Reverse engineering a car key fob signal

https://0x44.cc/radio/2024/03/13/reversing-a-car-key-fob-signal.html

by wolframio • 2 years ago

428 83 2 years ago

bombela• 2 years ago

I had to reverse engineer some cheap key fob purchased on AliExpress for an electronic project. It was simple enough that thanks to an oscilloscope and wikipedia I was able to do it after persisting long enough.

Next time I will try the method from this blog post. And maybe become a better hacker.

tiagod• 2 years ago

Note: Transceiver SDR devices do exist of course, but they tend to be very pricey

A HackRF clone is cheaper than a Flipper, and way more capable in my opinion. I would bet most flippers either lie in drawers or are used by stupid teenager kiddies for trolling.

stavros2• 2 years ago

A HackRF clone is cheaper than a Flipper

Yes, but a "HackRF clone, plus a Proxmark3, plus IR, plus whatever" probably isn't.

AnarchismIsCool3• 2 years ago

The flipper isn't really a full sdr though, it just has a very minimalist RF IC that has almost non-existent bandwidth.

For $400 you can get a limeSDR mini that can read and write 30MHz of spectrum at a time, ie the entire ham 70cm band all at once.

If you think a flipper is dangerous, plug in a dummy load and dump noise on L1 then watch your phones GPS stop working, or alternatively decide it's on another continent.

wallaBBB• 2 years ago

Why would you need such a stack? Article is analyzing unidirectional fobs, HackRF is half duplex so you could easily capture and analyze and/or replay the signal. Only additional thing you need is a PC.

One thing to consider is that the payload will be encrypted so you wont be really able to tell apart what is the rolling code. Hopefully fobs have stronger encryption so collecting enough sniffs and analyzing is insufficient (looking at tesla with their 64bit encryption, hopefully they upgraded).

Honda replay myth mentioned in the article is BS, it was popularized by ppl faking a simple replay attack while doing a more complicated one. If you record the fob command and the car never receives it, of course you can immidiatly after replay it to the car and car will accept it since RC is valid. But if you're sniffing while car is receiving it, RC gets updated. If Honda didn't have RC, it would have been far worse than the KIA boys (overriding immobilizer protection and hotwiring the car) issue that did a lot of damage to KIA in US.

alias_neo• 2 years ago

I would bet most flippers either lie in drawers or are used by stupid teenager kiddies for trolling

I find my FZ most useful, not for the radio stuff, but as a wireless (read: untethered) way to dump and write EEPROMs using a POMONA clip, otherwise, yes, it sits in a drawer.

alexb_• 2 years ago

Ok but do the clones have a cute dolphin? Very important feature

tibbon• 2 years ago

I've got a Flipper, LimeSDR (non-mini), some old-school ham equipment, a cheapo $10 RTL-SDR receiver, a few cheap HTs, some RFID tools, etc.

Each has their use. The Flipper is nice for quick and lightweight checking of things. LimeSDR is incredibly capable, but also a bit of a pain in the ass to use. Not something you'll flip out to quickly check something or run an experiment.

MuffinFlavored4• 2 years ago

checking of things

like what?

byteknight• 2 years ago

I have both. They both enjoy the warmth of my drawer :)

bongodongobob• 2 years ago

Realest comment here. I have a few drawers full of these kind of toys I used once and forgot about. Right next to my serial cables and bits of wire.

tivert• 2 years ago

These keys are generated and tracked using a counter which has to stay in sync between the remote and the car. This ensures that the car doesn’t reuse an old key, and that the remote always generates fresh keys.

Something I've always wondered about is, how do learning remotes defeat this?

My car has a couple of built-in garage door buttons, and I'm pretty sure I programmed it by just hitting the remote button in the garage while the car was in a learning mode. Is that a much more sophisticated feature than you would assume (e.g. decoding the signal, recognizing the type, then initiating a pairing with the opener, instead of just replaying the signal)?

seidleroni• 2 years ago

My understanding is that most garage door openers do not use rolling keys, they send the same code each time.

jjtheblunt• 2 years ago

They use rolling frequencies at least our 2021(?) garage opener does, to retry in the presence of narrow band noise from fcc violating devices. We seem to have neighbors blasting rf in the band that our previous builder installed 2014 model used, because it rarely worked unless super close.

mdip2• 2 years ago

They've been in use since the 90s, actually[0].

My understanding is that the earlier rolling code systems are easily defeated and I think this can be done (possibly with stock firmware) using a Flipper Zero.

