[Dylan16807]

> padding is one-to-one so there's still the same number of inputs

That part is debatable, if we imagine different padding schemes.

If the padding was just 0s, I would easily accept an argument that 111000 and 1110 are the same input giving the same hash.

You could also say you consider the extra '1' bit in SHA-256 as part of the payload, not truly 'padding' because it's mandatory, and make a similar argument, and it wouldn't be blatantly wrong.

[tialaramex]

> That part is debatable, if we imagine different padding schemes.

It isn't debatable because SHA-256 (and all these schemes) define the padding because it's necessary to have padding defeat the problem you're about to talk about and to do that they are in fact one-to-one:

> If the padding was just 0s, I would easily accept an argument that 111000 and 1110 are the same input giving the same hash.

As you might have guessed, this is not how it works.

SHA-256 appends that "extra 1 bit" you talk about, then zeroes until it is 64-bits short of a multiple of 512 bits, and then a 64-bit count.

So, as the earlier poster explained there are 2^257 -1 distinct hash inputs in abotsis' imagined set and thus there must be collisions by the pigeon hole principle, it really is that simple.

[Dylan16807]

> it's necessary to have padding defeat the problem you're about to talk about

There is no way it's "necessary" to avoid that particular problem, yet at the same time unnecessary to avoid length extension attacks.

> As you might have guessed, this is not how it works.

That's a weirdly hostile way to respond to a clearly counterfactual thought experiment.

> and thus there must be collisions by the pigeon hole principle, it really is that simple

Except they didn't just say there must be collisions in SHA-256, by my reading they were making an argument that the method of appending padding and stuff doesn't matter.

[tialaramex]

> There is no way it's "necessary" to avoid that particular problem, yet at the same time unnecessary to avoid length extension attacks.

Length extension attacks work because of a common misuse of these algorithms. If you use them properly you aren't vulnerable to a length extension attack whereas you would be in trouble with the terrible "padding" scheme you propose as a counterfactual. Algorithms like SHA-512/256 and Keccak are prominent because they're misuse resistant.

Consider the guillotine used to cut paper to size. Used correctly it's not very dangerous, but we don't let kids have access to a guillotine because it lacks misuse resistance. In contrast devices like a Rotatrim are safe for kids because they are resistant to misuse. It turns out most programmers are kids and we should not have given them C++ std::sort() or SHA-256 or ECB mode encryption because they will cut themselves.

> by my reading they were making an argument that the method of appending padding and stuff doesn't matter.

If your "method of appending padding" turns 2^257 -1 inputs into 2^256 outputs that was not, in fact, "padding" in the sense anybody knowledgeable in this field would use. You're supposed to be able to remove the padding in cryptography, if some of the "padded" structures are indistinguishable from each other you can't do that.

[Dylan16807]

If you had no size based padding, you could fake a document having a few more zero bytes at the end. That's almost never dangerous, and it also requires very specific (mis)use to cause problems, just like a length extension. Unless you can name a killer use case, I stand by length extension problems being a significantly bigger deal and more "necessary" to stop.

> You're supposed to be able to remove the padding in cryptography

It's a hash. You can never access the post-padding version of the input in the first place.

[tialaramex]

> Unless you can name a killer use case, I stand by length extension problems being a significantly bigger deal and more "necessary" to stop.

"it works as a cryptographic hash" is certainly a "killer use case" for a cryptographic hash.

Your "Dylan16807 hash" without working padding does not work as a cryptographic hash, it's useless.

[Dylan16807]

It would work just fine. And if you call those messages equivalent then there is no cryptographic failing. You'll need better than that argument.

Nobody complains that RSA gives you the same result for 45 and 0045 after all.

Better, think of it like a hash algorithm that takes bits vs one that takes bytes. They both work fine and are secure. If your byte happens to be a really wide byte, that's not a fatal flaw.