Trying to figure out how at the software and hardware level eSIMs work in a cellphone.
In: Engineering
Let’s start from the beginning.
A SIM card is a “subscriber identity module.” Required in all GSM, LTE, and 5G phones, it’s a chip that holds your customer ID and details of how your phone can connect to its mobile network, what radio frequencies to use, etc.
There’s really no reason to put this information in a circuit, though it does provide some protection from tampering. A more complex circuit could be built into the phone, what’s called an “eSIM”. The advantage to the phone company is that they can exchange encrypted messages with the phone over wifi to set it up. They don’t pay for a piece of plastic, or have to send it out, …. The advantage to the user is that a phone can hold lots of eSIMs, I think an iPhone can hold 8, and two can be active at a time – so two different numbers cause your phone to ring to get a SMS message.
A SIM card just authenticates you on the network, kind of like a password, except it’s stored on a little chip. eSIM just stores this password (chip) on your phone directly.
When you go to a carrier, they give you a SIM card registered to their network, but if you have a eSIM, you can transfer it between carriers so you tell them “this is my eSIM please use this to authenticate me” and they register your eSIM instead of giving you a physical SIM.
Nowadays you don’t even need to go to a carrier to activate an eSIM, you can just download an app which registers your eSIM with their mobile plan.
The technology is the same otherwise, it’s like a little SIM chip but built inside your phone. The main difference is that eSIM isn’t removable, so it’s designed to be transferable between carriers, while SIM is removable, but if you switch carriers you need a new one.
Basically, if your phone supports eSIM – it already has a built-in (hardware equivalent to) SIM-card inside. So when you buy a eSIM, you write some identification data into this built-in SIM-card and it’s done. When you buy a classic SIM-card, you actually buy a hardware with pre-entered data.
eSIM uses trusted computing feature of modern CPUs. Trusted computing allows to run small applications heavily protected from the user. Other major trusted computing applications before eSIMs have been DRM (copy protection; the owner of content does not want the user to be able to copy decrypted content) and corporate security and remote computer management (in this case corporate IT wants to be sure it can run an application even if the main OS has crashed or have been infected by a sophisticated virus that could have removed a regular security application).
Trusted computing applications run outside of the main OS and outside of the main RAM (random access memory). The CPU has a small embedded RAM inside the CPU die. Communications with the regular RAM can be easily listened to with inexpensive tools. Communications with the embedded RAM are extremely difficult to snoop. Since the CPU designers know the RAM is for trusted computing they mix RAM elements with other CPU transistors. Even if you could attach a tiny probe to the CPU die you wouldn’t know which line to listen to. They can also seal the die with an outer shell that makes it very difficult to attach a probe. Similarly the CPU has a small embedded flash memory for long term storage.
eSIM application uses the embedded flash to store cryptographic keys and subscriber id. When the phone connects to a mobile network it does what a regular SIM card does. It uses public/private key cryptography to confirm your subscriber id. Unlike a regular SIM card which has a subscriber id and keys stored at the factory, eSIM application can accept new keys and new subscriber ids. It can also store many key and id sets for multiple carriers.
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