David Buchanan
When you visually compare two hashes, how many digits do you check?

The strings hashed below are "retr0id_18b1f814a8e2d9c4fb9c" and "retr0id_1253dea672ebfa240e94", if you want to check for yourself.
Apr 19, 2023 at 2:05amWeb
Show threadDavid BuchananApr 19, 2023
This partial collision was computed in 10 minutes on a mid-range desktop GPU. There is no weakness in sha256 being exploited, just an application of the Birthday Paradox.

I used the methodology from this paper https://www.cs.csi.cuny.edu/~zhangx/papers/P_2018_LISAT_Weber_Zhang.pdf "Parallel Hash Collision Search by Rho Method with Distinguished Points" - but with a single GPU instead of a university HPC cluster!

By the way, the attack complexity would be the same no matter how long the prefix is. Furthermore, there could be two different prefixes (i.e. "good" vs "evil"), although the attack would get about 50% more expensive on average.
Show threadChristian DrexlerApr 19, 2023
@retr0id I didn‘t know that was possible, I usually compare the first and last 6 positions which wouldn’t have helped me with these.
Show threadAngeApr 19, 2023
@retr0id wrong address?
Show threadDougallApr 19, 2023
@Ange @retr0id Missing "www.": http://www.cs.csi.cuny.edu/~zhangx/papers/P_2018_LISAT_Weber_Zhang.pdf
Show threadAngeApr 19, 2023
@dougall @retr0id thanks!
Show threadWolf480plApr 19, 2023

@retr0id
> comllexity would be the same no matfer how long the prefix is

Even if the prefix is 256 bits long?

Show threadDavid BuchananApr 19, 2023
@wolf480pl even if the prefix is 256GB long
Show threadWolf480plApr 19, 2023
@retr0id oh you mean prefix of input not prefix of output?
Show threadWolf480plApr 19, 2023
@retr0id oh yoy meam the prefix of the data being hashed, not the colliding prefix of the hash?
Show threadDavid BuchananApr 19, 2023
@wolf480pl oh, right, yeah
Show threadrecursive 🏳️‍🌈Apr 19, 2023

@retr0id I used to just check the first 3 and last 3 hex digits but I've started to suspect that's a scheme that somebody's going to start to attack soon if not already

(Edit: oh, yeah, I see the image now)

Show threadNicolás AlvarezApr 19, 2023
@retr0id
Show threadDavid BuchananApr 19, 2023
@nicolas17 Did you mean to send that twice lol
Show threadNicolás AlvarezApr 19, 2023
@retr0id deleted and resent with alt text 👉👈
Show threadEli the BeardedApr 19, 2023
@retr0id Usually I check say five at start and five at end. Sometimes I'll go for the whole thing. I realize that birthday-paradox matches should be equally tricky for any N digits, but I figure people rarely aim for just middle different.
Show threadAMSApr 19, 2023
@retr0id First 24b, last 24b, rough shape of the middle, then look closer if they're shaped different. This passed the head and tail, but too many letters in one vs. the other.
Show threadGraham Sutherland / PolynomialApr 19, 2023
@retr0id would probably catch me out if I was tired but the "shape" of the characters in the middle is the biggest giveaway here
Show threadGraham Sutherland / PolynomialApr 19, 2023
@retr0id it'd be cool to see collisions that rely upon common character mutations (d/b, 3/8, 5/6, a/e/c) as a way to make it harder to spot that the middle part is wrong.
Show threadDavid BuchananApr 19, 2023
@gsuberland Yeah that's what I'm looking into next, I think I've come up with an efficient way of doing it
Show threadgenevieveApr 19, 2023
@retr0id this would totally have gotten me. good point
Show threadYaakovApr 19, 2023
@retr0id @mattcen not enough 😅
Show threadIgnas KielaApr 19, 2023
@[email protected] I caught it from the front, my mind merged the whole 22590 cluster into one from the first one, and it ended up being 2258 on the other side

