Explain Diffie$-$Hellman key exchange algorithm and solve the following scenario: In the Diffie$-$Hellman key exchange, users A and B agree on a common prime q$=11$ and a primitive root a$=8.$ If user A$\text{'}$s public key is YA$=$S$,$ what is the corresponding private key XA$?$ Similarly, if user B$\text{'}$s public key is YB$=4,$ what is the corresponding private key XB$?$ What is the value of the shared secret key between A and B$?$