[RFC PATCH 4/7] crypto: remove ARC4 support from the skcipher API

[Ard Biesheuvel]

On Thu, 2 Jul 2020 at 20:21, Ard Biesheuvel <ardb at kernel.org> wrote:
>
> On Thu, 2 Jul 2020 at 19:50, Eric Biggers <ebiggers at kernel.org> wrote:
> >
> > [+linux-wireless, Marcel Holtmann, and Denis Kenzior]
> >
> > On Thu, Jul 02, 2020 at 12:19:44PM +0200, Ard Biesheuvel wrote:
> > > Remove the generic ecb(arc4) skcipher, which is slightly cumbersome from
> > > a maintenance perspective, since it does not quite behave like other
> > > skciphers do in terms of key vs IV lifetime. Since we are leaving the
> > > library interface in place, which is used by the various WEP and TKIP
> > > implementations we have in the tree, we can safely drop this code now
> > > it no longer has any users.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> >
> > Last year there was a discussion where it was mentioned that iwd uses
> > "ecb(arc4)" via AF_ALG.  So can we really remove it yet?
> > See https://lkml.kernel.org/r/97BB95F6-4A4C-4984-9EAB-6069E19B4A4F@holtmann.org
> > Note that the code isn't in "iwd" itself but rather in "libell" which iwd
> > depends on: https://git.kernel.org/pub/scm/libs/ell/ell.git/
> >
> > Apparently it also uses md4 and ecb(des) too.
> >
>
> Ah yes, I remember now :-(
>
> > Marcel and Denis, what's your deprecation plan for these obsolete and insecure
> > algorithms?
> >
>
> Given Denis's statement:
>
>   It sounds to me like it was broken and should be fixed.  So our vote /
>   preference is to have ARC4 fixed to follow the proper semantics.  We
>   can deal with the kernel behavioral change on our end easily enough;
>   the required workarounds are the worse evil.
>
> I would think that an ABI break is not the end of the world for them,
> and given how trivial it is to implement RC4 in C, the workaround
> should be to simply implement RC4 in user space, and not even bother
> trying to use AF_ALG to get at ecb(arc4)
>
> (same applies to md4 and ecb(des) btw)
>
> There will always be a long tail of use cases, and at some point, we
> just have to draw the line and remove obsolete and insecure cruft,
> especially when it impedes progress on other fronts.
>

I have ported iwd to Nettle's LGPL 2.1 implementation of ARC4, and the
diffstat is

 src/crypto.c      | 80 ++++++++++++--------
 src/main.c        |  8 --
 unit/test-eapol.c |  3 +-
 3 files changed, 51 insertions(+), 40 deletions(-)

https://git.kernel.org/pub/scm/linux/kernel/git/ardb/iwd.git/log/?h=arc4-cleanup