The authenticate function computes an authentication tag for a message and a secret key, and provides a way to verify that a given tag is valid for a given message and a key.

The function computing the tag deterministic: the same (message, key) tuple will always produce the same output. However, even if the message is public, knowing the key is required in order to be able to compute a valid tag. Therefore, the key should remain confidential. The tag, however, can be public.

import Sel.SecretKey.Authentication qualified as Auth

main = do
  -- The parties agree on a shared secret key
  authKey <- Auth.newAuthenticationKey
  -- An authentication tag is computed for the message by the server
  let message = "Hello, world!"
  tag <- Auth.authenticate message
  -- The server sends the message and its authentication tag
  -- […]
  -- The recipient of the message uses the shared secret to validate the message's tag
  Auth.verify tag authKey message
  -- => True