Re: [gentoo-user] Securely deletion of an HDD
On Tue, Jul 14, 2015 at 6:21 PM, R0b0t1 r03...@gmail.com wrote: On Sun, Jul 12, 2015 at 7:18 PM, Rich Freeman ri...@gentoo.org 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
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 7:18 PM, Rich Freeman ri...@gentoo.org 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. That probably isn't practical with current technology, but I see no reason that it should be impossible. Magnetic force microscopy has a resolution fine enough to read any disk that can be created - they're just really expensive. On Sun, Jul 12, 2015 at 8:50 PM, Thomas Mueller mueller6...@bellsouth.net wrote: All that has been said on this thread supposes that the hard drive is still readable and writable. On Sun, Jul 12, 2015 at 6:22 PM, R0b0t1 r03...@gmail.com wrote: If you need to destroy a platter drive take it apart and sand the platters (probably the easiest). If it's solid state heat the drive over 150C-250C for an extended period of time or mechanically destroy the chips.
Re: [gentoo-user] Securely deletion of an HDD
Am Sun, 12 Jul 2015 22:43:44 +0200 schrieb Volker Armin Hemmann volkerar...@googlemail.com: http://www.howtogeek.com/115573/htg-explains-why-you-only-have-to-wipe-a-disk-once-to-erase-it/ Yeah, that was linked from the Arch wiki I looked at. http://www.vidarholen.net/~vidar/overwriting_hard_drive_data.pdf FWIW, Peter Gutmann doesn't have much good to say about that article (specifically, he wrote about the related blog article at [0] in his Further Epilogue at [1]). Regardless, the summary still seems to be: with modern high-density drives, there is *no* wiggle room outside for remnants of data to stick around after overwriting it, outside of some potential future method that is probably a) far enough away into the future that the data on the drive is uninteresting by then (if it ever was interesting to begin with!) and b) prohibitively expensive (at least at the start), which pushes the earliest time someone might ever look at my old hard drives even further back. This assumes that anybody is interested in developing something like that, if it's even possible. I can't help but wonder what the situation is like with tape, which still commonly used for backups. ISTR that huge densities are also the norm there, but that's about all I know. [0] https://web.archive.org/web/20090722235051/http://sansforensics.wordpress.com/2009/01/15/overwriting-hard-drive-data [1] https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html -- Marc Joliet -- People who think they know everything really annoy those of us who know we don't - Bjarne Stroustrup pgpQE7rExP0Zs.pgp Description: Digitale Signatur von OpenPGP
Re: [gentoo-user] Securely deletion of an HDD
On Mon, Jul 13, 2015 at 4:05 AM, Volker Armin Hemmann volkerar...@googlemail.com wrote: Am 12.07.2015 um 23:30 schrieb Rich Freeman: Impossible is a pretty bold claim. You need proof, not evidence that a particular recovery technique didn't work. I can demonstrate very clearly that I'm unable to crack DES, but that doesn't make it secure. they gave you the prove. Others have found the same. If you are unable to understand what they wrote, just say so. By all means point out more specifically where you think they made a theoretical argument. I see lots of talk of measurements and lots of empirical-looking numbers. Theoretical arguments tend to involve lots of h-bars over pis and such. As far as others finding the same goes, that also tends to characterize this as an experimental/practical argument. You generally don't tend to have publications of reproductions of theoretical arguments since about all you can do is either point out an error in the math or extend it. Such experiments are useful, but they're not airtight. It is the difference between AES and a one-time pad. The former has no known method of circumvention and seems really hard to attack, the latter is theoretically impossible to attack if correctly implemented, but probably impossible to truly implement correctly. I don't worry about using AES, but I'm not under any illusions that it is completely unbreakable. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
Marc Joliet mar...@gmx.de wrote:
Hi,
I have to failed drives that I want to give away for recycling purposes, but
want to be sure to properly clear them first. They used be part of a btrfs
The test patterns used on Solaris and marked with federal requirements are:
int purge_patterns[]= { /* patterns to be written */
0x, /* 10101010... */
0x, /* 01010101... == ... */
0x, /* 10101010... */
0x, /* 10101010... */
}
Jörg
--
EMail:jo...@schily.net(home) Jörg Schilling D-13353 Berlin
