Rupesh,
When you configure NTP authentication at the client you have to configure NTP authentication at the MASTER as well.
Router(config)#ntp master 1
Router(config)#ntp authentication-key 1 md5 JUSTKIDDING
Note: You can only configure this on the MASTER router. The client who hasn't been configured NTP authentication can synchronize to that MASTER as well. (grin)
Edit: You may know how to configure this at the client. I just want to share you what I have in mind. (grin)
Client(config)#ntp authenticate
Client(config)#ntp authentication-key 1 md5 JUSTKIDDING
Client(config)#ntp trusted-key 1
Client(config)#ntp server x.x.x.x key 1
HTH,
Toshi