Authentication between two sip clients

How to authenticate the calls between two SIP clients using Asterix as a SIP Server? For e.g. We have two SIP clients having extensions 100 and 200 . when extension 100 calls to 200, user have to enter the key and also a password to get started. After valid authentication, call goes to 200. Is it possible in SIP??
Is it done through sip.conf or extensions.conf?

You have already been told that SIP authentication is hop by hop and that end to end authentication can be done with the DISA or password checking applications. That should be enough for you to attempt a dialplan.