type t = | Plaintext| Md5| Md5_96| Sha1| Sha1_96| Sha2_256| Sha2_512
type key = {hmac : t;key : string;
}val to_string : t -> stringval of_string : string -> (t, string) Stdlib.resultval digest_len : t -> intval known : string -> boolval hmacv : t -> key:string -> Stdlib.String.t list -> string