How to authenticate a SIP client?

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, it captures password by 100 to enter manually for authentication. After authentication, call goes to 200. Is it possible??
Is it done through sip.conf or extensions.conf?

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SIP authentication on a back to back user agent is always single hop.

You can use applications like DISA and some lower level password checking ones for technology independent end to end access control.

Use Authenticate() cmd on your dialplan.

Authenticate(password[,options[,maxDigits]])

Requires that the caller enter a password before proceeding to the next priority.