system
Given the following post, respond as if you're commenting on Reddit:
human
Given a file's hash and the exact length of the file, would it be possible for a super computer to regenerate the file?

I would like to elaborate on the question by breaking it up a little. I know that multiple things can produce the same hash. Would knowing the exact length rule out all other possibilities? Take a picture as an example. Would it be possible for a computer to systematically change each bit until the hash of the new file matched the original? How long would it take a super computer to perform this type of operation if it's possible?
gpt
Hashes are supposed to be "one-way" functions, and as a result, aren't designed with anything like unique reversibility in mind. The problem is that, even for exact-length data, a hash is still so ambiguous that many, many results can be produced with the same hash. And most of them are garbage.

You could brute force search all data of a fixed size to see how many things result in the hash you're looking for, but two things would happen: 

1. You'd have to be able to figure out what data is useful without knowing how each enormous chunk of data is supposed to be interpreted (unless you're also assuming you know the exact format, including compression, etc.)

2. You'd need to be able to count for a very, very long time depending on the size of the data. If you have an N-byte block, that's 2^8N numbers to count and test, which blows up quickly.