joerg.schill...@fokus.fraunhofer.de (work) Blog:
http://schily.blogspot.com/
URL: http://cdrecord.org/private/
http://sourceforge.net/projects/schilytools/files/'
Re: [gentoo-user] Securely deletion of an HDD
Am Mon, 13 Jul 2015 01:50:57 + schrieb Thomas Mueller mueller6...@bellsouth.net: All that has been said on this thread supposes that the hard drive is still readable and writable. But the original post stated this was a failed drive. Then you might not be able to dd if=/dev/zero of=/dev/sdx .. or whatever else. You would be stopped by bad sectors. The two drives I'm referring to here failed in the sense that they have no more reallocation sectors available. Perhaps that will make it difficult to wipe them properly, but they were fine mechanically when I removed them. -- Marc Joliet -- People who think they know everything really annoy those of us who know we don't - Bjarne Stroustrup pgpm3N2V4UFQG.pgp Description: Digitale Signatur von OpenPGP
Re: [gentoo-user] Securely deletion of an HDD
Am Sun, 12 Jul 2015 18:32:39 +0200 schrieb Volker Armin Hemmann volkerar...@googlemail.com: Am 12.07.2015 um 14:35 schrieb Marc Joliet: Hi, I have to failed drives that I want to give away for recycling purposes, but want to be sure to properly clear them first. They used be part of a btrfs RAID10 array, but needed to be replaced (with btrfs replace). (In the meantime I converted the array to RAID1 with only two drives.) My question is how precisely the disks should be cleared. From various sources I know that overwriting them with random data a few times is enough to render old versions of data unreadable. I'm guessing 3 times ought to be enough, but maybe even that small amount is overly paranoid these days? As to the actual command, I would suspect something like dd if=/dev/urandom of=/dev/sdx bs=4096 should suffice, and according to https://wiki.archlinux.org/index.php/Random_number_generation#.2Fdev.2Furandom, /dev/urandom ought to be random enough for this task. Or are cat/cp that much faster? Any thoughts? Greetings actually 1 time is enough. With zeros. Or ones. Does not matter at all. If you look at my initial response to Rich, I already concluded that one time is enough, although I'm going to stick with whatever random data shred(1) produces. -- Marc Joliet -- People who think they know everything really annoy those of us who know we don't - Bjarne Stroustrup pgpykgWwMsVBc.pgp Description: Digitale Signatur von OpenPGP
Re: [gentoo-user] Securely deletion of an HDD
Am 12.07.2015 um 23:30 schrieb Rich Freeman: On Sun, Jul 12, 2015 at 5:20 PM, Volker Armin Hemmann volkerar...@googlemail.com wrote: read the second link I provided. I did. It contains no theoretical arguments against the possibility yes it does. of data recovery. Theoretical limits would be ones like the uncertainty principle. If a given amount of matter could only store a certain number of bits, and that number of bits is already being stored, then it would be clearly impossible to recover more. And then google for yourself. For what? Back then it was very hard. Today it is impossible. You toss a coin for every bit. And that is your chance to extract anything. Impossible is a pretty bold claim. You need proof, not evidence that a particular recovery technique didn't work. I can demonstrate very clearly that I'm unable to crack DES, but that doesn't make it secure. they gave you the prove. Others have found the same. If you are unable to understand what they wrote, just say so.
Re: [gentoo-user] Securely deletion of an HDD
Am 13.07.2015 um 03:50 schrieb Thomas Mueller: All that has been said on this thread supposes that the hard drive is still readable and writable. But the original post stated this was a failed drive. Then you might not be able to dd if=/dev/zero of=/dev/sdx .. or whatever else. You would be stopped by bad sectors. Or a hard drive might not be accessible at all through the computer interface. I heard something that sounded like a modem dialing, but had no such modem. Going around with my eyes and ears led me to determine that it was a hard drive whining in an external eSATA enclosure, no longer recognized or accessible from the computer. That was a Western Digital Green 3 TB hard drive that replaced, under warranty, a WD Green 3 TB hard drive that developed bad sectors. Fortunately I had no confidential data on that hard drive. So everything in this thread says nothing about if the hard drive failed due to a mechanical problem. Then the data could not be overwritten by ordinary means, but could still be read by techniques such as used by Drive Savers. in case of mechanical failure: open case, rub platters on the carpet. Done.
