What we are referring to here is a specific property of the document which is required to be verifiably unique. Examples are Currency, Airline Tickets, Retail Vouchers, anti-counterfeit labels, Cheques, sundry legal documents, lottery tickets etc.
This is distinct from Content validation in which our sole concern is that the document contains, verifiably, what it should contain. Nothing more and nothing less. The consumer's interest in Content validation, is the assurance that they are getting what they paid for or receiving the message that they were intended to read. The supplier's interest is in their intellectual property, reputation and clarity.
In contrast, their are many documents where it is desirable that only single copies of the document exist, or, more specifically, that only single copies are used. Each airline ticket can only be used by one person. We only want one £20 note or $100 bill to have a given serial number. Only one raffle ticket can win the prize etc.
Clearly, where uniqueness is required, it, too, benefits both consumer and supplier. The consumer knows that they will have a seat on the flight or in the theatre and that their money won't lose its value overnight. The supplier knows that their revenue stream is protected, they haven't overbooked and that the economy will not collapse through counterfeit currency.
Codel promotes Uniqueness first by recommending the appropriate choice of unique identifier. See, for example, the logic that underpins the Validation References we advocate for anti-counterfeit purposes.
The key concept in protecting a unique identifier is the inclusion of sufficient unpredictability in the id to make bulk forgery impossible and theft of data pointless.
This does not preclude, however, the continued use of convenient and user-friendly sequential numbering systems which make administration of many documentation systems much easier. All we need to do is add a further identifier whose sole purpose is protection of the unique identifier by making them unguessable.
Hence a series of tickets which might have been
12345
12346
12347 etc
might become
| 3KDFE ADKII 9WEQK FISAP WITTD | 12345 |
| OE8RG GLA4E RYGGY GAY12 DS346 | 12346 |
| NTHGA GHAPW GQRUT TATGW 58OLD | 12347 |
etc
where simple sequential monitoring is required, we only need to consult the last 5 digits, so the theatre still knows at a glance how many tickets it has sold. The counterfeiter, however, is stumped. Unless they have a copy of the original data, they don't know what to print on the ticket to make it pass muster.
Yet simply by storing the hash value of that identifier, the Codel system allows any issuer or user of the relevant document to validate simultaneously its uniqueness and its existence on the approved database. And they can perform this instant check anywhere in the world at the click of a mouse. If the ticket is printed to be machine readable (eg barcoded) then a typical online validation will take less than 5 seconds.
