How are devices like smattphones given unique IDs (IMEI numbers, MAC addresses) during the manufacturing process?
In: 55
You’re familiar with read-only chips and probably familiar with read-write chips (like those used for flash memory).
What they don’t talk about is write-once memory. You can write data to it a single time and that’s it.
How (in theory):
Imagine you have 8 fuses. We’ll label them 0 through 7. You want to store a binary representation of the letter A, which is 01000001. So you blow fuses 0, 2, 3, 4, 5, and 6, leaving only fuses 1 and 7 intact.
The thing is, there are two lines for each fuse: A read line and a write line. To read a fuse, you use the read line. To write to a fuse, you have to use both the read and the write line to burn the fuse.
On the write line, there another, higher amp fuse. After you’re done burning your fuses, you overload the write line without the read line and blow the fuse on the write line. No more writing. Only reading. (Also, you’d put a fuse on the read line to prevent someone from trying to dump a bunch of current there to change one of the bits.)
How (in practice):
When Foxconn or other manufacturers build devices, one of the steps with the partially assembled device connects a computer to specific connection points on the PCB which were placed there for this purpose specifically. These connection points are connected to the chip where the IMEI, Serial, MAC number, or other hardware ID is stored.
In older designs, you could later change the write-once content so long as you changed a 0 to a 1 only. While you couldn’t set it to any value you wanted, you could change the value to something other than what it was previously.
In more modern designs, the data is written and then current is passed through another connection point which disconnects the data lines for writing internal to the chip.
How (the bad way):
Write-once chips are pretty specialized which translates as “expensive in small quantities”. So, on cheaper devices or low-margin devices, the OEM just uses flash memory to store whatever and the software on the controller on the device simply won’t provide a way for the value to be changed. This doesn’t prevent someone with proper knowledge and equipment to change the content of flash chips.
This is how counterfeit SD flash cards and counterfeit usb flash drives are made. Someone buys a bunch of cheap 512mb cards and changes the content of the flash memory used by the controller so that the chip intentionally misidentifies itself to the computer as being 2tb. And so long as you don’t try to use more than 512mb of space and don’t format the card without using the Quick Format feature, you won’t notice. (Doing a ‘long format’ should catch this type of counterfeiting.)
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