Hashing

A hash function such as SHA-256 reads data of any size (a password, a file, a whole disk image) and outputs a fixed-size value: 256 bits for SHA-256, usually written as 64 hexadecimal characters. Change a single letter of the input and the output changes completely. A good hash is one-way, so the input cannot be worked out from the output, and collision-resistant, so in practice nobody can find two inputs that share a hash.

Hashes prove that data has not changed. Download pages publish checksums, Git names every commit by a hash of its contents, build tools put content hashes in file names so browsers can cache them safely, and digital signatures sign a hash rather than the whole file. MD5 and SHA-1 are broken for security use because collisions can be manufactured; SHA-256, SHA-3 and BLAKE3 are the usual choices today.

Hashing is not encryption: there is no key and nothing to decrypt. Fast hashes are also the wrong tool for passwords, which need deliberately slow password hashing.