The following protocol is designed for two parties $$ and $$ to establish a shared session key with

the help of a trusted server $.$ Initially, $$ shares a secret key $!"$ with $,$ and $$ shares a secret

key $$#$"$ with $.$ When $$ wants to establish a session key with $,$ first sends a request to $.$

The protocol is given below and is also shown in Fig. $1.$

$1.->$: $,,\{,\}$$$!"$

$2.->$: $\{!$#$\}$$$!"$

$,\{!$#$\}$$#$"$

$3.->$: $\{!$#$\}$$#$"$

Where $,$ and $$ denote the identities of the two parties and the server, $!$# is the established

session key for $$ and $,$ and $\{\}$$ denote the ciphertext when a message $$ is encrypted under

the secret key $$