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 10:39 AM, Marc Joliet mar...@gmx.de wrote: Am Sun, 12 Jul 2015 08:48:48 -0400 schrieb Rich Freeman ri...@gentoo.org: If it weren't painful to set up and complicated for rescue attempts, I'd just use full-disk encryption with a strong key on a flash drive or similar. Then the disk is as good as wiped if separated from the key already. Plus you don't have to worry about reallocated sectors (which might only contain single bit errors). Currently I'm planning on waiting for btrfs to support it. Chris Mason recently mentioned that it's definitely something they want to look at (https://youtu.be/W3QRWUfBua8?t=631), and it's not something that is so important to me personally that I have to have it right this instant. While some kind of native support would be nice, and likely more efficient in some ways, you could just layer btrfs on top of an encrypted loopback device. The problem is you'll need various scripts in your initramfs (or root partition if you don't bother to encrypt it) to actually set that up. In the event of a recovery situation you'll need to do all that setting up before you can run something like fsck on the disks and so on. In the event of a power loss I'd have to think through the failure modes, but I think you'd be fine as long as everything respected barriers, and btrfs/zfs already do checksuming. The typical approach is to use many rounds of encryption using a keyed-in password. That is a pretty good approach but obviously not nearly as secure as just using a completely random key with the full amount of entropy. A hand-keyed password with more entropy than the cipher uses would also be fine, but that would be a very long password (we're not just talking battery horse staple here). I guess you could just use a USB drive as your boot partition with the keys on it and keep a few copies of it, and with a decent grub setup on it that would also work for rescue purposes. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
On Sun, 12 Jul 2015 15:21:41 -0400, Rich Freeman wrote: While some kind of native support would be nice, and likely more efficient in some ways, you could just layer btrfs on top of an encrypted loopback device. The problem with that approach, if you use RAID, is that all writes must be encrypted multiple times, once for each disk, unless you use MD RAID between the disk and the encryption layer. The problem is you'll need various scripts in your initramfs (or root partition if you don't bother to encrypt it) to actually set that up. With a single device, Dracut handles all this automatically. I have such a setup on my laptop and used to use custom scripts to call cryptsetup at boot time, until I got fed up with you and Canek banging on about Dracut and decided to give it another go. With the right boot options, it just works. -- Neil Bothwick Any sufficiently advanced bug is indistinguishable from a feature. pgpLGZoheOcYT.pgp Description: OpenPGP digital signature
Re: [gentoo-user] Securely deletion of an HDD
Am 12.07.2015 um 21:14 schrieb Rich Freeman: On Sun, Jul 12, 2015 at 12:32 PM, Volker Armin Hemmann volkerar...@googlemail.com wrote: actually 1 time is enough. With zeros. Or ones. Does not matter at all. That depends on your threat model. nope. It doesn't. You believe in some urban legend you never dared to question. If you're concerned about somebody reading the contents of the drive using the standard ATA commands, then once with zeros is just fine. Secure erase is probably easier/faster. If you're concerned about somebody removing the disks from the drive and reading them with specialized equipment then you really want multiple rounds of complete overwrites with random data. Even then you run the risk of relocation blocks and all that stuff, so the secure erase at the end is still a wise move but it may or may not completely do the job. even then one time is enough. Links are below. If you're concerned about somebody leaving the disks in the drive but having access to directly manipulate the drive heads to possibly access data not accessible using the standard ATA commands then one pass is probably good enough, but I'd still use random data instead of zeros. The reason is that a clever firmware (especially on an SSD) might not actually record zeros to the regular disk space, but instead just mark the block range as containing zeros, leaving the actual data intact. For random data the drive has to actually store the contents as it cannot be represented in any more concise way. If I'm not in a rush I prefer to just do the multiple passes. Why take a chance? if you do it, it is your problem, but recommending something stupid is something else altogether. And of course full-disk encryption is the solution to all of the above, as it defeats any kind of attack at the level of the drive and is proactive in nature. cute. Unlike you, I read some stuff before posting. This is OLD NEWS: http://www.howtogeek.com/115573/htg-explains-why-you-only-have-to-wipe-a-disk-once-to-erase-it/ http://www.vidarholen.net/~vidar/overwriting_hard_drive_data.pdf to quote: Resultantly, if there is less than a 1% chance of determining each character to be recovered correctly, the chance of a complete 5-character word being recovered drops exponentially to 8.463E-11 (or less on a used drive and who uses a new raw drive format). This results in a probability of less than 1 chance in 10Exp50 of recovering any useful data. So close to zero for all intents and definitely not within the realm of use for forensic presentation to a court. 10^50. Think about that for a moment. And that is not 'all the data' but 'any useful data'.
