Introduction: A Universal RFID Key

Picture of A Universal RFID Key

RFID projects have been pretty prominent recently, ranging from projects here in Instructables, to our local Silicon Chip magazine in Australia publishing a RFID door lock project in their November issue.  Even I recently purchased a RFID door lock on eBay for $15 to lock my garage (so my front neighbor could get tools if he wanted to).

We have known that the cheaper RFID technologies were pretty insecure for a number of years.  Researchers have demonstrated cloners of all varieties, but simple RFID tags are still being used for access control.  Even my current employer uses them.

A while ago, I was looking at Hack A Day, and I saw an amazing project that somebody had made.  It was an RFID card with a keypad on it.  For the next couple of days, I couldn't get the image of the card out of my mind;  the project reminded me of how much I wanted to build a RFID spoofer myself.  The original author didn't release source code for their project, but they left enough clues that I could follow. 

So, in typical fashion,  I built my own reader hardware so I could have a look at the data from a card, and created my own version of the Universal RFID key.

The key I made works beautifully both on my garage door, as well as a number of other RFID readers I have tried!

I have decided to publish this, as more people should be aware of the design flaws that are inherent in older RFID implementations, and to allow others to make their own universal key.

Will this key let you into anybodies RFID protected office?  Yes it will, assuming a couple of things are true

  1)  The have to be using 125kHz RFID tags that use the same encoding standard as I have designed this project for, and,
  2)  You have to have access to the number printed on the back of the tag - with that number, you can simply key it into the Universal RFID key, and it will emulate that tag.

So there you go - I hope you enjoy making this project.  - And remember, with great power comes great responsibility!

Step 1: How Does RFID Work?

Picture of How Does RFID Work?

RFID, or Radio Frequency IDentification is the term used to describe a wide variety of  standards that allow data stored within electronic 'tags' to be read by a reader without using wires.  There are a number of standards, encoding formats, and frequencies in common use.   I will describe the 125 kHz standard that is common for access control mechanisms.

125 kHz RFID tags are commonly encased in a business card sized piece of plastic, or a round disk.  The tag consists of a coil of wire, connected to a microchip.  When the tag is brought into close proximity to a reader, energy is coupled inductively from the reader to the microchip within the tag. 

The energy from the reader has dual use; firstly, it provides power to run the card, and secondly, it provides a communication medium for data to be transmitted.  Once powered up, the tag modulates the bit pattern that is programmed into the tag using a signal that the reader can detect.  The reader then reads this bit pattern, and passes it onto the door controller.  If the bit pattern matches one that is authorised, the door will be unlocked.  If the bit pattern does not match an authorised one, then the door won't unlock.

In the RFID system I was playing with, the bit pattern looked like this;


I will describe what this pattern actually means in the next page.

One interesting feature of the data transfer between the card and the reader, is that data is encoded using Manchester Encoding, which is a way of encoding data so that it can be transmitted over a single wire ensuring that the clock information is able to be recovered easily.  With Manchester encoding, there is always a transition in the middle of a bit.  If you want to transmit a 1, the transition would be from low to high, and if you want to transmit a 0, the transition would from from high to low.  Because the transitions are in the middle of each bit, you can ensure that you have locked onto valid data.  For a detailed description, have a look a this page.

The actual data is transmitted by the card effectively shorting the coil out - this applies an additional load to the transmitter in the reader, which can be detected.

Step 2: Whats Stored on the Card?

Picture of Whats Stored on the Card?

I started by building a RFID card reader (more details in a  future article).  That showed me the data that was being sent when the card transmitted its information.

The RFID cards that I brought have numbers printed on the back of them.  This number says what data the card has included in it.

the card with  0007820706  119,21922 printed on it transmits this pattern:

The first set of 111111111 bits are the start sequence - it is used to tell the reader that a code is coming - the reader also uses the sequence to lock onto the card data.

Data stored is transmitted in groups of 4 bits, with a parity bit at the end of every group.

The data can be broken up as follows;
00101 11000 00000 00000 01111 01111 01010 01010 10100 00101 0110                0

If we ignore the parity bit at the end of every nibble we have

0010    1100    0000    0000    0111    0111    0101   0101  1010   0010    0110              0
     2          C        0           0            7          7          5          5           A          2     CHECKSUM   STOP

This code is  2c 0077 55a2  if we break the code into 3 groups, we have 2c, followed by 0077 (which is 119 in decimal), and finally 55A2, which is  21922 in decimal - this corresponds to the 119,21922. 

