The unique identifier attached to a protected item is known as its Validation Reference.
It can take any form provided it can be easily identified as unique. The VRs created by the CodelMark software for protecting Branded Goods use a particular form designed to ensure security as well as uniqueness. Rather than use simple sequential serial numbers (which are easy for a counterfeiter to guess or imitate), our VRs are 80% random (20% check digits) so no one can guess what the valid ones are. Then we take the further precaution of storing only their hash values on the authentication database. This ensures that if hackers break through our defences, they can't retrieve useful information.
Our recommended VR is 25 characters long. It is actually made up of 5 groups of 5 characters so that it looks similar to a Microsoft serial number.
This is what a valid VR might look like:
R36IJ GLVNY IN215 ZRHQZ WL1IA
The fifth character in each group is a "check digit". Its value depends on the all the previous characters. The check digit is calculated on the basis of the preceding random values.
This means when users go to the Authentication page and enter their VRs, they only actually need to enter the first 4 characters in each group. The check digit is recalculated and displayed when they enter the 4th character. Any errors made by the consumer are quickly spotted and can be corrected before the VR is hashed and sent to the Codel database for matching.
For a more detail, particularly regarding the logic determining the length of the VR, you need to read here
How Does the VR protect Anything?
How Does the VR protect the Consumer?
