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

From: Eric Biggers <ebiggers@kernel.org>
To: Denis Kenzior <denkenz@gmail.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Marcel Holtmann <marcel@holtmann.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 14:41:20 -0700	[thread overview]
Message-ID: <20190607214120.GE648@sol.localdomain> (raw)
In-Reply-To: <d394b421-799d-2019-fcf0-97ba0b2abb5f@gmail.com>

On Fri, Jun 07, 2019 at 04:28:59PM -0500, Denis Kenzior wrote:
> Hi Eric,
> 
> On 06/07/2019 04:15 PM, Eric Biggers wrote:
> > On Fri, Jun 07, 2019 at 03:45:45PM -0500, Denis Kenzior wrote:
> > > Hi Ard,
> > > 
> > > > 
> > > > Ah ok, good to know. That does imply that the driver is not entirely
> > > > broken, which is good news I suppose.
> > > > 
> > > 
> > > Not entirely, but we did have to resort to using multiple sockets, otherwise
> > > parallel encrypt/decrypt operations on the socket would result in invalid
> > > behavior.  Probably due to the issue Eric already pointed out.
> > > 
> > > No such issue with any other ciphers that we use.
> > > 
> > > Regards,
> > > -Denis
> > 
> > Okay, that sucks, so we do have to keep "ecb(arc4)" in the crypto API then.  And
> > we can't fix its name to be just "arc4".  It's odd that someone would choose to
> > use AF_ALG over writing a 20 line arc4_crypt() in userspace, but whatever.
> > 
> > Yes, "ecb(arc4)" isn't currently thread safe.  ARC4 uses a single key whereas
> > modern stream ciphers use a key + IV.  To comply with the crypto API it would
> > have to copy the key to a stack buffer for each encryption/decryption.  But it
> > doesn't; it just updates the key instead, making it non thread safe.  If users
> > are actually relying on that, we'll have to settle for adding a mutex instead.
> 
> Well the issue isn't even about being thread safe.  We run a single thread
> in iwd.  The details are a bit fuzzy now due to time elapsed, but if I
> recall correctly, even behavior like:
> 
> fd = socket();
> bind(fd, ecb(arc4));
> setsockopt(fd, ...key...);
> 
> sendmsg(fd, OP_ENCRYPT, ...);
> sendmsg(fd, OP_DECRYPT, ...);
> sendmsg(fd, OP_ENCRYPT, ...);
> 
> would produce different (incorrect) encrypted results compared to
> 
> sendmsg(fd, OP_ENCRYPT, ...)
> sendmsg(fd, OP_ENCRYPT, ...)
> 

That's because currently each operation uses the next bytes from the keystream,
and a new keystream is started only by setsockopt(..., ALG_SET_KEY, ...).
There's no difference between ARC4 encryption and decryption; both just XOR the
keystream with the data.  Are you saying you expected each encryption to be a
continuation of the previous encryption, but decryptions to be independent?

- Eric