Re: [gentoo-user] Securely deletion of an HDD
@topic: I would strongly suggest using a hardware key that also utilizes a passphrase. To delete, remove the key and/or don't tell anyone the passphrase. If you need to destroy a platter drive take it apart and sand the platters (probably the easiest). If it's solid state heat the drive over 150C-250C for an extended period of time or mechanically destroy the chips. It contains no theoretical arguments against the possibility of data recovery. The superparmagnetic limit sets the upper bound for storage density. It is impossible to store information inside the grain of a metal because it acts as if the magnetic moment is the sum of all of the atoms in the grain. At this size, the polarity of the magnet can randomly flip directions depending on the temperature. For ~2005 drives that was about 1Tbit/in^2 with ~850Gbit/in^2 used. Newer drives continue to have higher numbers but unless the efficiency drops there is not enough room to shadow all the data (you will need to calculate or find these numbers for each drive you are interested in). At best you could hope to recover some portion of it with magnetic force microscopy, which you can/should assume will read back at the maximum density available on the medium. But, simpler: if you combine a random stream of data with what is on the drive, the result looks just like random data. You need only overwrite the drive once.
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 4:43 PM, Volker Armin Hemmann volkerar...@googlemail.com wrote: Unlike you, I read some stuff before posting. This is OLD NEWS: No need to be rude. http://www.howtogeek.com/115573/htg-explains-why-you-only-have-to-wipe-a-disk-once-to-erase-it/ http://www.vidarholen.net/~vidar/overwriting_hard_drive_data.pdf to quote: Resultantly, if there is less than a 1% chance of determining each character to be recovered correctly, the chance of a complete 5-character word being recovered drops exponentially to 8.463E-11 (or less on a used drive and who uses a new raw drive format). This results in a probability of less than 1 chance in 10Exp50 of recovering any useful data. So close to zero for all intents and definitely not within the realm of use for forensic presentation to a court. 10^50. Think about that for a moment. And that is not 'all the data' but 'any useful data'. This really looks like a pragmatic argument, and not a theoretical one. I see no arguments based on hard laws of physics. This argument basically says that because this lab couldn't read the data with their equipment/methods, it is impossible for anybody to do it at any time in the future using any equipment. I'd say Schneier's Law applies. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 12:32 PM, Volker Armin Hemmann volkerar...@googlemail.com wrote: actually 1 time is enough. With zeros. Or ones. Does not matter at all. That depends on your threat model. If you're concerned about somebody reading the contents of the drive using the standard ATA commands, then once with zeros is just fine. Secure erase is probably easier/faster. If you're concerned about somebody removing the disks from the drive and reading them with specialized equipment then you really want multiple rounds of complete overwrites with random data. Even then you run the risk of relocation blocks and all that stuff, so the secure erase at the end is still a wise move but it may or may not completely do the job. If you're concerned about somebody leaving the disks in the drive but having access to directly manipulate the drive heads to possibly access data not accessible using the standard ATA commands then one pass is probably good enough, but I'd still use random data instead of zeros. The reason is that a clever firmware (especially on an SSD) might not actually record zeros to the regular disk space, but instead just mark the block range as containing zeros, leaving the actual data intact. For random data the drive has to actually store the contents as it cannot be represented in any more concise way. If I'm not in a rush I prefer to just do the multiple passes. Why take a chance? And of course full-disk encryption is the solution to all of the above, as it defeats any kind of attack at the level of the drive and is proactive in nature. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
