I meant as outside influences that could make using the reading as a cryptographic signature difficult. That is, they are measuring something to do with capaci

I meant as outside influences that could make using the reading as a cryptographic signature difficult. That is, they are measuring something to do with capacitance (at a very small "femto/10^-15" scale) at the place where they seal it up. Then you're supposed to be able to do that same measurement at the place where it arrives after shipping, and have identical readings. Even though origin and destination likely have different ambient temps, humidity, altitudes, and so on.