Custom cards and tags with Intratone

4 Byte serial numbers

Old Mifare Classic cards have a 4 byte serial number. If you read it with a card reader or just use a phone with an app such as NFC Tools you will get a Serial Number that looks someting like 12:34:ab:cd . This is a hex-representation of the 4 bytes in the serial number.

Test card with Serial Number 08:01:02:03

7 Byte serial numbers

More modern cards have longer, 7 Byte, serial numbers. These are seeemingly more problematic. Neither the app, nor the web gui allows you to enter 7 Byte strings into the fields. And the maximum decimal representation of a 7 Byte number is 70 110 209 207 109 375, which far exceeds what you can enter in the app or web gui. And even if you bypass the length-checks in the web gui by blocking some java scripts, they will only save a 10 charcater long string, so you basically can not enter such long serial numbers anywhere.

Comparison of 7 Byte and 4 Byte serial numbers. BCC is a checksum.

A small trick at the end.

If you have not figured it out already, a very easy (but time consuming and less fun) way to avoid all of this, is to just use the card reader on the door.

Two failed entries, with their serial numbers. Bonus question: Are these 4 Byte or 7 Byte cards?

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Ola Thoresen

Ola Thoresen

Noe over gjennomsnittlig interessert. Kjentmann i IP- og nettverksjungelen, og jobber i nLogic AS.