{"id":104,"date":"2011-09-19T13:33:25","date_gmt":"2011-09-19T11:33:25","guid":{"rendered":"http:\/\/blog.ulrichard.ch\/?p=104"},"modified":"2011-09-19T13:33:25","modified_gmt":"2011-09-19T11:33:25","slug":"make-an-rfid-tag-with-an-attiny-and-a-coil","status":"publish","type":"post","link":"https:\/\/ulrichard.ch\/blog\/?p=104","title":{"rendered":"Make an RFID tag with an AtTiny and a coil"},"content":{"rendered":"

When I stumbled across this blog post<\/a>, I was sure I have to try this at home. I had some interest in RFID for a while, but the Proxmark<\/a> was too pricy for me just to play with. So this experiment came just right.<\/p>\n

The attiny85’s were difficult to get in Switzerland, so I ordered them from Germany along with an ISP programmer<\/a>. I think it would also work with other AtTiny’s for example the 45 is available from Conrad. The hex file is just 1.7 kB, so the AtTiny45 should suffice.<\/p>\n

After learning how to use the programmer with avrdude, the chip was quickly programmed. But I couldn’t get it to work with the tiny fixed coils<\/a>. So I refreshed some theory about coils and LC filters, that I didn’t use in ten years, and wrapped a coil myself with 1 mH. ElectroDrod<\/a> simplified the caluations. With this coil connected to the IC, I got the same reading from my RFID-Reader<\/a> as for the reference tag.<\/p>\n

I don’t understand how the copy protection that some vendors advertise their system works. I always thought entrance qualification systems used some sort of cryptographic challenge response<\/a> scheme. But that would require two way communication… I wouldn’t consider these systems secure if it’s so easy to copy the keys.<\/p>\n

Now the interesting part was if my prototype would work not just with my RFID reader, but with a real entrance qualification system… And eureka, it works!! It needs half a second longer to charge, but then lets me in. The same housing also supports mifare<\/a> or legic<\/a> readers, but ours are only proX2<\/a> and the tags are EM4102<\/a>. While the other options are also possible to copy, that’s a lot harder.<\/p>\n

Here is the mterial list:<\/p>\n