We have had a requirement from a customer to make a number sequence, where a customer should not be able to calculate or find what will be the next receipt number. They are running a raffle draw, wherein a customer can participate by entering the receipt number of their purchase on their website.

They received many fraudulent entries. People were seen guessing the receipt number and checking if the receipt number was already registered. If the date and time stamp with seconds can be added to the receipt number the receipt number will be unique.