Each individual A and B picks a random number rA and rB which are only known to themselves and a fresh for session to be authenticated. There is shared key KAB which is not to be transmitted but A needs to know that B knows KAB and B needs to know that A knows KAB. The random numbers are known as challenges. |
rA |
Decrypt xA and see it gives rA |
Encrypt rA to give xA |
xA |
rB |
Encrypt rB to give xB |
xB |
Decrypt xB and see it gives rB |