Lets say that we use init vector IV, key K and HMAC key H. Message is M.
Mode of operation is CBC !!!
We usually encrypt as this
C = crypt(M,K,IV) #encrypted data R = C + MAC(C) # final result and + is for con cat
But we could use just:
R = len + crypt(M + H,K, IV) # notice M + H
Is it something wrong whit such scheme? It looks much faster for me and requires like twice less computation and complexity.