.e., factor out the core
arc4 code into crypto/arc4_lib.c, and make the existing driver a thin
wrapper around it, so that we can invoke the library directly.

> Someone could theoretically be using "ecb(arc4)" via AF_ALG or dm-crypt, but it
> seems unlikely...
>

Yes, that seems highly unlikely.

> As for removing the "cipher" API entirely, we'd have to consider how to convert
> all the current users, not just ARC4, so that would be a somewhat different
> discussion.  How do you propose to handle dm-crypt and fscrypt which use the
> cipher API to do ESSIV?
>

Without having looked in too much detail, ESSIV seems like something
that could be moved into the crypto subsystem, and be implemented as a
template.