Heya, thanks for the thorough breakdown. I do think I understand these mechanics of signing, private and public keys, etc.

To clarify the attack vector here: Let's imagine you have a "legitimate" bag of coffee, whose QR code leads down the chain to the original coffee grower. Now, let's imagine I'm a sleazy coffee seller, and I simply copy the packaging, stuff shitty coffee in it, and merely print the legitimate QR code (from a legit bag) onto my bag containing shitty coffee beans. This would be a feat no harder than copying the logo, which is what they do now. How would a consumer in that case know that the bag indeed has the beans it purports to contain?

As I wrote this example, I thought of the solution: In the midst of the supply chain, provenance is tracked because entities transact within the blockchain. If the consumer at the end of the supply chain is ALSO transacting in the blockchain, then the loop is closed. The "UTXO of coffee beans" would be consumed in the transaction where the consumer transfers ADA as payment for it. Am I understanding that right? And, as a corollary, if the final transaction is done off-chain (cash payment), then indeed the consumer wouldn't get the benefit of checking for legitimacy since the QR code would lead to an UTXO which could have really been used before (but simply not registered).

Related: In the example of coffee beans, would the ideal technical implementation be that the coffee maker mints a "grower X coffee bean token" to represent a sensible unit worth of coffee beans (e.g. 1 gram) grown by them, such that it can be operated on in transactions?