I don't know how what you describe works in cryptology, but that's definitely the vibe here. Something like an intersection of a series of "hashes". The art here is to find out the method of building them.
I am completely unable to guess, why you even mention direct download. The question wants to limit the download link. That's all. If you can spare 1 bit of data on that link, it counts as more performant, because that's how performance is counted in this case, per problem definition. Yes, it is so simple. For each such spared downlink bit cost can be counted in terms of extra uplink transfer and computation on the client and/or the server. So simple. And the "hash" you took the wrong direction. The point of any such "hashing" would be to decrease the exchange rate between spared bytes on the downlink and the cost on the uplink. I have an answer to this mystery already and will post it in a moment in bliako's subthread (the one with script code).