If the counterfeit gets used first, the stamp is marked as spent, and when the unlucky purchaser of the real stamp tries to use it, they're told theirs is "counterfeit" because it's flagged as "used" in the database?
I do wonder if this now means it is in theory possible to track items sent by letter post.
Also, an "is it used" database has to be kept to prevent an analog replay attack by reusing the same barcode. The most efficient way to keep track of used stamps would be a bloom filter. A poor implementation would lead to false positives, and mailers being accused of fraud. It also has should be highly reliable, highly available, and geographically disperse.
There's no need to have crypto if you're the authority on both assigning and verifying the barcodes. That way, no attacker will be able to create a barcode and have any hope of it working.
The spec is here: https://www.royalmail.com/sites/default/files/Royal-Mail-Mai...
This is exactly why gift cards have scratch-off material covering their secret keys, otherwise folks would just take a picture of the next card in the stack, wait for it to be funded, and then use it before the recipient does.
edit: And would you look at that, you can buy stamp sheets on Amazon.co.uk, and users have helpfully submitted a bunch of photos with barcodes visible.
Reused barcode does not mean counterfeit.
> Attempted reuse is exactly what would be prevented by scanning the barcode and seeing it had already been used.
Sure, but the alleged crime here is not attempted reuse. It is counterfeiting - as shown in the pic of the Royal Mail fee request label.
And the fact it did not tick the reuse box suggests it does not.