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I once read in a news article (can't find it now... sorry) that apparently if you overwrite data with other data on a hard drive that the previous data is unrecoverable. So, would overwriting the entire hard drive with cat videos be just as effective as all these other "professional" security protocols that are used?
janrinok: Data erasure is important when you want to prevent anyone from recovering whatever was written on the storage device in the first instance. But there are many potential problems including just how secure does the erasure have to be, what hardware is controlling the reading and writing to the disk, are you attempting to delete data on a spinning rust device, a more modern SSD , or a thumb drive, and who are you trying to prevent from reading the data? If you are just trying to prevent a regular Joe Soap from reading what you once securely stored on a hard drive then simple overwriting might be enough. However, if you are concerned that law enforcement or a government agency might be interested in the drive's contents then you will have to take more stringent precautions. Ultimately, many of the highest classifications of data can only be securely erased by full degaussing or the physical destruction of the device. The link details the various standards that are deemed as acceptable to securely erase data to meet specific documented requirements.
Presumably, if you are worried that someone might have access to your data then you have already taken the precautions of encrypting it. However, poor encryption is worse than no encryption at all - at least with the latter you know that your data is vulnerable. With a weak encryption you might incorrectly believe that your data is secure when, in truth, it is not. This might result in you taking risks that you wouldn't otherwise take with the physical protection of the drive itself. The military and government agencies often insist that drives are secured in an approved security container when not actually in use to prevent anyone actually getting to the data in the first instance. If at home you simply leave your drive in the computer or lying around in plain view then anyone entering your home can steal it. How much protection you need to give depends upon the value of the data to you and how much you need to ensure that no-one else can get to it.
Many proprietary encryption programs use an 'in-house' encryption scheme in the incorrect belief that it is more secure than the recognised encryption methods that have been rigorously tested and mathematically proven. Other systems might have back-doors or make the decryption algorithms available to LE or government agencies. I personally would strongly recommend against using these encryption systems because they might only be giving you a false sense of security. However, if your data is already encrypted with a recognised encryption system with a strong pass phrase and salt then you are well on your way to preventing anyone from ever getting access to the data even if they have the drive in their possession. Note that encryption that is 'unbreakable' today might not remain so with advances in computing and perhaps the discovery of encryption flaws. Essentially, if it is considered good enough for the military and government agencies then it is probably sufficient for your needs.
It is important to realise that, any time your data is inside your computer and viewable, then any encryption is already defeated. If you have valuable data that is protected by nothing more than a computer in hibernation then anyone who can awaken the computer has full access to the data.
So now we finally get to the question that the submitter asked. How secure is overwriting as a method of data deletion? If the data is already securely encrypted then perhaps no further action is required, or simply overwriting it with cat videos will probably be enough to prevent anyone but the most determined attacker from ever reading the data. It will certainly be enough to stop the vast majority of people from getting anything useful from the disk drive. If you believe that the data on the drive must never be recovered by anyone else then the physical destruction of the drive might be warranted. The actual requirement probably lies between those 2 extremes. Only you know the value of the data on the disk drive and how important it is that it is not disclosed.
I now invite everyone to contribute their own experiences, tips and advice regarding data erasure....
(Score: 2) by Dr Spin on Wednesday April 18 2018, @07:43AM
How can you tell? well the reality is that the head will not be exactly aligned on each pass over the track. When designing the servo system, you have got there when it aligns well enough to work reliably for the use case of reading and writing the users's data. If the head was a bit too far in when it wrote, and a bit too far out when it reads, then it will still work. However, if it is a bit too far in when it over-writes, and then, on a later pass, a bit too far out, it might be able to read some of the data.
This is extremely unlikely to be enough to recover the data, but it might be enough to demonstrate that there was a copy of that data on that disk.
This was a distinct possibility in the days of MFM recording, but I doubt it could be made to work with RLL and GCR, where N bits are encoded as N+1, or N+M, with multiple possible encodings. Also, I would point out that forward error correction is NEEDED in order to read modern disks. If there are more than a few bits in error, then the correction has equal probability of correcting or further messing up. Even if you were looking for one particular word, and knew which sector to look for it, this is NOT going to work on today's hard disks. (I cant speak for SSD's). Remember that, the erased data was probably not written on virgin disk anyway, so some of what you read outside the official track (maybe one or two bits, maybe whole sectors) might be from one, two or even a hundred writes earlier. On a Unix disk, in-place rewrites are not common, so some files (like those in /etc) constantly move about, over writing other deleted files. Others, like the cat video you never even got round to watching, stay where you put them for the life of the disk. I have no idea what happens with Windows - and no wish to know.
I spent a lot of years designing data separators which would out perform Seagate's, IBM's and Western Digital's (in the days of ST506/MFM) and no, you won't never get data back if over-written on anything ATA or later. With GCR and later, I expect that, with an entire team of the people who use electron microscopes to read ancient Sumerian cuneiform tablets, you could possibly prove there was data there before it was over written, but, no, not prove what the data was.
However, unless you fully understand the low level file system assembler code, you cannot be sure that "overwritten" means what it says on the tin. And the TLA's and data recovery firms of this world probably don't want you to know that.
Warning: Opening your mouth may invalidate your brain!