Security Access or 0x27 Ways to Have Fun
So vehicle manufacturers don’t like you… why? Because they know you want to modify your car in ways they never intended. They know you want to break your car and make them fix it under a warrantee claim. They know you want to Hack your car and make it do fun things. So they put in place a service that will deter you from accessing privileged functions on your vehicle.
What are these functions? Things like reflashing a controller. Not just anyone should be able to do this. Resetting the Odometer. Accessing control commands that would potentially stop a vehicle from functioning properly.
How do they stop you? Security Access a.k.a. Mode 0x27. This is the service that validates an application is authorized to do one or more of the privileged functions.
So how does it work? Well it varies a bit from OEM to OEM, but they all typically work the same. First you must be in a diagnostic mode (not all OEMs require this, but many do). So you have to use Service 0x10 – Start Diagnostics.
Start Diagnostics will typically take a Subfunction. This Subfuction tells the ECU which Level of Diagnostics it needs to go into. Some OEMs have two or three levels of Diagnostics: OBDII/Standard Diagnostics, Reflashing, and Enhanced Diagnostics. These subfunctions vary widely depending on the OEM so you’ll have to poke around to find out which your vehicle supports. Some good ones to try are 0x01, 0x02, 0x03, 0x80, 0x81, 0x82, 0x90). I’ve found these to be common. So an example is this: 0x7E0 02 10 82 00 00 00 00 00. If done correctly you will get back this: 0x7E8 02 50 82 00 00 00 00 00. If not you will likely get this 0x7E8 03 7F 10 12 00 00 00 00 00 meaning that the Subfunction is not supported, so you’ll need to try another. You might just want to increment through the entire range of 256 possible subfunction levels. Once you’ve found one that works, you can move on.
So Security Access is a Seed/Key authentication method. First you request a Seed from the ECU then you calculate the appropriate Key response then send it back to the ECU. Of course you likely don’t have the appropriate algorithm to successfully calculate the Key. So you may need to brute force it. (I’ve spoke about this in an early post, but I’m going to reiterate it here with some more general examples).
Brute forcing the key will take a while. How long? Well that depends on how wide the Key is (is it 2 bytes wide, 3 bytes, etc.). How do you know how wide it is? You have to make an educated guess. You do this by seeing how wide the Seed is. You know this by requesting the Seed from the controller (I’ll go into how to do this in a bit). So if the Seed is 2 bytes, likely the Key is too. If the Seed is 4 bytes or larger, the odds that you’ll be able to brute force it in any reasonable time is unlikely as manufactures add some simple steps to slow down the brute forcing method.
So how do you request a Seed? Send this 0x7E0 02 27 01 00 00 00 00 00 (do this within 3 seconds of the Start Diagnostics command). You should get back a Seed in the response: 0x7E8 04 67 01 XX XX 00 00 00 or 0x7E8 05 67 01 XX XX XX 00 00. Remember that the 0x04 indicates that the Seed will be two bytes wide (two bytes for the control information and two bytes for the seed) and 0x05 means the Seed will be three bytes wide. Also, you don’t have to send 0x01 as the subfunciton, but all Seed request have odd numbers (except 0xFF) as the subfunction. This is how you differentiate from a Seed Request from a Key (Key requests are even numbers and must be x+1 where the Seed request subfunction is x).
Now you have a Seed, that’s nice. But you have no way of calculating the Key so why even bother asking for one? Why not just send the Key? Because you can’t, you must ask for a Seed before you can send a Key. The system requires it.
Now you need to send the Key.. but wait not yet. Because there is one thing you need to determine first. Is the Seed static or dynamic? You want to know this because this will let you know if you are going to increment your key or not. If the Seed is static, then you’ll need to change the Key when you are brute forcing the system. If the Seed is dynamic, then you’ll want to keep the Key Static. So send another Seed Request. Did the Seed change? If so it’s a dynamic Seed. If not it’s static.
Now we need to send a Key. To do this we send 0x7E0 04 27 02 XX XX 00 00 00, where XX XX is the Key (remember 0x02 subfunction is x+1 of the Seed request). Likely we will get a negative response because the odds of us guessing the exact Key for the Seed we received is 1/(2^key width). So if key width = 65536 then we have a 1/65536 chance of getting it right. Now if we increment through all possible Keys then our odds of getting the response approach 100% quickly (see the birthday problem). But if the Seed is dynamic we don’t want to increment the Key. So this is a much different problem.
However those pesky engineers at the auto manufactures thought of this brute force method and took some steps to slow us down. How? By making it so that we after 3 or 4 attempts we get locked out. How do we know that we’ve reached this condition? They let us know by sending this: 0x7E8 03 7F 27 36 00 00 00 00. Which means “Exceeded Number of Attempts.” This means we have to someone reset the controller so we can try again. This can be achieved by either cycling the power. You can do this by finding the fuse for the controller and simply pulling it. Now this could be cumbersome so you may want to automate it using your favorite open source embedded controller. Or we may be able to reset it using the ECU Reset Service a.k.a. Mode 11.
ECU Reset is the funnest service ever… because it allows you to tell a controller to cycle its power. (Don’t try it while you are actually driving bad things may happen……). The problem with ECU Reset is there are so many different permutations of it, it’s hard to describe universally. So you’ll have to do some experimenting on how your vehicle has it implemented. But here is an example of how it may work: 0x7E0 02 11 01 00 00 00 00 00. Because you did a reset, you may not get a response from the controller at all if you did it correctly. However if it didn’t like the request, it will give you a negative response 0x7E8 03 7F 11 XX 00 00 00 00, where XX is the negative response code. You’ll have to parse the code know how to handle the exception. If the NRC is 0x12 just keep trying all subfunctions until one of them works.
Once you’ve reset the controller, send another Seed request. If you still get “Exceeded Number of Attempts” as a response. Then the reset didn’t work. Keep trying more subfunction of the reset command until you’ve exhausted them all. If still no luck, you’ll have to do a hard reset of the power to the ECU (pulling the fuse). Now try to send another Seed request. Likely if you do this quickly (within 10 or so seconds of the ECU powering up), you’ll get another negative response, 0x7E8 03 7F 27 37 00 00 00 00, “Required Time Delay Not Expired.” Meaning that you have to wait a few seconds longer. Keep trying until this error goes away. This is just another way that the manufacturer has made it difficult for you to brute force the system. Because of this delay, the brute force will take much longer. But if you’re motivated you’ll eventually get there.
How will you know you’ve got it? You’ll see this: 0x7E8 02 67 02 00 00 00 00 00. Then you can be sure. And if you missed that message you can always send another Seed request. If you get Seed of Zero that means that the ECU security is bypassed.
Don’t let the window close. If you want to keep the ECU “unlocked” then you’ll need to maintain this state. To do this simply keep sending a Seed request or better, send a Tester Present message: 0x7E0 02 3E XX 00 00 00 00 00, where XX is the subfunction (you’ll have to test this first to see what works) or 0x7E0 01 3E 00 00 00 00 00 00.
Good Luck and as always feel free to contact me if you have any questions.