Recreating the Half Coins of Jin-Qua [closed]

Question

In the Asian Saga series by James Clavell, the Struan family gave away the half-coins of Jin-qua: in exchange for a desperately needed loan, the pirate Dirk Struan received 4 halves of 4 coins. The Chinese merchant, Jin-qua, kept the remaining halves. The repayment of the loan came in the form of an oath: Dirk Struan and his descendants would grant a boon to whoever presented one of the four halves, with no limits and no conditions. Those who wish to become head of the Noble House, the family’s powerful trading business, must swear to uphold this promise. When fulfilling the bargain the Struan descendant gets out his original coin and checks it against the one presented to him. He keeps the coin in exchange for the favor.

How would someone make a bargain like this nowadays? The system with the original half coins could be easily broken using a modified key copier. Admittedly only four times (at most, perhaps less if they get suspicious), but for carte blanche you only need one.

The key:

• Cannot be copy-able (see below).
• Must be a physical object, though it may contain digital parts.
• Can be anything up to briefcase-sized.
• Must survive at least 5 years at room temperature and preferably 10
• Must survive rough handling to the level of an airport carry-on (but does not necessarily have to be shippable or legal to take on planes). For example, laptops meet this criteria but very thin china does not.
• Must be verifiable with a high degree of confidence. The only limit on verification procedure is that it should not take more than a week and preferably much less than that.

Let’s say the threat model is that faced by the Noble House: well-funded private companies and individuals, but not the CIA or magical assassins.

Answer 1

First off, the half-coins will probably work as well today as in the novel. The point wasn't that they were not copyable, but that they represented a shared secret - the boon was due to the person who had a matching coin; thus it was essentially a secret key.

As each coin was unique it could be used only once. If you had your half stolen or copied, you would in effect lose your boon as it would be "used up".

But let's go with your original requirement: The object should not be copyable. Obviously, any object can probably be copied or faked, given enough resources and time. However, there are possibly some that make it hard enough.

• A disk of wood would prabably work fine. It would be quite impossible to create a piece of actual wood with matching growth lines that would in all respects appear to be from the same tree. There may be other natural objects (e.g. crystals) with a unique structure that cannot be copied.

• If you want to use something more high-tech, a hardware token like a Yubikey would also work. It would be possible to copy using enough resources, but would probably take a long time and you'd need to destroy the original.

Answer 2

It sounds like the only attack you're worried about is that someone might counterfeit one of the IOUs. Therefore, I need to pick something for my IOUs that is hard to counterfeit. This suggests that money is still a good model.

I could just say "I'll hire an engraver to engrave some very detailed and intricate plates, mint four certificates from those plates, and use those certificates." That's what actual nation-states do, when they want to create tokens that are exchangeable for goods and services — excuse me, "favors." Such tokens are non-counterfeitable because hiring an engraver is expensive — like, state-level expensive — and we're not worried about that level of adversary.

However, hiring an engraver would also be very expensive for me. That's not good. Is there some way for me to piggyback on someone else's expensive engraver?

Here's what I'd do. I go into my wallet and pull out four 20-dollar bills. (100s if you've got 'em. Fives are probably okay. Probably don't use singles.) I write down the serial numbers on a single piece of paper. I give this paper to my descendants. I give the 20-dollar bills to my creditors. I tell my descendants, "If anyone ever presents you with a 20-dollar bill with one of these serial numbers on it, you do what they say, okay?"

If my descendants are really paranoid, they take the presented 20-dollar bill to the bank and have a teller look at it, before they do what the guy says.

In fact, if I and/or my creditors are smart, we'll take the bills to the bank and have them examined (after I write down the serial numbers, but before I hand them out).

The above scheme is foolproof for as long as

• 20-dollar bills are legal tender, and

• your descendants have access to a bank that accepts U.S. currency (which admittedly might be tricky if you are a family of pirates).

The Fed estimates that in circulation, any U.S. bill will last at least 4 years; and if you treat it like the average $100 bill and keep it "as a store of value," it'll last at least 15 years.

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Observe that a U.S. bill shows its serial number twice — on the left and right sides. You might be tempted to rip your $20 in half, give half to your creditor and keep half for your descendant. The problem with this idea is that two-halves-of-a-ripped-$20 are not legal tender, and so you lose your safety net: if you suspect forgery, you can no longer ask a teller, because they might just say "Nah, we don't take ripped $20s," and refuse to speculate about whether both halves were originally real or what. So, don't rip that bill!