The export below is available to both client and server:

exports.sha256:Hash(text)

From http://pastebin.com/gsFrNjbt linked from http://www.computercraft.info/forums2/index.php?/topic/8169-sha-256-in-pure-lua/

Adaptation of the Secure Hashing Algorithm (SHA-244/256) Found Here: http://lua-users.org/wiki/SecureHashAlgorithm

Using an adapted version of the bit library Found Here: https://bitbucket.org/Boolsheet/bslf/src/1ee664885805/bit.lua