§ 01
Definition
SHA-2 was designed by the US National Security Agency and published by NIST in 2001; the current specification is FIPS 180-4. It replaced SHA-1, which researchers showed to be practically breakable with a public collision in 2017.
§ 02
How it works
A hash function processes data in blocks and outputs a digest (256 bits for SHA-256). Changing one character of the input produces a completely different digest, and it is infeasible to find two inputs with the same output. VPNs use SHA-2 inside HMAC, certificate signatures and key derivation.
§ 03
Why it matters
When a VPN lists 'SHA-256' or 'SHA-384', it is naming the hash used for integrity and certificates. Any setup still relying on MD5 or SHA-1 for signatures should be treated as outdated.