This protocol assumes that encryption is commutative, i.e.
{{x}y}z = {{x}z}y.
The initiator A encrypts his message M by his secret key Ka,
then B encrypts the message he received by his secret key Kb.
Since {{M}Ka}Kb = {{M}Kb}Ka, the agent A can decr
ypt it and
send {M}Kb to B. Then, using Kb, B can retrieve M.