Prior to that, garage doors had a set of DIP switches (16, or 32, I can't remember). You matched the switch configuration on your opener with the switch configuration on the controller. And as you might imagine, in a typical suburban area about 80% of the garage doors are set to all zeros.

Because the range of the devices was "lucky if you can open the door from the bottom of your driveway", most people didn't notice this. Of course that meant you could open a large number of garage doors by sending the "0" signal for each manufacturer with enough wattage.

Compatible models are made by reverse engineering each individual model's rolling code implementation (in the early days) and making an accessory that had the necessary seed value or other component to allow it to be "paired" with a compatible door head unit. Considering it wasn't uncommon for the higher-end models to charge $150 for an accessory remote, manufacturers had a bit of incentive to roll their own slightly incompatible implementations.

This is from memory and minimal memory at that, but -- late 90s or early 00s, I think, "HomeLink" was created, which basically allowed car manufacturers to integrate a door opener into the car. If you bought a higher-end model, your visor might have the buttons in it. I believe licensing allowed third-parties to easily create fully compatible accessories at that point (pay a fee, get the patent license/datasheets sort of arrangement).

[0] Genie thinks they were first in 1995 but I seem to recall we had a rolling code door installed as early as 1993.

porkbeer• 2 years ago

They have been rolling codes since the early 2000s. 80s and most 90s doors are static codes.

tyingq• 2 years ago

Mine has rolling codes and a "learn button" on the head end to accept a new remote.

throwaw33333434• 2 years ago

my 2016 gate use rolling codes.

tkems2• 2 years ago

The largest garage door manufacture in the US uses the Security+ and Security+ 2.0 algorithms that are rolling, but can be fairly trivially decoded to gain the serial number and rolling value of a remote. [0] This is how the flipper zero decodes remotes for playback later.

[0] https://github.com/argilo/secplus

tkems2• 2 years ago

This sounds like HomeLink and is indeed more complex. My understanding of it is that they partner with lots of companies to support their rolling/fixed codes and remotes so that they can be paired to your garage door.

I linked this in a sub comment, but the largest garage door maker in the US is Chamberlain [0] (which owns a ton of other brands) and uses known rolling code algorithms that can be decoded. [1]

[0] https://www.chamberlain.com/ [1] https://github.com/argilo/secplus

sgerenser2• 2 years ago

You have it backwards, the main garage opener is doing the learning, the cars button is just transmitting a signal. Doing this process, you’re telling your garage door opener “hear this new remote? Please allow him to open the door as well.” Presumably the car side buttons just cycle through a few common protocols (realistically there’s only 4-5 ones in common use, almost all garage door openers I the U.S. are made by Chaimberlain/Liftmaster or Genie).

sgerenser2• 2 years ago

Actually I take it back, there might be learning on the car side as well, where you press a button on an existing remote, but all that does is tell the car “Oh, this is a Chamberlain SecurityPlus 1.0 remote, I’ll start acting like one of those.” The actual rolling code and security algorithm is never decoded/cloned on the car side.

tzs• 2 years ago

Let's start with fixed code systems. In those the remote sends the same signal every time. There will usually be some way to customize this signal, such as a set of DIP switches in the remote and in the head unit so that you can set your system to a different fixed signal than you neighbor's fixed signal.

A learning remote for such a system could work by simply replaying whatever it heard when it recorded during learning. So if you press the "learn" button, press the button on your current remote, and then do whatever tells the learning remote you are done it could save everything its radio picked up during that few seconds, and then play that back whenever you press the open/close button.

You probably wouldn't want to take that bare bones approach though. The recording might include other things. Suppose while you are setting it up someone happens to come to your house and press the button on your wireless doorbell, which just happens to operate in the same band as your garage opener. That gets picked up and is part of your recording.

You don't notice this when you test the recording afterwards because you are in the garage and can't hear the doorbell, but your spouse inside is getting spurious doorbell dings. It might take a while for someone to realize that your plague of spurious doorbell rings is connected to the garage door.

You'd want to at least look at the recorded signal and crop it down to just the part with the actual door signal. If the signal has both a door signal and something else like the aforementioned wireless doorbell the new remote might pick the wrong one, but in that case you'll know right away and can try again.

Better would be if the new remote could actually decode the signal to get the code, and then generated a new signal with that code every time. The record and replay method, even if cropped down to the right signal, might have recorded a weak or noisy signal which might be hard for the header for end to handle. By figuring out the code and generating a clean strong signal every time you'll get better performance.

That approach does require understanding the details of the code systems of each manufacturer whose remotes you want to be able to clone.

