The fundamental problem with this proposal is that it relies on “security through obscurity”
I don’t think that description is accurate. Like digital signatures, it relies on asymmetry between verification and duplication.
Criminals synthesizing the chemical compounds is virtually guaranteed because it’s very difficult to distribute a chemical to every gas station in the country and keep it secret.
Distributing the substances to every gas station is unnecessary. Transportation pipelines are expensive infrastructure projects with very high throughput, so there are only a few of them to guard. The real difficulty is that they’re very, very long and the criminals can tap at any point, so it’s infeasible to police the entire stretch 24⁄7 and it’d be great if we could reduce the policing to merely the few end points. The “last mile” is much less problematic since it’s close to population centers where witnesses are plenty and police reaction time is short (a tapping operation takes ~40 minutes).
Also, we’re not keeping the usage of chemicals a secret any more than we’d keep the existence of passwords a secret.
At the same time, criminals are good at synthesizing weird substances.
That’s because these weird substances have different purposes, e.g. narcotics. Here, we are optimizing for nothing but the asymmetry between verification and duplication, because a signature is all the substance is. A closer analogy would be banknotes; they’re also specifically optimized for difficulty of duplication. And bank note printers have thus far been very successful, else cartels would not bother smuggling drugs when they could just print money, and paper-currency based economy would become impossible.
What are some example substances you could use for chemical compounds A and B?
I don’t have the relevant expertise, unfortunately. I’m thinking protein sequence or some other biomolecule.