provides hashing functions (SHA-1 at present) Constructor - creates new digest object, which you use to build up a digest by repeatedly calling zdigest_update() on chunks of data. Destroy a digest object Add buffer into digest calculation Return final digest hash data. If built without crypto support, returns NULL. Return final digest hash size Return digest as printable hex string; caller should not modify nor free this string. After calling this, you may not use zdigest_update() on the same digest. If built without crypto support, returns NULL.