One phrase I've used with success in the past is that I have secure governmental cryptographic codes owned by someone other than myself in my laptop/bag/iphone

One phrase I've used with success in the past is that I have secure governmental cryptographic codes owned by someone other than myself in my laptop/bag/iphone and am not allowed under law to disclose them to unknown parties such as yourself. Most first level TSA/DHS people measure the cost/benefit of my assertion and back off.