Hash function

A hash function is a function that takes a variable-length input and produces a fixed-length output, also called a digest (or just a "hash"). Hash functions should be quick to compute, and different inputs should as far as possible produce different outputs (this is called collision-resistance).

Hash functions have both cryptographic and non-cryptographic uses. Outside cryptography, for example, hash functions can be used to generate the keys for an associative array such as a map or a dictionary.

The digest() function of the SubtleCrypto interface makes various hash functions available to web applications.

Cryptographic hash functions

In cryptography, hash functions have many applications, including digital signatures, and message authentication codes.

Not all hash functions are appropriate for cryptography. To be used for cryptography, a hash function must be:

quick to compute
one-way: given the output, it should be impractical or impossible to regenerate the original input
tamper-resistant: any change to the input leads to a different output
collision-resistant: it should be impractical to find two different inputs that produce the same output

The hash functions most commonly used in cryptography are from the SHA-2 (Secure Hash Algorithm 2) family, whose names are "SHA-" followed by the length of the output digest in bits: for example, "SHA-256" and "SHA-512".

SHA-2 is the successor to the SHA-1 algorithm, which is no longer considered secure and should not be used in cryptography. Note that the MD5 algorithm is also considered insecure.