We can still have laws, e.g. that using someone else's certificate (or knowingly giving them your certificate) would constitute fraud.

We have laws against kids buying alcohol, even though kids can (and do) try to get adults to buy them booze, but I don't think that's a good reason to say we shouldn't have laws against kids drinking.

Sure, I agree. But we're talking about a claim by GP that his cyrpographic system prevents this behavior.

If it can't, it's no better than a button that says "I'm 18"