Am 12.07.2015 um 23:10 schrieb Rich Freeman: On Sun, Jul 12, 2015 at 4:43 PM, Volker Armin Hemmann volkerar...@googlemail.com wrote: Unlike you, I read some stuff before posting. This is OLD NEWS: No need to be rude. http://www.howtogeek.com/115573/htg-explains-why-you-only-have-to-wipe-a-disk-once-to-erase-it/ http://www.vidarholen.net/~vidar/overwriting_hard_drive_data.pdf to quote: Resultantly, if there is less than a 1% chance of determining each character to be recovered correctly, the chance of a complete 5-character word being recovered drops exponentially to 8.463E-11 (or less on a used drive and who uses a new raw drive format). This results in a probability of less than 1 chance in 10Exp50 of recovering any useful data. So close to zero for all intents and definitely not within the realm of use for forensic presentation to a court. 10^50. Think about that for a moment. And that is not 'all the data' but 'any useful data'. This really looks like a pragmatic argument, and not a theoretical one. I see no arguments based on hard laws of physics. This argument basically says that because this lab couldn't read the data with their equipment/methods, it is impossible for anybody to do it at any time in the future using any equipment. I'd say Schneier's Law applies. read the second link I provided. And then google for yourself. All that 'overwritte many times' crap came from people who never read Guttman's original paper closely. Back then it was very hard. Today it is impossible. You toss a coin for every bit. And that is your chance to extract anything. There are better chances to extract the key you used to encrypt your data from RAM than to extract any useful data from a harddisk that was overwritten once.
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 5:20 PM, Volker Armin Hemmann volkerar...@googlemail.com wrote: read the second link I provided. I did. It contains no theoretical arguments against the possibility of data recovery. Theoretical limits would be ones like the uncertainty principle. If a given amount of matter could only store a certain number of bits, and that number of bits is already being stored, then it would be clearly impossible to recover more. And then google for yourself. For what? Back then it was very hard. Today it is impossible. You toss a coin for every bit. And that is your chance to extract anything. Impossible is a pretty bold claim. You need proof, not evidence that a particular recovery technique didn't work. I can demonstrate very clearly that I'm unable to crack DES, but that doesn't make it secure. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
All that has been said on this thread supposes that the hard drive is still readable and writable. But the original post stated this was a failed drive. Then you might not be able to dd if=/dev/zero of=/dev/sdx .. or whatever else. You would be stopped by bad sectors. Or a hard drive might not be accessible at all through the computer interface. I heard something that sounded like a modem dialing, but had no such modem. Going around with my eyes and ears led me to determine that it was a hard drive whining in an external eSATA enclosure, no longer recognized or accessible from the computer. That was a Western Digital Green 3 TB hard drive that replaced, under warranty, a WD Green 3 TB hard drive that developed bad sectors. Fortunately I had no confidential data on that hard drive. So everything in this thread says nothing about if the hard drive failed due to a mechanical problem. Then the data could not be overwritten by ordinary means, but could still be read by techniques such as used by Drive Savers. Tom
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 6:22 PM, R0b0t1 r03...@gmail.com wrote: But, simpler: if you combine a random stream of data with what is on the drive, the result looks just like random data. You need only overwrite the drive once. 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. That probably isn't practical with current technology, but I see no reason that it should be impossible. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
On Sun, Jul 12, 2015 at 8:35 AM, Marc Joliet mar...@gmx.de wrote: My question is how precisely the disks should be cleared. From various sources I know that overwriting them with random data a few times is enough to render old versions of data unreadable. I'm guessing 3 times ought to be enough, but maybe even that small amount is overly paranoid these days? As to the actual command, I would suspect something like dd if=/dev/urandom of=/dev/sdx bs=4096 should suffice, and according to https://wiki.archlinux.org/index.php/Random_number_generation#.2Fdev.2Furandom, /dev/urandom ought to be random enough for this task. Or are cat/cp that much faster? I'd probably just use a tool like shred/wipe, but you have the general idea. I'd probably follow it up with an ATA secure erase - for an SSD it is probably the only way to be sure (well, to the extent that you trust the firmware authors). If it weren't painful to set up and complicated for rescue attempts, I'd just use full-disk encryption with a strong key on a flash drive or similar. Then the disk is as good as wiped if separated from the key already. -- Rich