The same number is also written in another way on these cards 0007820706 (in decimal) is simply the hexadecimal number 7755A2.

WOOT we now understand how the data is stored.

2C is a constant code that is sent with all of the cards.  It is simply a facility identifier for this RFID system.

How does the parity and checksum work?

One final piece of data that the card transmits is a checksum word - this is used to ensure that all of the data has been received successfully.  Firstly, the parity bit at the end of each nibble of data is Even parity - this means that the transmitter will add a 1 to make sure that each block of data has an 'even' number of '1' bits -  So if we look a the '2', which is 0010 in binary - the parity system would detect that there was an odd number of '1' bits, and would add one to compensate.  Compare that to the 'C' which is 1100, the parity system would detect that there are an even number of '1' bits, so it would add a zero.

00101 2
11000 C
00000 0
00000 0
01111 7
01111 7
01010 5
01010 5
10100 A
00101 2

0110 checksum    + 0 stop bit

Finally, the checksum is an even parity bit applied to each of the vertical row bits.  This way, there is a horizontal and vertical check of every bit sent - everything has to line up, or the reader will simply reject the transmission.

When I decoded the data for my work prox card, it followed a similar sequence here, but (for obvious reasons) I won't actually publish the numbers.  Again, part of the sequence was a facility code, and the rest of the sequence held the same number that was printed on the back of the card.

Step 3: How Do We Emulate a Card?

Picture of How Do We Emulate a Card?

So the next step was to identify how to pretend to be a card - I wanted a card that I could type a card number into, so it had to have a microprocessor on it, was well as a keypad to allow the data to be keyed in.

The ATMega  manipulates the 125kHz RF field by using a bridge rectifier.  When the output of the micro is low, the diodes in the bridge are allowed to be turned on by the current induced in the coil,  this effectively short it out.  The reader detects the additional load, and a bit transition is detected.

The job of the micro is simply to turn the output on and off in a way that makes sense to our reader.  So I created a board that had the micro, a power supply, keypad, and some status LEDs on it.

The attached PDF is the full schematic of the project.

You may notice that c6 is 0pF -  That is intentional  c6 is a placeholder component allowing me to either use a 1000pF surface mount cap, or a 1000pF through hole cap.

The coil is 100 turns of fine wire would on an open former that is just smaller than the card border.

Step 4: The Software - Entering Data Into Our Card

The software was next.  Using the Arduino IDE,  I implemented a simple menu system that allowed me to enter the relevant facility and CardID data directly from the keypad.  I also provided a way of displaying the data using the LEDs that I mounted on the board.

One problem I came across, was when I was calculating the card data (parity and checksum) on the fly - To be read successfully, the card has to output data in real time (most readers need a number of sequential valid reads), and adding subroutine and calculation delays caused the card to output invalid data as far as the reader was concerned.  I worked around this problem by populating an array of bits that gets sent when the card is in transmit more.  That way, the calculations are done only once.

When the card is powered up, it waits for the 'mode' button to be pressed.  The current mode number is displayed using a set of 4 LEDs.  Each press on the 'mode' button increments the current mode.  Once the correct mode is displayed, then the 'enter' key starts that function executing.

MODE 1 - Enter low power (sleep) mode

The card enters a low power mode, waiting for the reset button to be pressed to re-awaken it

MODE 2 - Enter a Hex Facility ID

The card waits for 2 digits to be entered signifying the facility code for this system (In this case, it is 2C) - The software defaults to 2C - so this does not need to be entered.

MODE 3 - Decimal Card ID
The card waits for 8 digits to be entered signifying the CardID for the card to be spoofed (In this case, it is 07820706)  - This is the long number printed on the back of the card, not the 119,21922 number.

MODE 4 - Dump the facility and Card ID
The Facility and Card ID are Dumped as Hex numbers using the 4 Leds at the top of the card.

MODE 5 - Emulate a card
The card enters emulation mode - all LEDs are turned off.  Emulation mode can only be exited by pressing the reset button.

The software relies on Mark Stanley's and Alexander Brevig's Keypad Library

Step 5: Etching the PCB

Picture of Etching the PCB

As per standard, I used toner transfer onto magazine paper to etch a board.  If you want to see the details, have a look here.

The etched PCB had its edges cleaned up a bit using a file, and holes were drilled for the IC legs.

Attached are the PDF files that I used for the Toner Transfer.

