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> Flash storage, unlike traditional hard drives, can really be completely and irrecoverably erased, with no lingering magnetic ghosts that a sufficiently determ

> Flash storage, unlike traditional hard drives, can really be completely and irrecoverably erased, with no lingering magnetic ghosts that a sufficiently determined person could in theory reconstruct. I think people vastly overestimate the possibility to recover any data from a wiped HDD. See for example this article by SANS [1]. To recover a single bit you have a better chance of flipping a coin to see if the bit was a zero or a one then you have when you are actually trying to reconstruct it with lab equipment. [1] https://web.archive.org/web/20140621130410/https://digital-forensics.sans.org/blog/2009/01/15/overwriting-hard-drive-data/ https://web.archive.org/web/20140621130410/https://digital-f...

germanier 4y ago
This can't be true. Any method must be at least as good as a coin toss. If it's worse you can just bit flip the result and end up better than the coin.
- detrites 4y ago
  Unbelievably, the paper actually makes that claim: > That is, if you toss a coin, you have a 50% chance of correctly choosing the value. In many instances, using a MFM to determine the prior value written to the hard drive was less successful than a simple coin toss. More believably, the paper was written by Dr Craig Wright.
  - kryps 4y ago
    The paper does not make that claim. According to the paper the probability of recovering the correct value of a single bit is 96% for a pristine drive and 56% for a used drive, both better than a coin toss. Of course the probability gets worse if one tries to recover more bits (but still better than n coin tosses for n bits).
  - marginalia_nu 4y ago
    Right, if you flip a hundred coins, about half of them land heads and the other half tails. The correct coins we'll call oracles, and the incorrect ones liars. Now you perform a hdd recovery that is successful 70% of the time. That is still worse than the performance of the oracle coins which were correct, and if you invert the performance of the liars, they also beat the performance of the recovery. If you have two coins, one which is always correct and one which is always incorrect, you can flip one of them to decide whether the other one is correct (and pick the opposite). Check mate, statisticians!
- mecsred 4y ago
  I don't think that follows? Just because a better method exists (the inverse) doesn't make the original perform better. Imagine the following method: take whatever the bit was, and set it to the opposite of that. 0% < 50%. Of course you can invert and get 100%, but that's a different method. It doesn't make the original any better.
- DSMan195276 4y ago
  The article isn't super clear, but I think what it's saying is that the data recovery method they were testing (when looking at recovering a single bit) was as good as 92%, and as bad as 32%, depending on how the data was overwritten and other factors like the specific drive. Thus you likely can't just flip the result to get better odds because in a real scenario you wouldn't know which category you fall into (whether it's really good, or really bad), and overall if you don't know that information then the odds are close to 50%. Edit: And of course even a high of 92% is abysmal for a single bit, since the combined odds of multiple bits quickly goes to zero for any sizable amount of data.
- kmeisthax 4y ago
  Bits are correlated: we care about the order of the bits, not just their population count. Furthermore, <50% is only an antipredictor for a single coin flip. If you have two bits - two coin flips where you care about the order - then there's four values, and you have to be <25% to be an antipredictor. Even then, an antipredictor only removes one possibility[0], so flipping the bits will only slightly improve your terrible chances of recovering the data. [0] https://en.wikipedia.org/wiki/Monty_Hall_problem https://en.wikipedia.org/wiki/Monty_Hall_problem
fdeee 4y ago
I think you are right in that it is quite impossible to read data that has been overwritten from a sufficiently modern HDD. But all modern HDDs retire sectors that exhibit a higher correctable error rate and replace them by reserve sectors. Those retired sectors then still contain readable data (maybe with some bit errors) if you know the appropriate factory mode commands. Overwriting the entire disk also won't overwrite retired sectors, except if the disk has an explicit function for it and you trust that function to work properly. In short, you always need full disk encryption.
- Dalewyn 4y ago
  Or a low level format.
deleted 4y ago
[deleted]
throw0101a 4y ago
> See for example this article by SANS [1]. According to NIST 800-88 Rev. 1 a single overwrite is fine: * https://en.wikipedia.org/wiki/Data_remanence#Feasibility_of_recovering_overwritten_data https://en.wikipedia.org/wiki/Data_remanence#Feasibility_of_...
bcoates 4y ago
Also it's not obvious that it works the other way. The only requirement of the flash reset operation is that it consistently causes bits to read as 1 and that the normal write operation allows them to read as 0 again; it doesn't actually follow from that that an erased-but-not-reprogrammed block will have cell charge levels that are indistinguishable from each other based on the pre-erasure charge level.
Thaxll 4y ago
All those articles are very theoretical, in reality 1 pass of dd /dev/zero makes things unrecoverable, for recent SSD since trim is enabled by default you can't recover anything, I deleted a document by mistake on an SSD and it was impossible to get back, the moment I pressed delete on windows the trim command was sent and everything was erased on the ssd. Pro recovery compagny that are not shady know that SSD recovery with TRIM is impossible.
- luma 4y ago
  So ... you didn't read the article then?
  - Thaxll 4y ago
    The article is light on details and assume a lot of things. he doesn't prove anything, show us that after succesful trim you're able to recover anything. From my experience it's impossible.