Nearly every home garage door opener company in the US switched to rolling codes for their new models during the '90s, and so if you are dealing with any system installed less than ~25ish years ago it almost certainly uses rolling codes.

These typically work like this. They have a pseudorandom sequence determined by a seed value. The seed is determined by the remote. I don't know if the seed is built in at manufacturing time or selected at random by the remote when it is first powered up or what. The remote generates the sequence and keeps track of where it is in the sequence. Each press of the open/close sends the next value.

The head end has a "learn" mode. You put it in learn mode and then press the open/close button a couple of times on the remote. The head sees those, recognizes they are in the right format for its system, and is able to figure out what seed would have had to been used for that system's sequence function to produce those two consecutive values. It adds that to a table of known remotes.

In operation when you press open/close, the head decodes the signal to get the sequence value, checks its table to see if that matches the value it expects from any of the known remotes. If it does the door operates and it updates its idea of where that remote is in its sequence.

There's some slack in there so that it will accept a sequence value several values ahead of what it last saw from a remote so if say your bored kid on a trip pressed the remote button a dozen times you won't get home and find your garage won't open.

You could make a learning remote that could clone an existing rolling code remote by following the same procedure the head follows for learning a remote. The learning remote would have to know the rolling code systems for all the systems it supports, but that would be doable.

The problem would be if you got the learning remote because you wanted an additional remote. To the head end the cloned remote is the original remote. Say you have one remote and two people, and you clone the remote this way. As long as neither of you goes too long without using their remote it would be fine. But if for some reason one of you doesn't use yours for a while the other might advance the sequence past the slack that I mentioned earlier and yours would stop working.

You'd have to re-pair it with the head end. How well that would work would depend on details of the system. If the pairing works by recovering the initial seed value, and that is used as the key for the head's table of remotes, it might cause problems because the two remotes have the same seed but are farther apart in their sequences than the slack.

(I suppose you could clone the old remote, then take the battery out of the old remote long enough for it to re-seed when you put the battery back in--assuming the seed isn't fixed at manufacturing time--and then pair it with the head).

The way every universal remote for rolling codes that I've seen works is that you don't use an existing remote to teach them. They have a way to tell them what your head end is. You do that, then you pair them with your head just like you would pair a new remote bought from the head's manufacturer.

These things don't usually have rich user interfaces. Telling it what system you have will probably mean looking up a number in a table in the manual, and then pressing some button on the remote (probably hidden in the battery compartment so it won't accidentally be hit), then pressing the button you want to program N times where N is the number you looked up in the manual.

You might need several tries because some manufacturers have changed code systems multiple times. If you don't know what year yours was made you might have to try them all. Even knowing the year might not help. Genie for example has one from 1995, one from 2005, and one that I think is from 2011, and they didn't stop using the old ones when the new ones came out.

It could be a nice feature if the universal remote could look at the signal from your existing remote to learn what rolling code system your head end uses. But it would also mean the universal remote would need a receiver, and that would be the only thing the receiver is used for so it is hard to justify. Pairing with the head and telling the head to open/close the door is strictly one way: remote to head.

bigiain• 2 years ago

Samy Kumar did a project about 10 years back, where he worked out how to brute force a 12 bit garage door code in under 10 seconds, using a childs toy: http://samy.pl/opensesame/

My garage door opener uses a 12 but dip switch config (and my last place used an 8 dip switch config, and I'm pretty sure still does).

Re reading that OpenSesame post was fun. It reminded me of a few names I need to go find out what they're up to these days (Travis Goodspeed and Michael Ossmann are names I remember seeing doing/writing-up some really cool stuff), and that the Mattel IM-ME toy he was using uses that same CC1110 "sub gHz"

0xfeba• 2 years ago

What a refreshing article. One I can understand for a change.

mdip2• 2 years ago

Just keep hanging around here and you'll start understanding more of them ;)

elif2• 2 years ago

Why bother intercepting, decoding, and encoding your own signal when you can just use a big antenna and MITM the fob and the vehicle and convince them they are closer than they really are?

orra• 2 years ago

I find it wild how pervasive passive keyless entry is. Completely form over (security) function.

SR2Z• 2 years ago

I wouldn't be so quick to say that - it's unquestionably more convenient than old-fashioned transponder keys in a few really important ways. You can't lock your keys in the car, you don't need more than one free hand to open a door (and sometimes not even that), and you don't need to deal with a massive bundle of keys jangling against your knees.

elif2• 2 years ago

tumber locks are built on even more hopes and dreams than security.

proper PSK cryptographic locks can (and are) implemented for cars already, just not all cars.

mmh0000• 2 years ago

Honestly... As an end user, I prefer convenience over security in my everyday life. I have insurance for the rare instance someone steals it.