Step 6: Mounting the Components

Picture of Mounting the Components

To keep the project the same size as a normal prox card, I decided to make it on a small PCB that was the same size as a business card.

I decided to use surface mount push buttons that I brought from eBay, so that meant that all of the components must be soldered onto the copper side of the PCB to allow the buttons to be mounted and labeled.

I started by soldering the push buttons, then I mounted the LEDs, resistors and capacitors.  I had to install the 16MHz crystal on the bottom of the PCB, as I did not have a surface mount crystal.  I also installed 12 jumpers on the back of the card to connect the key columns together.

The ATMega168 was mounted next.  I did not use a socket, as I wanted to reduce the board thickness. 

Next, I wound the coil - I used a piece of scrap timber, with 4 screws mounted on it, and counted 100 turns of 0.25mm diameter coil winding wire.  Before I removed the coil from the mounts, I wound a small amount of clear tape around each edge to make sure that the coil didn't unwind.

Then, I mounted the coil on the back of the PCB, along with a small battery holder.

I was pretty happy with the result of my handiwork.

Step 7: Programming the Micro

I used a standard 6 pin header mounted on the PCB to allow a FTDI 5V USB-232 cable to be used to program the chip in-situ - this was especially important, as the ATMega chip is soldered directly to the PCB, so it couldn't be removed for insertion into a normal Arduino PCB- This is a small price to pay to have a nice compact project.

The chip was programmed using the .pde Arduino sketch that was supplied in Step 4 - using the normal Arduino IDE.

The .PDE file that I have provided is tailored to the standard cheap eBay RFID systems.  It is not the version for the other IFID readers I have access too..... (I just thought I would mention that :-) )

Step 8: Testing the Project

Picture of Testing the Project

Testing was a breeze - I typed the relevant code into the keypad, swiped the board against the reader, and was rewarded with a satisfying 'BEEP' indicating that the read was successful.

Testing at the other readers I have access to was just as rewarding, and scored infinite geek points!!!

Step 9: Further Steps

This was a 'to prove I could do it' project - I have completed it, so it now sits on my shelf at work to remind others that simple RFID systems are simply not secure.

You are welcome to adapt the project however you would like to, and while you may have the skeleton keys to the kingdom, you still need the little numbers on the back of the access card before you can use the key yourself.

I have considered modifying my card so that it works as all of the compatible RFID tags that I hold.  In my job, I need have access to multiple work sites, and it would be great to use the one card, but I don't think that would be a great idea..........

Will this work on all RFID sytems?

No it won't. This is a good thing.

The first RFID systems deployed years ago used very simple protocols, based on the intelligence of the chip in the card - They also used a low frequency (125kHz) carrier.

More modern systems use a number of techniques to ensure security, such as one time codes; cryptography; use bi-directional communication; use internal passwords, and use much higher frequencies. So spoofing these systems is a lot more work.

But there are a large number of low tech systems in place now.

What can I do to protect my system?

Firstly, don't equate cards to physical keys - in simple systems they are not equivalent.

Don't give out visitor cards - They are easily duplicated - I f you do need Visitor cards, then implement a system where they are only active when they have been issued.

Enable Pass Back systems - If the card system believes you are in a particular room, make sure that the card can't be used in other rooms at the same time.

Remove the numbers from the back of the cards - while they may make it easier to enter card details, but they also make it easy for somebody to use the details for their own purposes.

Finally, look at how to upgrade your access system to a card system that is not trivially spoofed using $15 worth of parts.  And - No, purchasing a new system from eBay for $15 is not the answer....


edg62 (author)2017-09-03

awesome. Im impressed and inspired by how you reversed engineered and integrated these components! Bravo brother!

Magragh (author)2016-12-29

Sooo... this could be used to easily hijack a pushbutton car?

fixfireleo (author)2016-06-16

so, is this basically the technology used by theives to steal people's credit card info remotely??

Fiernocht (author)fixfireleo2016-11-14

The only way that it is similar is that they both use RFID but, most credit cards do not use RFID because it is so insecure. Credit/debit cards use Pin and Chip technology which requires the card to be inserted much like the magnetic swipe, as the information is read from the chip. However, if the card reader is compromised, you are out of luck. Also. This tech is more similar to the RFID implants you have on your pets or it is exactly like those cards you see at low security doctors offices and what not.