germanier 4y ago

This can't be true. Any method must be at least as good as a coin toss. If it's worse you can just bit flip the result and end up better than the coin.

detrites 4y ago
Unbelievably, the paper actually makes that claim: > That is, if you toss a coin, you have a 50% chance of correctly choosing the value. In many instances, using a MFM to determine the prior value written to the hard drive was less successful than a simple coin toss. More believably, the paper was written by Dr Craig Wright.
- kryps 4y ago
  The paper does not make that claim. According to the paper the probability of recovering the correct value of a single bit is 96% for a pristine drive and 56% for a used drive, both better than a coin toss. Of course the probability gets worse if one tries to recover more bits (but still better than n coin tosses for n bits).
- marginalia_nu 4y ago
  Right, if you flip a hundred coins, about half of them land heads and the other half tails. The correct coins we'll call oracles, and the incorrect ones liars. Now you perform a hdd recovery that is successful 70% of the time. That is still worse than the performance of the oracle coins which were correct, and if you invert the performance of the liars, they also beat the performance of the recovery. If you have two coins, one which is always correct and one which is always incorrect, you can flip one of them to decide whether the other one is correct (and pick the opposite). Check mate, statisticians!
mecsred 4y ago
I don't think that follows? Just because a better method exists (the inverse) doesn't make the original perform better. Imagine the following method: take whatever the bit was, and set it to the opposite of that. 0% < 50%. Of course you can invert and get 100%, but that's a different method. It doesn't make the original any better.
DSMan195276 4y ago
The article isn't super clear, but I think what it's saying is that the data recovery method they were testing (when looking at recovering a single bit) was as good as 92%, and as bad as 32%, depending on how the data was overwritten and other factors like the specific drive. Thus you likely can't just flip the result to get better odds because in a real scenario you wouldn't know which category you fall into (whether it's really good, or really bad), and overall if you don't know that information then the odds are close to 50%. Edit: And of course even a high of 92% is abysmal for a single bit, since the combined odds of multiple bits quickly goes to zero for any sizable amount of data.
kmeisthax 4y ago
Bits are correlated: we care about the order of the bits, not just their population count. Furthermore, <50% is only an antipredictor for a single coin flip. If you have two bits - two coin flips where you care about the order - then there's four values, and you have to be <25% to be an antipredictor. Even then, an antipredictor only removes one possibility[0], so flipping the bits will only slightly improve your terrible chances of recovering the data. [0] https://en.wikipedia.org/wiki/Monty_Hall_problem https://en.wikipedia.org/wiki/Monty_Hall_problem

fdeee 4y ago

I think you are right in that it is quite impossible to read data that has been overwritten from a sufficiently modern HDD. But all modern HDDs retire sectors that exhibit a higher correctable error rate and replace them by reserve sectors. Those retired sectors then still contain readable data (maybe with some bit errors) if you know the appropriate factory mode commands. Overwriting the entire disk also won't overwrite retired sectors, except if the disk has an explicit function for it and you trust that function to work properly. In short, you always need full disk encryption.

Dalewyn 4y ago
Or a low level format.

deleted 4y ago

[deleted]

throw0101a 4y ago

> See for example this article by SANS [1]. According to NIST 800-88 Rev. 1 a single overwrite is fine: * https://en.wikipedia.org/wiki/Data_remanence#Feasibility_of_recovering_overwritten_data https://en.wikipedia.org/wiki/Data_remanence#Feasibility_of_...

bcoates 4y ago

Also it's not obvious that it works the other way. The only requirement of the flash reset operation is that it consistently causes bits to read as 1 and that the normal write operation allows them to read as 0 again; it doesn't actually follow from that that an erased-but-not-reprogrammed block will have cell charge levels that are indistinguishable from each other based on the pre-erasure charge level.

Thaxll 4y ago

All those articles are very theoretical, in reality 1 pass of dd /dev/zero makes things unrecoverable, for recent SSD since trim is enabled by default you can't recover anything, I deleted a document by mistake on an SSD and it was impossible to get back, the moment I pressed delete on windows the trim command was sent and everything was erased on the ssd. Pro recovery compagny that are not shady know that SSD recovery with TRIM is impossible.

luma 4y ago
So ... you didn't read the article then?
- Thaxll 4y ago
  The article is light on details and assume a lot of things. he doesn't prove anything, show us that after succesful trim you're able to recover anything. From my experience it's impossible.