Hash

Cryptographic hash functions take arbitrary binary strings as input, and produce a random-like fixed-length output (called digest or hash value).

It is practically infeasible to derive the original input data from the digest. In other words, the cryptographic hash function is one-way (pre-image resistance).

Given the digest of one message, it is also practically infeasible to find another message (second pre-image) with the same digest (weak collision resistance).

Finally, it is infeasible to find two arbitrary messages with the same digest (strong collision resistance).

Regardless of the hash algorithm, an n bits long digest is at most as secure as a symmetric encryption algorithm keyed with n/2 bits (birthday attack).

Hash functions can be simply used as integrity checks. In combination with a public-key algorithm, you can implement a digital signature.

SHA-2

SHA-2 (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001