When a person asks for something that appears suitable only for nefarious purposes, they need to be prepared to explain why they need such a thing. Your question is suspiciously like "What gun do the police have the hardest timing getting fingerprints from?"
Wow, totally echoed my thoughts as I read that. All kinds of things spun through my mind.
Soggy, no offense intended, but this does sound like something sinister in the works.