The hash values we talk about in our chain-of-custody guides — computed here, locally, for any file. Nothing is uploaded.
A hash is a fixed-length fingerprint computed from a file's exact contents: change a single byte and the hash changes completely. In forensic practice a hash recorded at acquisition, and re-verified at every handover, is what proves a working copy is identical to the source — the backbone of Chain of Custody. Two files with the same SHA-256 are, for all practical purposes, the same file.