PPP chap authentication

Chap authentication is unidirectionnal. A router always answer to a challenge even if ppp authentication chap is not configured. So for a one-way authentication R5 authenticating R4 :

On R5
username ROUTER4 password 0 CISCO
interface Serial1/1
ip address 132.1.45.5 255.255.255.0
encapsulation ppp
ppp authentication chap

On R4
interface Serial1/1
ip address 132.1.45.4 255.255.255.0
encapsulation ppp
ppp chap hostname ROUTER4
ppp chap password CISCO

The 2 ways authentication :

On R5
username ROUTER4 password 0 CISCO
interface Serial1/1
ip address 132.1.45.5 255.255.255.0
encapsulation ppp
ppp authentication chap host
ppp chap hostname ROUTER5

On R4
username ROUTER5 password 0 CISCO
interface Serial1/1
ip address 132.1.45.4 255.255.255.0
encapsulation ppp
ppp authentication chap
ppp chap hostname ROUTER4