Hi, I am working on a web application- I have a node created in browser and talking with another in-browser node.
How can we authenticate that we are talking to right node?
You are already authenticated! 
The multiaddress of the node (Qm…) is derivated from its key pair, and everything is signed with it.
So when node B get a packet supposedly from node A, it can check that the address, the signature and the payload match.