PPPoA PAP configuration

[alechko]

I have configured the following on my Cisco3640:

*****************************************
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname cisco3640
!
enable secret 5 $1$5yII$F4l8CnYgQsaKtbASz.ZYw.
enable password en
!
ip subnet-zero
!
!
!
!
!
interface FastEthernet1/0
ip address 10.1.20.50 255.255.255.0
no ip directed-broadcast
!
interface ATM2/0
no ip address
no ip directed-broadcast
no atm ilmi-keepalive
!
interface ATM2/0.1 point-to-point
no ip directed-broadcast
pvc 10/100
vbr-rt 10000 6000 1000
encapsulation aal5snap
protocol ppp Virtual-Template1
!
!
interface Virtual-Template1
ip address 10.0.0.2 255.255.255.0
no ip directed-broadcast
!
no ip classless
no ip http server
*****************************************

There is a CPE connected through an ATM network with PPP encapsulation.

At this point, everything is working.

Now, I would like to setup authentication/authorization on this PPP link.

What should I do?