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How secure is merely discarding (TRIMing) all of a SSD's blocks?
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Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#42Earlier quoted context omitted.
Delete all the files then "dd" from the random device until the drive is full? Must we always jump straight to the hydraulic press?
As mentioned in the article, that's not reliable. Drives have lots of overprovision blocks and there's no way of consistently forcing them to be used; and drives might mark blocks as too questionable/worn to reuse and never reset them. Also a standard failure mode of SSDs is refusing to accept writes, so you'll have to handle that case.
Encryption plus a single rewrite is likely many times more sufficient than ever needed.
Even if there is a massive 10% block reserve, you looking at a random 10% of an entire encrypted drive.
Figuring out how to just decrypt those blocks is a very hard challenge, then figuring out the SSD block mapping is going to be very challenging, and then for a chance to maybe get something valuable from that 10%.
Multiple very difficult challenges for only a chance of maybe recovering something useful.
Related, I just had a hard drive head crash. I was able to use dd_rescue to read out 40% of the drive before it fully gave out. It's amazing, but at least NTFS stores data everywhere. I recovered a few small plain text file, but not a single image was recoverable. It was pretty shocking to me.
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#43As others have mentioned, use encryption. But that has gaps as someone can hold onto the drive and wait for impossible to break to become somewhat expensive to break to broke by mistake through bad encryption implementation . Put the encrypted drive into a hydraulic press and flatten it into a thin sheet, then run that sheet through a chop saw, or just use tin-snips if your hydraulic press was powerful enough. A hydr…
Delete all the files then "dd" from the random device until the drive is full? Must we always jump straight to the hydraulic press?
> the random device
/dev/zero would be sufficient.
Also easier to confirm things worked (if desired) since you know what to expect on read-back.
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#44> 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 wa…
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.
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.
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#45> 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 wa…
> 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_...
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#46> 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 wa…
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.
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#47Most relevant ssds are always encrypted.
I don't think it's necessary to delete it.
But if you want, write the disk fully
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#48I'm surprised people actually think TRIMming is in any way a "secure delete" operation. It's basically the "quick format" of SSDs: "hey, all those blocks here, I don't care about them anymore, use them as if they were free..." Without OS-side encryption, Secure Erase is the only reliable option (SSD encrypts everything with an internal key which then gets deleted by the secure erase command, leaving only random data…
> It's basically the "quick format" of SSDs: "hey, all those blocks here, I don't care about them anymore, use them as if they were free..."
Due to implementation details, SSDs can only use a block if it is clear. Thus trimming a block and waiting for long enough would be sufficient to erase the data if the driver actually enforced that the block will be reused.
So, it's not good enough, but this is not obvious at all.
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#49As others have mentioned, use encryption. But that has gaps as someone can hold onto the drive and wait for impossible to break to become somewhat expensive to break to broke by mistake through bad encryption implementation . Put the encrypted drive into a hydraulic press and flatten it into a thin sheet, then run that sheet through a chop saw, or just use tin-snips if your hydraulic press was powerful enough. A hydr…
Delete all the files then "dd" from the random device until the drive is full? Must we always jump straight to the hydraulic press?
Hydraulic press is too obscure, .45 ACP works just as well.
But it depends on your risk profile, for most people a wipe-and-reuse is going to be fine, especially if the "reuser" is already a trusted entity.
Re: How secure is merely discarding (TRIMing) all of a SSD's blocks?
#50This is why you always want to use full disk encryption (FDE). Then it doesn't really matter whether some blocks are recoverable after TRIM. It also doesn't really matter whether the Secure Delete function works. It also doesn't matter whether the very expensive security-buzzword-compliant secure deletion snakeoil software really works. Also, TRIM is only possible with drives that are still in working order. Very oft…
TRIM and FDE isn't perfect though as it leaks information about what parts have been written etc. And if you turn off TRIM you get a massive performance decrease.
Why would someone turn off TRIM?