As a supplier of NXP technology cards we have no control over what UID numbers are on the cards chip. This is the case for most suppliers of Mifare cards.
When you inspect a card on your system the UID may show as decimal UID value but it is not the case that a UID viewed in decimal always has a set length of digits.
The actual mifare/desfire UID is a 4 byte for Mifare or 7 Byte hexadecimal value for Desfire (e.g 4E DC F0 81 on a 4 Byte card, see attached screen shot from the NXP tag info android app available to download for free on Google Play ),
The decimal value (e.g 1567935364 )is a conversion of the true hexadecimal value.
This number can vary in length so if we supply a box of cards some UID's may be longer than others when viewed as a decimal value.
The mifare Classic EV1 cards we supply have 4 byte NUIDs’ and we can source 7 Byte UID if required. The Desfire cards have a 7 byte UID.
From our experience some access control systems do not read the UID in the native hexadecimal value or do not read the complete hexadecimal value. This can introduce a scenario
Where duplicate numbers appear on the system.
Here is a sequence of 7byte UIDs I made up. Cards in a box will never be in sequence but they are not completely random numbers, there is some pattern to the numbering.
45 D3 F4 11 B1 AC 56
45 D3 F4 11 B1 AC 57
45 D3 F4 11 B1 AC 58
45 D3 F4 11 B1 AC 59
45 D3 F4 11 B1 AC 60
45 D3 F4 11 B1 AC 61
If a system can only read a 4byte UID (eg mifare 1k not desfire) it may only see these numbers for each card…
45 D3 F4 11
45 D3 F4 11
45 D3 F4 11
45 D3 F4 11
45 D3 F4 11
45 D3 F4 11
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