Two plugins for pidgin that are both portable. Even though it seems obvious to me, maybe I should explain that they are as portable as the program that they were designed for.
Pidgin Off The Record(OTR).
Pidgin OTR stores keys in the profile directory whereas Pidgin Encryption give you a choice to store them in the profile or elsewhere. PE also lets you choose the size(512-4096) of your keys as well. PE will of course automatically generate keys for accounts that already exist where as OTR requires manual generation. Both are good. Just a matter if you want AES or RSA as your means of encryption.