So it does not encrypt the message, just creates a hash, so when I send a message to my second arduino (B), the B creates the hash (based on the same secret key I have got) upon the message, the B sends me the hash back, and I may compare the B-hash with my hash I did for the same message before. When my compare of both hashes match, I know a) the B is my friend, b) the message has not been altered by somebody during the transport to the B. Is my understanding correct please?