Re: [gentoo-user] Securely deletion of an HDD
On Sunday 12 Jul 2015 13:35:25 Marc Joliet wrote: Hi, I have to failed drives that I want to give away for recycling purposes, but want to be sure to properly clear them first. They used be part of a btrfs RAID10 array, but needed to be replaced (with btrfs replace). (In the meantime I converted the array to RAID1 with only two drives.) My question is how precisely the disks should be cleared. From various sources I know that overwriting them with random data a few times is enough to render old versions of data unreadable. I'm guessing 3 times ought to be enough, but maybe even that small amount is overly paranoid these days? As to the actual command, I would suspect something like dd if=/dev/urandom of=/dev/sdx bs=4096 should suffice, and according to https://wiki.archlinux.org/index.php/Random_number_generation#.2Fdev.2Furan dom, /dev/urandom ought to be random enough for this task. Or are cat/cp that much faster? Any thoughts? Greetings I use urandom a couple of times (3 to 5), because random takes too long and I don't store state secrets on my disks. Then I dd onto it a final round of /dev/zero. Finally, run hdparm to securely erase it for good measure.[1] All of this could be an overkill, but do it out of habit these days. It is worth saying that I use haveged to increase entropy: [I] sys-apps/haveged Available versions: 1.5 ~1.7a 1.7a-r1 ~1.9.1 Installed versions: 1.7a-r1(12:46:23 04/21/14) Homepage:http://www.issihosts.com/haveged/ Description: A simple entropy daemon using the HAVEGE algorithm I should clarify that disks which contained financial data are dealth with a high speed angle grinder, after I remove the outer casing of the drive and don a pair of goggles.[2] *Only then* do I recycle the bits left. ;-) [1] https://ata.wiki.kernel.org/index.php/ATA_Secure_Erase [2] You can also use a hammer, a drill, or any similar implement which will completely break the physical disk platters to bits. -- Regards, Mick signature.asc Description: This is a digitally signed message part.
Re: [gentoo-user] Securely deletion of an HDD
Em 12/07/2015 10:03, Mick michaelkintz...@gmail.com escreveu: On Sunday 12 Jul 2015 13:35:25 Marc Joliet wrote: Hi, I have to failed drives that I want to give away for recycling purposes, but want to be sure to properly clear them first. They used be part of a btrfs RAID10 array, but needed to be replaced (with btrfs replace). (In the meantime I converted the array to RAID1 with only two drives.) My question is how precisely the disks should be cleared. From various sources I know that overwriting them with random data a few times is enough to render old versions of data unreadable. I'm guessing 3 times ought to be enough, but maybe even that small amount is overly paranoid these days? As to the actual command, I would suspect something like dd if=/dev/urandom of=/dev/sdx bs=4096 should suffice, and according to https://wiki.archlinux.org/index.php/Random_number_generation#.2Fdev.2Furan dom, /dev/urandom ought to be random enough for this task. Or are cat/cp that much faster? Any thoughts? Greetings I use urandom a couple of times (3 to 5), because random takes too long and I don't store state secrets on my disks. Then I dd onto it a final round of /dev/zero. Finally, run hdparm to securely erase it for good measure.[1] All of this could be an overkill, but do it out of habit these days. It is worth saying that I use haveged to increase entropy: [I] sys-apps/haveged Available versions: 1.5 ~1.7a 1.7a-r1 ~1.9.1 Installed versions: 1.7a-r1(12:46:23 04/21/14) Homepage:http://www.issihosts.com/haveged/ Description: A simple entropy daemon using the HAVEGE algorithm I should clarify that disks which contained financial data are dealth with a high speed angle grinder, after I remove the outer casing of the drive and don a pair of goggles.[2] *Only then* do I recycle the bits left. ;-) [1] https://ata.wiki.kernel.org/index.php/ATA_Secure_Erase [2] You can also use a hammer, a drill, or any similar implement which will completely break the physical disk platters to bits. -- Regards, Mick A physical damage is what I guess be the best choice for sensitive data. I use to disassemble the HDD and rub a strong magnet over the disks' surfaces. Just my 2c. Best regards, Francisco
