[SOLVED] What is hash number and should I check it when downloading?

[LAPIII]

What is hash number and should I check it when downloading ?

[Nemo]

If you want a simple answer I guess it's safe to say that a hash code is a fixed length code that only corresponds to a specific file or directory.

Hashing is creating that code using an algorithm.

Every .torrent files contain a hash code and the algorithm used to make it (by the uploader), so that your torrent client can always verify that your downloading the right file and not just a same size file with the same name.

When you add a torrent and you already have the file (cross-seeding or uploading) your client will hash the file using the algorithm in .torrent to make sure it's the right file.

You don't need to check it yourself as far I would say. If you try to add a torrent with the same hash number it will ask you to overwrite an existing one automaticly.

Quote from Wiki:

Hash
The hash is a string of alphanumeric characters (typically hexadecimal) in the .torrent file that the client uses to verify the data that is being transferred. "Hash" is the shorter form of the word "hashsum".
Torrent files contain information like the file list, sizes, pieces, etc. Every piece received is first checked against the hash. If it fails verification, the data is discarded and requested again.
Hash checks greatly reduce the chance that invalid data is incorrectly identified as valid by the BitTorrent client, but it is still possible for invalid data to have the same hash value as the valid data and be treated as such. This is known as a hash collision. Torrent and p2p files typically use 160 bit hashes that are reasonably free from hash collision problems, so the probability of bad data being received and passed on is extraordinarily small.

See here for other terms and explanations: https://en.wikipedia.org/wiki/Glossary_of_BitTorrent_terms