TonyD155 (author)Fiernocht2016-11-20

Any credit/debit card that is tap enabled uses RFID (very common in Canada). Pin and Chip far exceeds the swipe method that was very easy to skim.
Most banks will limit the risk of Tap by setting the transaction limit to $100.

mikefromspace (author)fixfireleo2016-06-16

Basically, it's a reader for id chips which includes but is not limited to credit/debit cards. Simply owning or using such a device on a stranger is most likely illegal in all states, but is a useful tool in discovering someone's identity, for example. This nation (U.S.) was founded on the principle of overthrowing any government which condemned the people from the freedoms of the constitution. We are thereby granted the right by founding fathers to hold equal technology and protection against any government. Anyone who otherwise affects these freedoms should be imprisoned and or deported for treason against the American people.

Fiernocht (author)mikefromspace2016-11-14

it's not a reader at all.. It's a transmitter. you have to manually input the data into it. However, as far as I know it's legal status is a grey area

flagstone78 (author)fixfireleo2016-06-19

This guy is a pretty well known electrical engineer that explains rfid theft/proptections pretty well:

KryptoTSD (author)fixfireleo2016-06-16

Hope not...

Fiernocht (author)2016-11-14

I have NEVER heard of a dentist or ENT doctor putting an RFID chip in a patient. And I'm hearing impaired. However, I know some doctors do and a lot of wetware body hackers do...I havent ever heard of anyone using those chips to torture anyone either though... I mean you would have to know the person has the chip for one...Not saying it isnt a possibility, just that ive never heard of it. I do know the chips are becoming more popular though. but I dont think you can actually block an object from inducing a current in a chip with anything short of a faraday cage around the chip you want to protect. but then that would make it unreadable period.

KurtPH (author)2016-09-26

Seriously? Please come up with one reference. I am a family doctor, and I have never heard of doing this. Chipping a pet, yes.

Marcel Varallo (author)KurtPH2016-10-06

Hi, my name's Marcel and here is my chip

farahxx (author)2016-09-26

wow it seems like something in the movie. i thought it's just an imagination

BuyThisComputer (author)2016-09-09

Hello, do you thin that it's possible to make a generic hotel RFID keycard (to keep aircon and plugs ON when i go out for a short time) or possible to do this with a mobile ?


JanP103 (author)2016-08-29

Do you think it's possible to program your phone to do the same thing? Like by using a bluetooth or wi-fi antenna at lower frequencies?

anagliph (author)2016-08-13


You mentioned you might publish an article on an RFID reader using an arduino, did you so?


baicker (author)2016-06-20

sorry for that, it was for my friend's birthday gift, not a school project

philpp (author)2016-06-18


etmilosk (author)2016-06-17

This is a good project and a great article! Very well written and documented. I'm surely going to build one when I find some time. Thanks!

dshavr (author)2016-06-16

nice one :)

ChristopherW41 (author)2016-06-16

What about placing a rfid reader into the existing rfid so it a log of all scanned cards so you can then enter them into the emulator and when someone figures you out you can just switch to a different number

gtoal (author)2016-06-16

"2) You have to have access to the number printed on the back of the tag - with that number, you can simply key it into the Universal RFID key, and it will emulate that tag." - or you can brush past them and sniff their card with an RFID reader, as is being done en masse nowadays:

xvicente (author)2016-06-16

Man you look just like Flynn in the movie TRON with that

itninja (author)2016-06-16

Is there already a solution that pairs RSA hash generation with RFID?

sacarlson (author)2016-06-16

Very interesting idea. I wonder if you could use it to setup one time use RFIDs. So preload a list of 1000 ids (or a non linear sequence), and each time you use it, that one is removed from being valid. The would be preknown on the reader side. Unlike rolling codes in the garage door, there is no reset code.

RyanC100 (author)2016-01-20

By any chance would anyone be selling one of these. I would love one of these but dont have the paitence so I would happily buy one.

RyanC100 (author)RyanC1002016-01-20

Just to add this is to test my home locks. I have 4 on rfid and well this key looks much better than a white card for when I have gloves on my xEM implant wont read through leather glove for obvious reasons.

neumanngregor (author)2015-10-12

This is great. if it cud be made smaller and rugged, and with a way of "spiting" more codes , stored before, at a press of a switch, a dream. I'm a careless person and i kill around 4-6 RFID tokens / year, the dumb thing is, sometime i get them not working on Saturday or Sunday :P

taifur (author)2015-09-18


yaly (author)2015-09-12

can we make a pcb antenna on the back side ?

yaly (author)2015-09-12

SMD component sizes please ?

