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> Using a hashing algorithm to protect sensitive data (should I hash passwords with SHA-1?)

You're almost certainly fine! A collision oracle doesn't actually make it any easier to conduct a preimage attack.

We normally recommend against using hashing algorithms with known collision attacks for password hashing because (a) these hashes aren't well-suited to password hashing in the first place (b) because having a collision attack implies structural weakness and you don't really want to be caught relying on that hash if a preimage attack does turn up (c) as a fashion statement.

> using it as a convenient lookup optimization (should I use SHA-1 to identify git references?)

Now this is a bad idea, because having collisions in the wild means that someone can swap out one for the other without the hash catching it, so now you need to build in extra complexity to mitigate that.

(Of course, collisions always exist in theory, so you want to design around the possibility. The difference having collisions in the wild makes is that as long as they're only theoretical, you can get away with e.g. screaming loudly about running into an impossible situation and guarantee that you don't do something actively dangerous, or falling back to something horrendously slow but always correct; once collisions exist in the wild, that's a denial-of-service.)

> Now this is a bad idea

Somebody better tell Linus? I kid. But yeah the answer is you use it as part of your lookup and then mitigate collisions with more expensive comparisons.

He was told weeks after git was published, I guess 2005. Sorry, no link handy.

They are working on it since 2017. https://github.com/bk2204/git/commits/transition-stage-4/Doc...

Not that it's particularly urgent...

It's a non-issue he's addressed many times. What happens after you collide a single hash, you need to have a useful commit that isn't going to destroy the ability of the code to run AND do something useful to you, etc etc? Also what happens after the next commit, where all the hashes recalculate? It's possible to be an issue, but highly improbable. More likely your creds are hacked or the fact that no one signs their commits finally bites us on a wide scale.

Well, it's well known since basically as soon as git was published and determined to have not been a big deal, with work on the way ever since an actual collision was published.

Anyway, practically you're going to write a collision fallback anyway, the difference is that as long as collisions aren't likely, you can be pretty sure that collision handling is a very cold code path that's never going to have more than a handful of items in a bucket.

The point I'm trying to make, though, is that while practical collision attacks do have practical implications, they aren't relevant to password hashing. People routinely assume that collision attacks make a hash unsuitable for password hashing. They're not exactly wrong, but it's kind of a non sequitur. I felt that this was a point that had to be made because the framing of parent ("what the consequences of a collision are") implied that the expected answers were exactly the opposite.

One concrete example is using hashes as identifier for objects.

It is possible that this a bad idea because of collisions. However, if you have for example a federated system you don't have a central instance issuing identifiers for objects. So you create a hash. In reality it doesn't matter if you used SHA1, MD5 or even a random identifier. As long as you have enough bits, a collision becomes unlikely. However again, if you have many objects consider the birthday paradox.

The interesting thing here is that this is an issue in any case: Even if you have an extremely secure hash algorithm, in the end you have less bits than the original data, so collisions must occur, even if they are immensely improbable.

If you then decide that you ignore collisions because after considering the birthday paradox collisions won't occur practically it can be worth thinking about what would happen if such a collision does occur.

Let's say your application executes a database update and has a hash collision. Then another object would be overwritten.

This is a use case of not mixing object creation and object update. If you program carefully such that you never overwrite another object if you update your object you'll get an internal error if you have a hash collision. That's better than losing unrelated data.

However this is difficult to test. And I am sure that other bugs are a lot more probable.

But this thinking shows me that it is useful to program defensively.

If you have assumptions, encode them in assertions. So even if you don't have tests for this rare problem, your chance of being lucky is higher.

However, if you have for example a federated system you don't have a central instance issuing identifiers for objects. So you create a hash. In reality it doesn't matter if you used SHA1, MD5 or even a random identifier.

But it does matter. Usually it's only a big problem in edge cases, but sometimes those edge cases are extremely important. TLS certificates use a hash as their identifier in a distributed/federated model like you describe. By generating collisions, one can submit a normal signing request to a trusted public CA, but then use the resulting signature with a second certificate that has malicious properties (being for a domain name belonging to someone else, having additional properties like it being trusted to sign other certificates, etc.).[1] This is why you can't get new MD5 or SHA1 certificates signed by public authorities anymore.

As you say, there is always going to be at least an infinitesimally small chance that two realistic inputs to the same hashing function will produce the same output, but IMO virtually any system that uses hashes as identifiers will have its security degraded in some way if it's practical to generate a collision.

If you program carefully such that you never overwrite another object if you update your object you'll get an internal error if you have a hash collision.

It sounds challenging to handle this gracefully in a distributed system without some fairly complex logic. Alice uploads a PDF of sensitive data that hashes to value A on node 1. Node 1 grants Alice full access to the file with that hash. Bob uploads some meeting notes that also hash to value A, but on node 2. Node 2 grants Bob full access to the file with that hash. When the nodes are synchronized an hour later, the system detects a collision. What does it do?

[1] https://www.win.tue.nl/hashclash/rogue-ca/

The interesting thing about your comment is that you view password hashing as needing more security but git commit hashes to be less of an issue. I couldn't disagree more.

First, for a password hash function to matter, the hashed password must be compromised. If you don't have the hash it doesn't really matter all that much. Yes, hashed passwords do get compromised but many don't.

Second, I can't think of many bigger security threats than intentionally creating an SHA1 collision for a git repository. Poisoning a Linux git repo could be truly disastrous. Remember too that if you can poison a git repo, you can also make the binary output have the same size and hash too. I mean that's much more difficult but not impossible and it wouldn't surprise me if state actors have dedicated a lot of computing power to this.

And "is it possible?" is by definition always "yes".

> The interesting thing about your comment is that you view password hashing as needing more security but git commit hashes to be less of an issue.

I do? I should have been more clear that I think this is another case which very much depends on the other considerations I addressed and probably many more.

> And "is it possible?" is by definition always "yes".

You’re not the only person to say so, and… I know. I left the possibility open in my response mainly because I wanted to focus on the other considerations without dismissing the parent comment. Even if the answer was less definitive I believe considering level and degree of risk should be a bigger factor in these conversations than what’s theoretically possible.

The issue is that your commit still has to do something useful and have the proper colliding hash, which dramatically increases the effective collision complexity for something that is already nearly impossible to occur. Sure you could have some garbage commit with a colliding hash, but all that means is an inconvenience while they remove that commit.

Well, you can just comment out the garbage you create to match the hash so that's not a big problem in the case of source code. Creating a correct binary file may or may not be a bigger problem, depending on the file format.

You're correct and this should be obvious. It's demonstrated already by 'shattered' where the colliding PDFs both render properly.

> for something that is already nearly impossible to occur

As the link suggests, it is nowhere near impossible, and well funded attackers can produce collisions extremely quickly.

Git also uses sha1 for signing commits, which is another area where it relies on it for security.

It is always possible to generate a collision for any hash function since they are not injective by definition.

My point wasn’t to address whether any hashing technique might be collision-free, but that assessing that fact is not a good metric for the fitness of a given hashing technique.