Pidgin has a plugin that allows peer to peer encryption across any IM protocol. So Yahoo, Skype, Jabber etc. so long as the other party is also using Pidgin. As it is available for both windows and Linux that is a majority of users (at least ones I usually talk to) It uses RSA Encryption and key lengths up to 4096 bits. For more information please feel free to visit the Pidgin Encryption site to learn more.