Is my phone behaving similar to proximity cards?

[Jaime Stuardo]

Hello,

I need to understand this in order to use it in my work.

I work with access control systems. One of the hardware I use is proximity card readers, which read both 13,56 MHz and 125 kHz cards.

Accidentally I put my Samsung S20 phone near the reader and my surprise was that the reader did some readings.

Then I have opened Notepad and put the phone near the reader again and I saw that reader have read several numbers. For example, I have just put my phone in the reader and this was read:

4029234184
3629474568
1516677640
3332058120
2247351560
1729290504
0046941192
0967781384

It was like I have placed 8 proximity cards but in this case, all are contained inside my phone.

Can anybody explain me how this is happening? Is there an application that is allowing the phone to act as proximity cards? numbers read are always different every time.

The number of cards read depends on how long I keep the phone near the reader.

Thanks in advance,

Jaime

 

[hallux]

This is probably the NFC functionality that allows for contactless payments in stores that accept them. The reader is "tricking" the phone into trying to send that information (though I'm surprised it's not showing on the screen, it should).

 

[Jaime Stuardo]

Thanks for your reply, but I need to know how the phone behaves like that. Is there an app that generates random numbers continually so that if I place the phone near a reader, those random numbers are read?

I am a developer who has a system that uses proximity cards. If I can develop an app that sends, via NFC, always the same number (for example, the person's DNI number) I could allow the customers to use their own mobile phones instead of purchasing proximity cards.

Regards
Jaime

 

[hallux]

What about allowing the numbers to be stored in GPay, which also includes the membership card functionality? GPay is usually the default NFC app, for making mobile payments easier.

Why the random numbers? I'm not 100% certain but it's possible GPay works like a rolling-code garage door, updating the "token" for the card on each use. Maybe not. This would be some research you, as a developer, might be better able to do than I can as the information might be available in developer resources (or maybe Google has it squirreled away for security).

 

[Jaime Stuardo]

Hello! Yes... you are right. By mean of phone settings, i disabled NFC payment and no random numbers were generated anymore. By default GPay was selected, but I am wondering what is the logic or algorithm of that number generation? (for me they are random numbers, but there should be some algorithm for sure)

If I develop my own app that sends the person's DNI number. when reading those numbers on the other side, how can I separate the NFC payment numbers read from the DNI number? or should I teach the end user to disable mobile payment before using my app?

Regards
Jaime