PPP link encryption?

Is there any software out there on the net for doing real-time, transparent encryption of a PPP link? Also, if this link is running at 600kbit/s to 1Mbit/s, how much processing power would be required to keep up (assuming we're using, say, the IDEA cipher)? Would a dedicated 386DX/33-based router on each end be sufficient? Finally, could someone make a ballpark estimate as to the amount of additional latency that would be added? thanks, - alex