bit-torrent regarding file security

[LovesTheSun]

I don't understand much how bit-torrent protocol regarding hash-checks and file security work. So here is a few questions which will help me understand I hope.

Can a 2nd gen seeder (not the roiginal seeder) seed halfway to downloaders, and then make changes to some files ie. remove the exif data certain pictures. and continue seeding the whole torrent, without downloaders/clients knowing about it, or having to redownload the torrent-file for their download to continue?

I ask this bc I'm paranoid 2nd gen seeders could modify files and still seed the torrent without downloaders realiszing it. if this is at all possible.

[kluelos]

Nope. This is flatly impossible.

Take a look at a .torrent file. After some preliminary stuff like filenames, the rest of it is a list of long hexadecimal numbers. These are the piece hashes. The torrent as a whole also has a hash which is used to uniquely identify it.

Whenever bittorrent downloads a piece, it computes the hash for the downloaded piece and compares it to the hash in the .torrent file. If they match, great. If they don't the piece is deleted and bitttorent downloads the piece again. If one source gives too many hashfails, bittorrent finds a different source. Any piece containing part or all of a changed file would fail the hash check.

This makes it impossible for anyone, even the original seeder, to change a torrent after it is made, even for good and innocent reasons. No torrent can be fixed. The only solution, if you left something out, is to make a new torrent and try to steer people away from the old one.