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BitTorrent creates a hashes of your file. The file is broken down into certain size chunks and hashes taken of each chunk. If you already have a piece with a given hash, you won't download it. If a hash differs, it'll be transmitted. The receiver verifies the hash. If it fails it re-requests that piece.

I assume the sync protocol works the same. It hashes the file, checks if any chunks have changed, sends the hash meta-data to the recipient, who then requests and pieces that have changed.




What about hash collisions?


SHA-2 has no collisions found as of yet [1]. The collision attacks on md5 required a lot of junk data, so not sure how much of a problem that would be.

[1]: https://en.wikipedia.org/wiki/SHA-2#Comparison_of_SHA_functi...




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