Is there a way to enable a keysigning in my Canon SLR? How do I install my private key?
No, it's a hardware module that you connect to the camera.
http://www.canon.co.jp/imaging/osk/osk-e3/index.html
Apparently it was broken in 2010 (found while googling for this).
The otherwise is already covered by DSLRs. Canon has it, and I'm sure Nikon does. Law enforcement has been using this stuff for years.