I have working wireless network with two nodes and one backbone link. I want to encrypt the backbone wireless link as I noticed that my passwords are constantly being stolen (I guess someone is catching the packets on the link).

So how do I do that? Create security profile with only WPA2 enabled and Pre-shared key ?
What mode should be used? I am looking for simplest method.

Just tell me what I need to do on both sides so the link continues to work.

Thanks

Is the wireless link between two mikrotik routers using WDS?

no everything is working fine right now. I just want to encrypt the data traveling across.

It is a simple setup currently - AP bridge on one side and AP client on the other side... no WDS no anything else

basically yes - create profile with WPA2 and preshared key, and set it up as security profile for interface, do not forget to add that on all ends of wifi links.