class mac_final_triple : ?iv:string-> ?pad:Cryptokit.Padding.scheme->block_cipher->block_cipher->block_cipher-> Cryptokit.hash
c1, c2 and c3. The input is run through c1 in CBC
mode, as described for Cryptokit.Block.mac. The final
initialization vector is then super-enciphered by c2, then
by c3, to provide the final MAC. This construction results
in a MAC that is as nearly as fast as Cryptokit.Block.mac
c1, but more resistant against brute-force key search
because of the additional final encryption through c2 and
c3.