class type digest =object..end
method name : stringThe name conventionally follows the <uppercasestring>-<size> format,
e.g. "MD5-128", "SHA1-160", or "SHA2-256".
method iana_hash_fn : iana_hash_fn optionWhether registered at IANA, and if so, which ID we use here
method iana_name : string optionThe name as registered by IANA (if registered)
method oid : int array optionThe OID (if registered)
method size : intThe size of the digest string in bytes
method block_length : intThe block length of the hash function (for HMAC) in bytes
method create : unit -> digest_ctxStart digestion