Since IPSec defines two protocols: Authentication Header (AH) protocol and Encapsulating Security Payload (ESP) protocol.
I find I am some how not getting my mind around why cisco combine two different protcols like this command below:
RTA(config)#crypto ipsec transform-set secure ah-md5-hmac esp-des-hmac
As you can see AH protocol is used for authentication and ESP is used for encryption!!!