OmarM27 (author)2015-08-11

Hello drj113. I am trying to use this page to make the key. Can you list all the things you used in this project? Thanks.

OmarM27 (author)OmarM272015-08-12

bottom right*

OmarM27 (author)OmarM272015-08-11

I also want to ask, how did you program the microcontrolller? Do i need to buy an Arduino Dumaelianove? And what is that green thing at the bottom left?

sekarganesh93 (author)2015-06-27

Awesome ,, Bro for we enter the data , we must know the what is the data it should be accept for that we have to know the data .... instated of we have to change the emulator into a reader and emulator and as well as key pad in the meaning we have the chance to know what is the data their using and would we emulate the same through our;s circuit

qq135791050 (author)2015-06-20

that is great . i want to make it fo myself

AmpOwl (author)2014-10-19

Theoretically, could you write the code so that it runs through every possible rfid code combination (similar to a password hacking program) until one of them works, or are there too many combinations for that to be efficient?

Assuming a standard 18-number code for a lock, there would be 158,789,030,400 possible combinations to it. An RFID reader, on average, takes 5 seconds to register a card, verify it's credibility, and reset the lock for a different pass code. It would take 25,559 years to enter all of the possible codes into the lock. Technically, it's possible, but not entirely feasible.

When I was playing with this project, I found it easier to stand beside people, reading their cards, and then programming this card to duplicate theirs. I never implemented a rolling code as it would take too long.

jerbs (author)2014-10-17

I bought a RFIDler at Derbycon a couple of weeks ago and having trouble getting started with it, I really wish someone would make an instructable for one of those.

Vadzz (author)2014-02-06

Hi. Sorry for bad English.

Noticed some mistakes. The diagram shows a ATmega8 microcontroller, in the article you say that ATMega168 was mounted on board, but the image on the finished board I see Atmega328 installed.

I understand that for the project is suitable for mega168 and mega328, but on Schematic - ATmega8 microcontroller will not work and it is misleading. Please correct the error, or specify exactly what MCU you used.

Thank you!

BeerLogic (author)2014-01-05

It is interesting that *ANY* of the actual encoded number was printed on the card it's self. Twenty years ago I was designing systems that used the original Wiegand cards (the protocol that the cards use, 36 bit) and they had protocols that ensured that the printed numbers in no way matched the encoded numbers. It is not unusual that the facility code is not printed on the cards. - RJ

drj113 (author)BeerLogic2014-01-05

Yea - I would have thought that Security 101 would be to not disclose the numbering... But sadly, in 3 samples that i have decoded thus far, that simply isn't the case.

usbg3rd (author)2013-03-07

hi, excellent project there... but i have a few questions
1- i have a card,with the number on the back , i can decode most of the portion of the number but how do i find the facility code? and the total number of bits including the starting bits and the ending bits.
2- i don't have the card reader so how can i see the bit pattern sent by that RFID card.
3- is there any way i can receive the whole bit stream sent by my rfid card?

drj113 (author)usbg3rd2013-03-08

Cool - Thanks for the questions.

The only way of getting the facility code is to read a card. It is rarely printed on the card itself

The only way to see the bit pattern is using a card reader. I built my own - there are lots of simple designs.

usbg3rd (author)drj1132013-03-09

Hey thanks for the reply , but i wanted to know how can i build a card reader (without using any module) using only micro controllers (pic AVR etc) and stuff.. can u give me a link to it
and just wandering if 8051 can be used in making the reader ?

usbg3rd (author)usbg3rd2013-03-11

helo ? u there ?

drj113 (author)usbg3rd2013-03-11

Sorry for the late reply, I was away.

You can certainly use an 8051 to read a card - You have to build all of the electroinics yourself though.

Here is a link to a project that I found helpful.

Soirry, but my reader is not a completed project that is at the stage where it can be released as an Instructable.

prayalone (author)2013-02-10

I'm experimenting with the RFID.
with my little knowledge ^ ^'.

Please help me comparing your design using diode bridge and some other design using a transistor. (like this one : what the difference (pros/cons)?

I'm see that using transistor is more simpler but I don't know if there is any trade of?

Thanks for your contribute :D

drj113 (author)prayalone2013-02-10

That is an interesting way of doing it.

The thing to consider is that the output of the micro already has a transistor anyway... So this is simply duplication.

About This Instructable




Bio: I have a background in digital electronics, and am very interested in computers. I love things that blink, and am in awe of the physics ... More »
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