I don't know enough specifics about sha-1 or bit torrent, but it would seem to me that it depends on the data that is chosen for the base ("leaves") of the tre

I don't know enough specifics about sha-1 or bit torrent, but it would seem to me that it depends on the data that is chosen for the base ("leaves") of the tree: if it just consists in taking the hashes, concatenating them, and move higher until you only have one hash, then yes. However, if the data itself is concatenated for the first step, that might not be the case (depending on the sha-1 algorithm). I seem to recall that md5 operates on 512 bit chuncks (padded if necessary), and takes the result to seed the next computation. So, with md5 (if I recall correctly, still), the attack could work if each part of the torrent was a multiple of 512 bits, Merkel or not, and regardless of the base. Of course, you cannot change the whole torrent in that case, as it would be prohibitively expensive. And you still have to keep the same file size. I would love to hear a bit more on the topic from someone more knowledgeable about hashes and torrents.