[saulpw]

If that mechanical process is not reversible, then it's not a copyright violation. For instance, I can compute the SHA256 hashes for every book in existence and distribute the resulting table of (ISBN, SHA256) and that is not a copyright violation.

[dkjaudyeqooe]

That's actually within the other fair use factors. So your hash table is fair use because its transformative and doesn't substitute for the original work.

I edited my post to make it a bit clearer.

[anticensor]

It's actually even less than fair use, it's non-copyright use: one-way hashes are intentionally designed to eliminate the creative element and output random looking data.