Analysing An Active RFID Card

The next device from the box of bits was a pair of active RFID cards from a top of the range vehicle security system used around 2001-2003. These tags were required to be on your person to start the car in addition to a key. This is just an analysis of the cards RF transmissions and not the system itself as I don’t own one.

Popping open the two cards we can see they have similar layouts but the newer board on the right doesn’t look as clean and does not have any PCB labelling. The keys each have a serial number sticker on the back of the PCB as well as having it printed on the front of the card.

The two RFID tokens.

The two RFID tokens

There are not many components on the card, we can see an SMD EEPROM along with a small SAW device and the PCB antenna. Looking up the SAW device documentation indicated that it will most likely be transmitting in the 433Mhz ISM band on 433.900.

The batteries in the cards had long run out, so we just hooked them up to an external power supply and monitored with the HackRF and GQRX, this would have worked fine with the RTL SDR device too. We see a short transmission on 433.920Mhz roughly every 5 seconds but it does vary a bit. The frequency reading is wrong below as the screen shot was captured off an IQ replay.

Clear signal.

Clear signal.

On looking at this a bit closer with baudline it unsurprisingly looks to be an on off keying AM signal. Importing a saved audio clip in to Audacity shows the signal, the first capture looked a bit odd with dips and other strangeness but it turned out I was a bit off frequency, fixing the frequency gave a clearer capture.

Unclean off frequency capture.

Unclean off frequency capture.

The first thing I wanted to do was level off the signals to allow for an easier visual comparison, the capture below shows the original signal up top and the altered one below. I used a the same script I used with the previous doorbell project and we can count 78 peaks in each tokens transmission.

Full capture with the second channel prettified

Full capture with the second channel prettified

Comparing transmissions showed that the token is beaconing the same pattern over and over so maybe they are easily repayable as with the doorbell. Interestingly when we look at the two tokens with different serial numbers next to each other we can see a quite long period at the start where they roughly match followed by not too dissimilar patterns after. At this stage I’m guessing the serial numbers are likely just being beaconed following the preamble/synchronisation signal.

The signals patch prior to the red line.

The signals match prior to the red line.

The timings were not particularly consistent even in the initial preamble to allow them to be lined up easily and my previous script to guess binary output based on the period of time (NRZ) was not working perfectly due to this.

There appeared to be two spacing times, one around 18 samples and one around 45 samples but these varied by +/- 10 samples which added up to a large error by the end of the 78 state changes. In order to sort this out I added an option to the script we created for the doorbell to make the sample spacing gaps even. So in this instance a sample space of >30 = 40 and <30 = 20 gives us an easy to compare output.

A resized comparison showing some matching digits

A resized comparison showing matching digits

Now that the WAV file has evenly spaced samples, we can compare them easier as can be seen in the image above of the two tokens beside each other. The initial preamble matches perfectly between each transmission up to the previously indicated red line and would seem to have 5 8 bit components going by the peak count. Handily enough, the tokens 6 digit serial numbers share one digit in the same place and we can see where these match between the two tokens, we can also see this digit repeated again in two other places in the second serial number, these are indicated in the image above.

Now on to the actual decoding! The number of peaks, 78, doesn’t really tally with the indication from the preambles/synchronisation that each part is 8 bits. We now have to turn to a different encoding mechanism commonly used for this sort of cheap OOK device, Manchester encoding. This is better explained somewhere like http://www.quickbuilder.co.uk/qb/articles/, but the idea is that instead of counting the peaks and troughs as a high or low as you might when taking a look at it, we use the transition between high and low values in the middle of a set time period to define the 1 or 0.

As we have already altered our file to have a consistent spacing of 20 or 40 samples we can script reading this Manchester encoding easily by starting at the first state change position, measuring forward 39 samples and detecting whether the value increases or decreases in that time period and repeating this process until the end of the file.

This gives us the following binary output which looks very promising, we can see the matching patterns for the shared digits in the serial numbers between the tokens. The first 5 preamble bytes are the same, 6 is different, and 7 is the same matching the digit 0 in each of the tokens at position 2 with token 2 having two further digit 0’s all in bold.

Token 1: 11111111 01010101 10101010 01010101 00110110 00110001 00110000
Token 2: 11111111 01010101 10101010 01010101 00110110 00110100 00110000
Token 1: 00110001 00111001 00110011 00110010 00000000 10111011
Token 2: 00110000 00110111 00110000 00110011 00110000 11100100

Now converting each of these from binary to hexadecimal we get a match for ASCII characters so by adding an extra decode step to the script processing the file containing the resized signal, we can see the serial numbers they are broadcasting!

stats.py -i gqrx_20140905_134055_433900000-tag-101932.resize.wav -m
Manchester encoding, channel: 0 offset: 40 boundary: 40
ASCII: �U�U6101932

stats.py -i gqrx_20140905_134055_433900000-tag-400703.resize.wav -m
Manchester encoding, channel: 0 offset: 40 boundary: 40
ASCII: �U�U64007030�

So we have managed to go from a captured signal to decoding a beacon from an active RFID card in not too many steps. I’m not going to go in to any more details but it doesn’t seem a particularly secure security device on the evidence we have gathered here.

Now this would be a lot nicer if I could do it with GNU Radio as it is happening, but small steps!

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