At the recipient end
The above process shows how authenticator is calculated by the program for sending a message. Now, to show how the authenticator helps to make the recipient sure if the message has been altered or not, let’s assume that the message from the sender has been altered to 0000000b(hex). The message received will be 0000000b and the authenticator from the sender is F1EEC67D.
We know that both the sender and receiver will share the secret key, in this case J=00000001 and K=00000001. At the recipient end, the above 2 figures are same for calculating authenticator at his side. Then, he inputs the message he has received, 0000000b and calculates the authenticator as shown in the figure below.
Fig5: Calculation of authenticator at the recipient end. The key is same for the sender and receiver. The input message is the altered message 0000000b which he received. Finally, he gets the authenticator value Z=3FACF04A calculated at his side. Since it doesn’t equal to the authenticator value from the sender Z= F1EEC67D, he becomes sure that the message has been altered since it has left the sender.
Note: If the message was not altered, he’d get the same authenticator value as sent from the sender, and the message would be assumed to be intact.
