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Comment by verygoodname | Hacker News Reader

> Having the explicit length is quite superfluous

Not entirely. It makes it so that, to achieve a "full" collision, you have to ensure that the sets of data collide both in SHA hash and in length, helping to prevent attacks that rely on appending/prepending/removing data (for example, "length extension attacks" involve manipulation of the hash by appending data).

TL;DR: It is harder to find a collision SHA(B) for SHA(A) if you add the additional constraint that the length of B must match the length of A.

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This is completely wrong.

The known collision attacks for the MD-family and SHA-1 all in fact produce collisions with the exact same length. The method used necessarily does this.

> This is completely wrong.

Which part? The fact that storing "length" along with a hash is not superfluous?

You can probably find many things which have a SHA hash of "ca978112ca1bbdcafac231b39a23dc4da786eff8147c4e72b9807785afee48bb" (infinite things, if we assume arbitrary-sized inputs), but you can only find ONE thing which has that hash and has length 1. I just made it impossible (not just unlikely) for you to find a collision.

> The known collision attacks for the MD-family and SHA-1 all in fact produce collisions with the exact same length.

Emphasis mine. And note that I did not claim otherwise in my comment.

> Which part? The fact that storing "length" along with a hash is not superfluous?

The part where you make a false claim out of ignorance.

> You can probably find many things which have a SHA hash of "ca978112ca1bbdcafac231b39a23dc4da786eff8147c4e72b9807785afee48bb"

No reason I should go looking for such things. You're the one making the false claims, if you have found "many things" with that hash then list them to prove your point, otherwise go away.

> The part where you make a false claim out of ignorance.

Which false claim did I make? I'm still waiting...

> No reason I should go looking for such things. You're the one making the false claims, if you have found "many things" with that hash then list them to prove your point, otherwise go away.

You don't need to look for those things. By definition, you know they exist. I don't need to find or enumerate all primes to know that an infinite number of them exist.

For more information, see here: https://en.wikipedia.org/wiki/Pigeonhole_principle

By definition, assuming arbitrarily-sized inputs, there are infinite messages that collide to the same hash value.

But, don't worry... it is clear you have no actual meaningful point to add, so I won't continue this conversation with you any further. Have a nice day.

You are misrepresenting or more likely have simply misunderstood the Pigeonhole Principle. Which I guess makes sense for somebody who didn't understand why length extension matters. It does not prove that any particular output will recur, and what you've got here is one very particular output.

Again, you need actual examples. Not handwaving, not the unwavering yet entirely unjustified certainty that you're correct, you need examples. And you don't have any.

Again, which false claim have I made? Be specific and quote me: you need actual examples, not handwaving.

Until you do that, I'm not pursuing this conversation any further. Have a nice day.

EDIT: Also, if you do want to have a conversation, make sure to stick to HN rules and talk about what is being discussed, rather than about me. Thanks.

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