I think you could try to generate hashes such that the first few "blocks" that the mind segments the hash into match though, but need to get at least a heuristic on how those blocks would be segmented like.
Show thread💬Apr 19, 2023
@retr0id @fincham first 5-ish, last 5-ish, then pick 5-ish from the middle of one and confirm it appears in the middle of the other
Show threadrobbystkApr 19, 2023
@retr0id Not enough, evidently.
Show threadAriadne Conill 🐰Apr 19, 2023
@retr0id @RichiH i don’t. i use sha256sum -c.
Show threadmirabilosApr 19, 2023

@retr0id uhm yeah.

The first and last couple, but I also put them under each other so I’d get a visual disturbing if there’s a mismatch in the middle, sometimes I even use uniq

Show threadblackstream #RIPNatenomApr 19, 2023
@retr0id By arranging the checksums directly below each other, you can spot the differences much easier:
Show threadDavid BuchananApr 19, 2023
@blackstream If you're going to the effort of doing that, why not *actually* compare them?
Show threadblackstream #RIPNatenomApr 19, 2023
@retr0id that would be the best variant indeed.
Show threadDave HollandApr 19, 2023
@retr0id @blackstream To be fair the original post explicitly asked about visual comparison. But yes this is a good wake-up call that proper comparison is becoming essential.
Show threadDavid BuchananApr 19, 2023
@davebiff @blackstream Right, for me at least, I only do visual comparison when there's no convenient alternative - e.g. the hashes are on two different devices, or maybe one is a VM without clipboard integration.
Show threadSaagar JhaApr 19, 2023
@retr0id This is why you can hash the hash,
Show threadDavid BuchananApr 19, 2023
@saagar if I know what second-hash you're using I can target that instead :P
Show threadPaul WarrenApr 19, 2023

@retr0id So, I was primed by the toot, but after checking the first and last ~6 that I can read in one go, i rely on the sort of shape of the rest. At the 2258 v s 2259 spot, the shapes differ enough that I checked closer.

But I thought the whole point of shas256sum was to use it computationally, like with sha256 -c or something, it's not meant to be easily human comparable.

Show threadfelix (grayscale) 🐺Apr 19, 2023
@retr0id If the hash is unimportant, I just look at 6-8 digits at the beginning or end, whichever is more convenient. If it's important and passes the sniff test, I check every digit. I know it's extremely unlikely to be a near-collision, and it's tedious to do, but the exercise feels important. And if I keep doing it, I'll get better at it, or find better ways of doing it.
Show threadNicolas SAPA Apr 19, 2023
@retr0id It is easier to check every digit when the hash are vertically aligned. (command_that_generate_checksum); echo expected_checkum
Show threadDidier RaboudApr 19, 2023
@retr0id when in a hurry, 4-5 first digits and (importantly) 4-5 last digits.
Show threadMarc Kleine-BuddeApr 19, 2023
@retr0id Fixed that for you: 😀
Show threadAndy HerdApr 19, 2023
@retr0id always “git diff --no-index” for this exact reason
Show threadRyan Castellucci (they/them) Apr 19, 2023
@retr0id Is that 80 bits colliding?
Show threadDavid BuchananApr 19, 2023
@ryanc yup! I will hopefully have a demo of 96 bits colliding by this time tomorrow, depending on how lucky I get
Show threadRyan Castellucci (they/them) Apr 20, 2023
@retr0id We should talk... this reminds me of an old project I tried to do almost a decade ago.
Show threadSiguzaApr 19, 2023
@retr0id ever since I've had to do this in bulk, the only way for me now is to line them up one below the other. And if there's more than two, I run them through sort | uniq -c | sort
Show threadbarstewardApr 20, 2023
@retr0id So after checking n bytes at the beginning and end, we need to do the same with a hash of each hash. Let’s see how long that takes to calculate a collision!