Pbkdf.SHA256Sourceval pbkdf2 :
password:Tezos_base.TzPervasives.Bytes.t ->
salt:Tezos_base.TzPervasives.Bytes.t ->
count:int ->
dk_len:int32 ->
Tezos_base.TzPervasives.Bytes.tpbkdf2 password salt count dk_len is dk, the derived key of dk_len octets.