From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lists.gentoo.org (pigeon.gentoo.org [208.92.234.80]) by finch.gentoo.org (Postfix) with ESMTP id CDEFB138D18 for ; Wed, 15 Jul 2015 12:29:48 +0000 (UTC) Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id 0C6C4E0851; Wed, 15 Jul 2015 12:29:42 +0000 (UTC) Received: from mail-qg0-f49.google.com (mail-qg0-f49.google.com [209.85.192.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by pigeon.gentoo.org (Postfix) with ESMTPS id F380BE0827 for ; Wed, 15 Jul 2015 12:29:40 +0000 (UTC) Received: by qgef3 with SMTP id f3so17090476qge.0 for ; Wed, 15 Jul 2015 05:29:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:date:message-id:subject :from:to:content-type; bh=lhcF4F6O6bGlMJ1kUp8PFY2Y9rtX+dOTOXP1EGcIkVo=; b=vklHNhVAQr517DUlR3yQbZ4+MTArzcUusS8yAD8NDeX1qAEp9JasuDe4IzlB7U1U2y YTM1TJcZFcf9UDluySgL+6M1WYIVG9zXnH/hr+xVDgOFGpbW+N5FPUtmZreTvzupFnN6 4Ta9niU3CbX+Ol4vA8Sva29zz11le86Ca32R/zTGTU8X0Pbh6NMgzo+qNLOzWY+ehWrT C1odeS9jkufgQLTZQy2rbq85LMq31Oi3EQyRuNAG5A0xD3K6LI/tfjG+liv1UKbCfFwu XtBsvtIng/5NX9ncYLkmK0S/VHD7Lwpsd4fu/fsHWU7lkZFFPn6Ld+DB2k72heBe5eEr 2AdQ== Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-user@lists.gentoo.org Reply-to: gentoo-user@lists.gentoo.org MIME-Version: 1.0 X-Received: by 10.140.85.85 with SMTP id m79mr7910041qgd.37.1436963380170; Wed, 15 Jul 2015 05:29:40 -0700 (PDT) Sender: freemanrich@gmail.com Received: by 10.140.34.68 with HTTP; Wed, 15 Jul 2015 05:29:40 -0700 (PDT) In-Reply-To: References: <20150712143525.07b6bdf5@thetick> <55A296A7.5070301@googlemail.com> <55A2D180.2030109@googlemail.com> Date: Wed, 15 Jul 2015 08:29:40 -0400 X-Google-Sender-Auth: fbcrTCCD_fhRdbTLcN8tmVKRGTg Message-ID: Subject: Re: [gentoo-user] Securely deletion of an HDD From: Rich Freeman To: gentoo-user@lists.gentoo.org Content-Type: text/plain; charset=UTF-8 X-Archives-Salt: 31f3ec0b-643e-4427-94d7-959695154098 X-Archives-Hash: 85aa2acb9d72e44e3922b686d36978ac On Tue, Jul 14, 2015 at 6:21 PM, R0b0t1 wrote: > > On Sun, Jul 12, 2015 at 7:18 PM, Rich Freeman wrote: > >I think that assumes that the two get averaged together in some way > >and cannot be separated. If you could determine the orientation of > >individual magnetic domains it is possible that you might be able to > >determine which ones are which. For example, if in a given location > >you found 90% of the grains had one orientation, and 10% had another, > >you might be able to infer that the 10% was the previous value of that > >location. > > Every bit on the disk will have this ghost inverse behind it. If you > flip bits at random - what overwriting the drive with random data > effectively does - then it's impossible to tell which ones were > flipped recently and which ones were flipped before the last write. If a disk head moves across a track and lays down a pattern of magnetic fields, I imagine that the intensity of those fields will vary with distance from the head. If the head makes a second pass writing a different pattern of magnetic fields following a path not identical to the first, I imagine that those field intensities will also vary with distance from the head, but particles on ones die of the track will probably retain more of the former pattern and particles on the other side of the track would tend to retain more of the second pattern. I'm just not seeing anything that suggests that such an attack is physically impossible. It might be impractical today. It might be impractical forever. However, impossible is a very high bar to clear. Whether somebody with a technical capability so advanced that it is so debatable today fits within your threat model is a different story. Clearly these techniques are not available commercially/etc. If you're afraid of the NSA and you have unencrypted data on a disk in the first place they've probably already defeated your security in 100 different ways already. So, I'll agree there is a practical argument to be made. However, I can't really agree that something is physically impossible unless you can prove it from first principles. -- Rich