This makes it invulnerable to changes during distribution by torrent nodes, but it won't completely avoid distributing two different / precalculated files with the same hash it seems to me. It seems bittorrent distributes the hash outside of the torrents itself. by verifying / validating a certificate chain). To solve this using cryptography you'd use a signature, verified by the public key in a certificate that is trusted in advance (e.g. If you see such a hash on sites then the hash has two functions: making sure that the file was fully & correctly downloaded and making sure that the file is correct when downloaded from a less trusted mirror.

Hosting it next to the data makes no sense.

Sharing a hash only makes sense if that shared hash can be trusted with the receiver. Then they could execute specific actions depending on the document that was used. It is also possible to simply use two known files with the same SHA-1, such as the PDF documents from the SHAttered attack. It is possible to pre-calculate two (or more) messages so that they have the same hash, i.e.

No, they can only tamper the pre-image if they previously inserted a known week message.

Add torrent by hash utorrent