Consider the following protocol for sending an encrypted message from Alice to Bob in the presence of a Key Distribution Center (KDC).
? Alice sends to KDC { Alice;Bob;E_KDC(R) }. That is, her identity, Bob's identity and also a random session key encrypted with the KDC's public key (of which it has a trusted copy).
? The KDC decrypts the random session key R and then encrypts the same with Bob's public key (of which it has a trusted copy) and sends to Alice { E_Bob(R) }.
? Alice sends to Bob E_R(M), that is the message M encrypted with the random session key R and also { E_Bob(R) } which it received from the KDC.
? Bob decrypts { E_Bob(R) } to get R and then ER(M) to get M.