[JoshuaDavid]

The digest is 64 characters long, so on average you should get 4 positions where your guess and the digest are the same, which would narrow it down to (1/16)*4 of the possibilities, corresponding to "peeling off" 16 bits of entropy.

Figuring out how to enumerate only those values which generate a hex digest that matches the known characters in the hash is left as an exercise for the reader.

[3np]

You may be trolling, but that "exercise for the reader" does not have a known solution. Anyone who found one may wish to keep it secret to get rich on Bitcoin mining...

[sva_]

I think he meant to do it offline via brute force, then entering it

[3np]

The same applies. You can't "pin" part of the hash when attempting a brute-force - that's part of what it means to be a cryptographic hash function.

[cestith]

There are two layers of entropy in what I'm looking at, but I only got like two hours of sleep last night.

There's the entropy of the password from which the hash is generated, which is clearly what you're addressing.

But in the game I'm seeing, the hash itself is unknown but the game gives you feedback on the contents. So pinning characters of the hash cuts down on that search space. Then there's still the matter of finding a plaintext that hashes to that value, which as you've said should evade this sort of analysis.

[sva_]

He didn't say you could "pin" the hash. He said you could eliminate all hashes, that don't contain the positions known, and just enumerate those which contain the known positions (perhaps by bruteforce), therefore reducing the search-space. It'd still be ridiculously expensive, of course (as in, implausible to compute in this universe). Unless I'm misunderstanding something here.

[thaumasiotes]

> Figuring out how to enumerate only those values which generate a hex digest that matches the known characters in the hash is left as an exercise for the reader.

It's always bothered me that the standard security jargon for an oracle for some information is to call it "enumeration". Will your service confirm whether or not a particular email address is associated with a current account? User enumeration!

In my view, it's only enumeration if I can make the service give me the email address without me having to know the address independently. :/