What are hash functions?

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Hash functions are used to provide data integrity. Hash functions are used to map things like sound files, video files, documents, code, text, digital signatures, and so on into fixed length size value.

For example a 2GB video file that is hashed might look similar to this value:

78AE647DC5544D227130A0682A51E30BC7777FBB6D8A8F17007463A3ECD1D524

Any changes to the video file will result in the hash values changing, hence the use of hash functions provides data integrity.

Also see:
https://en.wikipedia.org/wiki/Hash_function, https://en.wikipedia.org/wiki/Cryptographic_hash_function