From: Herbert Xu Subject: Re: [PATCH] crypto/arc4: convert this stream cipher into a block cipher Date: Tue, 23 Feb 2010 08:15:20 +0800 Message-ID: <20100223001520.GB9118@gondor.apana.org.au> References: <20100209073718.GA17612@gondor.apana.org.au> <20100209145705.GA20421@Chamillionaire.breakpoint.cc> <20100209204519.GC26258@gondor.apana.org.au> <20100209211238.GC21548@Chamillionaire.breakpoint.cc> <20100209214522.GA27002@gondor.apana.org.au> <20100212084228.GA1535@Chamillionaire.breakpoint.cc> <20100216125125.GA390@gondor.apana.org.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Sebastian Andrzej Siewior , mbroz@redhat.com, dm-devel@redhat.com, agk@redhat.com, linux-crypto@vger.kernel.org To: Mikulas Patocka Return-path: Received: from rhun.apana.org.au ([64.62.148.172]:54115 "EHLO arnor.apana.org.au" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750766Ab0BWAP2 (ORCPT ); Mon, 22 Feb 2010 19:15:28 -0500 Content-Disposition: inline In-Reply-To: Sender: linux-crypto-owner@vger.kernel.org List-ID: On Mon, Feb 22, 2010 at 02:40:49PM -0500, Mikulas Patocka wrote: > > > So what we could do is simply add a new blkcipher arc4, alongside > > the existing cipher arc4. Then we can convert the existing users > > across, and finally remove the old arc4. > > arc4 can't be used as a block cipher --- see this paper > http://www.wisdom.weizmann.ac.il/~itsik/RC4/Papers/Rc4_ksa.ps , it says > that initialization vectors on RC4 are unreliable, if you use (unknown key > concatenated with known IV) or (known IV concatenated with unknown key) as > a RC4 key, the RC4 state can be exposed and the cipher is broken. What we call a blkcipher is not really a block cipher. In fact, what we call "cipher" is really a block cipher. So we're actually changing arc4 so that it doesn't get used as a block cipher, i.e., you will no longer be able to say "cbc(arc4)" or some such. I know it's confusing and perhaps one day we will rename blkcipher to skcipher and cipher to blkcipher. Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt