Seems like it would only work in a very benign network environment. The first thing 4chan would do is to write a "peer" that answers yes for every SHA it gets a

Seems like it would only work in a very benign network environment. The first thing 4chan would do is to write a "peer" that answers yes for every SHA it gets and then sends over porn instead. Hope all your network software re-verifies the hash for every file you get! There is also the massive privacy leak of asking for the SHA of a certain file only found on specific websites and then seeing who has it. HTTP is just inherently server-client, decreasing load on servers by sharing between clients was not a design goal.