From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from v6.tansi.org (ns.km31936-01.keymachine.de [87.118.116.4]) by mail.server123.net (Postfix) with ESMTP for ; Sun, 8 Feb 2015 10:23:35 +0100 (CET) Received: from gatewagner.dyndns.org (77-57-54-224.dclient.hispeed.ch [77.57.54.224]) by v6.tansi.org (Postfix) with ESMTPA id BBC7020DC1F9 for ; Sun, 8 Feb 2015 10:23:34 +0100 (CET) Date: Sun, 8 Feb 2015 10:23:34 +0100 From: Arno Wagner Message-ID: <20150208092334.GA20982@tansi.org> References: <20150205115435.GA4093@tansi.org> <20150205235135.GA21304@tansi.org> <20150206140140.GA16920@dashborg.com> <20150206182729.GB7283@tansi.org> <20150207172747.GA26528@dashborg.com> <20150207180356.GA4982@fritha.org> <20150207231624.GA23872@citd.de> <20150208081954.GA2856@fritha.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150208081954.GA2856@fritha.org> Subject: Re: [dm-crypt] plain: opening with a wrong password List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: dm-crypt@saout.de On Sun, Feb 08, 2015 at 09:19:54 CET, Heinz Diehl wrote: > On 08.02.2015, Matthias Schniedermeyer wrote: > > > > You need something to compare the passphrase to, and that's the hash. > > > How would you check the validity of the entered passphrase otherwise? > > > A plain text comparison is obviously impossible. > > > With Plain the password can't be verified, the dm-crypt device is setup > > and if the password was wrong, the "decrypted" device contains garbage. > > Containers usually have a means to test if the password is correct, > > plain does not. > > I tried to keep it simple in my example. Although you're (of course) right, I > didn't write about "plain encryption" or "plain dmcrypt", but plain text > comparison, in order to explain why there is the need for e.g. a hash. > As you point out, with plain dmcrypt the only possibility is actually > using the password and checking if the "decrypted" data based on it > makes any sense. Form a purely practical perspective, the difference usually negligible. Wile plain dm-crypt mounting fails at the mount-stage due to wrong filesystem signatures, LUKS mounting fails at the decrypt stage. >From an attacker's perspecive, the difference is also small, except that all the iteration in LUKS adds a massive amount of computational effort. The data in the LUKS header does not help the attacker at all. It does take a look at the details (as so often in crypto protocols) to see that though. Arno -- Arno Wagner, Dr. sc. techn., Dipl. Inform., Email: arno@wagner.name GnuPG: ID: CB5D9718 FP: 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718 ---- A good decision is based on knowledge and not on numbers. -- Plato If it's in the news, don't worry about it. The very definition of "news" is "something that hardly ever happens." -- Bruce Schneier