(** {{:https://tools.ietf.org/html/rfc7914} The scrypt Password-Based Key
     Derivation Function} specifies the password-based key derivation function
    scrypt. The function derives one or more secret keys from a secret string.
    It is based on memory-hard functions which offer added protection against
    attacks using custom hardware. *)

val derive :
  password:string ->
  salt:string ->
  n:int ->
  r:int ->
  p:int ->
  dk_len:int32 ->
  string
(** [derive ~password ~salt ~n ~r ~p ~dk_len] is [dk], the derived key of
    [dk_len] octets. [n], the cost parameter, must be larger than 1 and a power
    of 2. [p], the parallelization parameter, must be a possitive integer and
    less than or equal to 2^32 - 1 / (4 * r)
    @raise Invalid_argument when either [n], [p] or [dk_len] are not valid. *)