I'm curious if you looked at Oneswarm[1], as it's built on the azureus platform (i.e. java) and utilizes PKI for authentication. It's not exactly lightweight, but I have setup ~30 friends, that I know and trust, on it in one tightly secure network.

For lightweight, on my router I have a transmission package (.opkg[2]) that handles files not available on my small network.

[1]http://oneswarm.org

[2]http://en.wikipedia.org/wiki/Opkg

EDIT: Running Openwrt (Backfire, 10.03) on Asus RT-N16