So encrypt large files not blocks. Then you ask for random segments and either the file is stored, or the cheater would need to re-encrypt a large file over and

So encrypt large files not blocks. Then you ask for random segments and either the file is stored, or the cheater would need to re-encrypt a large file over and over, which would be much more expensive than storage. (and if it's not more expensive, make the files larger, the requests more often, etc...)