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Comment by unwind | Hacker News Reader

Yeah, and it's not enough to do e.g.

    d = random_block()
    for block in capacity:
      block.write(d)
      if block.read() != d:
        fail("fake disk")

Since that would be fooled by a device mapping all reported blocks to the same actual storage. I guess you basically have to write unique data to all reported blocks first, then read them all back to verify.

Since storage devices are often bigger than RAM, the data is probably best to generate procedurally (but deterministically) from the block number.

That sounds like fun!

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I believe f3 (Fight Flash Fraud) already does random numbers with a preset seed: https://github.com/AltraMayor/f3/blob/master/f3read.c#L144

So they put that code on the drive controller, detect when f3 is being used, and generate the pattern dynamically in the drive controller.

Yes, that would be possible, but that would require effort on the counterfeiter's part. Any additional hardware in the drive controller is going to reduce the counterfeiter's profits. So why would they bother? H2testw, which f3's algorithm is based upon, is more than a decade old at this point with the same RNG generator and no one has made such a drive controller to trick it.

Ooh, I just did something with this, and I should finish the draft blog post for it:

* write encrypted zeroes

* decrypt it and count the zeroes

I think cbc mode is enough. I see a possible computational loophole, which I want to check before I post.

You could do that, but doesn't it seem easier to: 1) initialize a CSPRNG to a known seed; 2) generate random numbers and write them sequentially; 3) initialize a new RNG to the same seed and compare numbers you generate to numbers you read from the storage? That way, you even get to point at the exact byte where the first divergence happens, which might let you say, "this drive which claims to be 30TB seems to be 1GB".

I think that would avoid the potential loophole I was worried about! I'd be careful with the periodicity of the RNG though.

I was on a machine with limited user space. I used OpenSSL, no new binaries, reading and writing at close-to-wire speeds. "Counting zeroes" would track down where they don't match, doesn't it?

CSPRNGs don’t have that issue.

Once you find the exact byte where you know the storage ends, you can actually format the drive it's completely usable. F3 can do this with f3probe and f3fix.

https://fight-flash-fraud.readthedocs.io/en/latest/usage.htm...

I've used it on a 2GB microSD that came with a 3d printer that turned out to be 100MB. I continued to use it to transport files to my 3d printer for a bit with this.

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