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Comment by jsiepkes | Hacker News Reader

> 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-f...

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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.

Or a low level format.

> 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_...

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.

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.

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).

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!

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.

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.

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

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.

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.

So ... you didn't read the article then?

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.

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