Re: [gentoo-user] Securely deletion of an HDD
(Thanks to everyone for the replies so far!) Am Sun, 12 Jul 2015 08:48:48 -0400 schrieb Rich Freeman ri...@gentoo.org: On Sun, Jul 12, 2015 at 8:35 AM, Marc Joliet mar...@gmx.de wrote: My question is how precisely the disks should be cleared. From various sources I know that overwriting them with random data a few times is enough to render old versions of data unreadable. I'm guessing 3 times ought to be enough, but maybe even that small amount is overly paranoid these days? As to the actual command, I would suspect something like dd if=/dev/urandom of=/dev/sdx bs=4096 should suffice, and according to https://wiki.archlinux.org/index.php/Random_number_generation#.2Fdev.2Furandom, /dev/urandom ought to be random enough for this task. Or are cat/cp that much faster? I'd probably just use a tool like shred/wipe, but you have the general idea. Ah, I overlooked that shred can operate on device files! Thanks. I especially trust shred, since my main source was an article by its author (https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html). With regards to the other replies: I think physical destruction is unnecessary, and I don't really want to go through the trouble. The key bit in the above article is: [...]. If these drives require sophisticated signal processing just to read the most recently written data, reading overwritten layers is also correspondingly more difficult. A good scrubbing with random data will do about as well as can be expected. And this was in 1996! Drives have only gotten denser since then (e.g., perpendicular recording), and the epilogues (which reiterate the above) suggest that nothing has changed to make old data more recoverable. I noticed that the info manual to shred even says: On modern disks, a single pass should be adequate, and it will take one third the time of the default three-pass approach. The Arch wiki also arrives at the same conclusion (see https://wiki.archlinux.org/index.php/Securely_wipe_disk#Residual_magnetism), and provides some additional references. I'd probably follow it up with an ATA secure erase - for an SSD it is probably the only way to be sure (well, to the extent that you trust the firmware authors). Yeah, that sounds like a good idea. In the case of HDDs, even if I can't trust the firmware, I've already wiped what I can. With regards to SSDs, I've been meaning to read http://www.cypherpunks.to/~peter/usenix01.pdf. So my intermediate summary is: I'll probably use shred with one pass, followed by ATA (Enhanced) Secure Erase to erase the reallocated sectors (though I'll have to fiddle with my BIOS to do that). I'll be sure to read https://ata.wiki.kernel.org/index.php/ATA_Secure_Erase first. If it weren't painful to set up and complicated for rescue attempts, I'd just use full-disk encryption with a strong key on a flash drive or similar. Then the disk is as good as wiped if separated from the key already. Plus you don't have to worry about reallocated sectors (which might only contain single bit errors). Currently I'm planning on waiting for btrfs to support it. Chris Mason recently mentioned that it's definitely something they want to look at (https://youtu.be/W3QRWUfBua8?t=631), and it's not something that is so important to me personally that I have to have it right this instant. -- Marc Joliet -- People who think they know everything really annoy those of us who know we don't - Bjarne Stroustrup pgp3zot4xIl6d.pgp Description: Digitale Signatur von OpenPGP
Re: [gentoo-user] Securely deletion of an HDD
Am 12.07.2015 um 14:35 schrieb Marc Joliet: Hi, I have to failed drives that I want to give away for recycling purposes, but want to be sure to properly clear them first. They used be part of a btrfs RAID10 array, but needed to be replaced (with btrfs replace). (In the meantime I converted the array to RAID1 with only two drives.) My question is how precisely the disks should be cleared. From various sources I know that overwriting them with random data a few times is enough to render old versions of data unreadable. I'm guessing 3 times ought to be enough, but maybe even that small amount is overly paranoid these days? As to the actual command, I would suspect something like dd if=/dev/urandom of=/dev/sdx bs=4096 should suffice, and according to https://wiki.archlinux.org/index.php/Random_number_generation#.2Fdev.2Furandom, /dev/urandom ought to be random enough for this task. Or are cat/cp that much faster? Any thoughts? Greetings actually 1 time is enough. With zeros. Or ones. Does not matter at all.
