Reverse patch: seemingly stupid idea (or just straight stupid)

[len...@gmail.com]

Hi,

Well I know it's not possible to fully recreate file A from file A_new and A's signature. But wouldn't it be possible to just try to recreate it partly and fill the remaining bytes with 0's?

Use case: Cross-seeding torrents

Imagine that A has a torrent file and the full content for it. B has almost the same content except some metadata is changed in his/her file so his/her file does not match with the torrent file. Now he wants to join to seed the torrent with the least bandwidth usage possible. Fortunately A distributes his/her file's rdiff signature.

Now if B could compute a "reverse delta" against this signature he/she could patch it against his/her file and could start seeding. Some of the torrent's chunks might be missing so he/she could download these from the torrent.

It's really a specific use case and maybe it could be implemented in torrent clients as some "smart cross-seed" feature. I just ask if something like this is theoretically possible.

Best regards,
Lénárd Szolnoki

[Martin Pool]

Hi, yes, that should be pretty possible. You can probably do it with a small program on librsync by calculating delta(A_sig, A_new) and looking for the copy instructions, then essentially playing them backwards. 

If you don't expect sub-block-sized insertions or deletions you might be able to able to do this directly on the hashes in the torrent file.

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Martin