The same goes for my house. I could live in a concrete bunker with no windows and steel doors, but I would much rather live in a home with large windows and a door with a crummy deadbolt.

The risk of someone stealing my car or breaking into my house is low. If that risk increases (and thus the area's overall quality decreases), I'll move to a different location.

hnick2• 2 years ago

Ultimately, if it were a severe problem, wouldn't insurance premiums reflect this?

bdavbdav• 2 years ago

Having had it on my last few cars, I wouldn’t go back.

MuffinFlavored4• 2 years ago

what kind of consumer level antenna can forward/amplify key fobs (in the gigahertz range, no?) without causing excess “signal to noise” ratio that the car can detect?

VBprogrammer• 2 years ago

It's a super common form of theft in the UK at the moment. Not sure exactly what equipment they are using but it's clearly consumer level.

ihattendorf• 2 years ago

One example: https://youtu.be/uxzm_6SYBFo?si=BSav_oEKiFBT8cf8&t=41

topspin2• 2 years ago

I think your conception of the sophistication of all this is a good deal too high. Fobs are extremely low power devices with truly terrible (undersized) antennas. Fabricating a digital repeater to produce a modest amplification is not difficult. The high frequencies involved are a benefit to the attacker because a high gain antenna remains reasonably portable. The active bits are low cost, widely available COTS digital transceivers and MMICs; the same stuff the fob and vehicle is made from.

A obvious countermeasure for such attacks would be to have the car measure the RTT between the car and fob, exchanging some cryptographic credential. If it takes too long the fob is too far away and/or an attackers repeater is adding delay.

JosephRedfern• 2 years ago

There's also a gnu-radio flow graph which serves a similar purpose: https://github.com/bastibl/gr-keyfob.

Presentation here: https://www.fleark.de/keyfob.pdf

pajko• 2 years ago

And there's a multiformat receiver block too: https://github.com/merbanan/rtl_433

zzz999• 2 years ago

Just buy a fob from eBay and program it using your car... Instructions can easily be found online

platz• 2 years ago

429 Too Many Requests = no images lololololol

lukasm• 2 years ago

Receiving/analyzing raw signals

Stock Flipper can receive raw signal.

mordae• 2 years ago

https://www.ti.com/lit/ds/symlink/cc1101.pdf

You might be able to get it to output raw demodulated FSK or OOK data without further processing, but I really doubt you are getting raw IQ samples from it.

gigel82• 2 years ago

I wish car manufacturers would start making tiny (maybe RFID) remotes I could stick in my (minimalist) wallet. Alternatively, looking forward to a tiny Flipper-like (credit-card sized) that can achieve the same result.

Seriously, the car fob is the largest thing in my pocket after the phone (thickness-wise at least).

0x4574• 2 years ago

I think what you wish is your phone to be your car key.

imp0cat• 2 years ago

And you can already have that!

Grimblewald• 2 years ago

the show it's always sunny in philadelphia has a good episode covering this idea, the episode is 'dennis takes a mental health day'

fileoffset• 2 years ago

New BYD EVs come with a credit card sized NFC key which can be used to unlock and start the car...

rainbowzootsuit• 2 years ago

Interesting related development that access to key programming is being put behind some more "security" due in part to easier access of key programming devices, but it's on the manufacturer to say what's part of the "security" system. Not just keys but can extend to tons of modules.

It's arguable if this would have any effect on criminals who are known to follow rules (/s), but will definitely have an impact on some businesses.

A criminal record can disallow participation. One way for people who have a record to enjoy success after serving their sentence is to start and run their own business, but I guess they are screwed. <shrug-emoji></shrug-emoji>

https://wp.nastf.org/?page_id=367

https://wp.nastf.org/wp-content/uploads/2023/07/ApplicationC...

swamp40• 2 years ago

He decoded everything, but he didn't actually open a car door. He still has to defeat the rolling code. It's not like you can add 1 to it and resend it. From the outside world, the next rolling code should appear random.

paledot• 2 years ago

That's right there in the lede, "not as insecure as you think".

ff7c11• 2 years ago

Is it hard to brute force? If so, it is still something that can be sniffed, though you can't unlock the car without first recording a genuine button press.

solaarphunk• 2 years ago

What’s more interesting is that if you get into a car now, there are OBD tools that just let you program a new key and drive off, which is wildly insecure.

Reverse engineering a car key fob signal

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