If need be, just grind and cut till you are back to the one silicon wafer that contains crypto keys, then rebuild the whole lot from scratch, add your evil parts, then re-pot it.
The best I've done is to modify a potted chip by very careful thermal imaging to figure out where the part was that blocked the feature I wanted to unlock (market segmentation) and then drill through the case to a depth just higher than the chip. You'd have definitely been able to spot the difference. This was at the millimeter level. Recently there was a post about a group doing this at the chip level, which I thought was extremely impressive, and again, even with the recipe out there duplicating it would still be very hard.
https://www.xda-developers.com/how-drill-defeated-the-xbox-3...
Sounds like a good approach when you only have a single (or just a few) of the devices so you need to be careful.
With hardware wallets (aka substantial incentive), then the attackers could order a bunch of them for sacrificial zero-ing in the exact places to drill with a CNC/microscope/etc.
Yes, initially I had just the one. But once I had it figured out there were lots of other people that wanted their device 'improved' so that jig got used plenty of times. Even so working on other people's hardware always felt very scary, especially poking around with a drill in a sensitive piece of hardware. Even so, they all worked, it was a neat little trick.
What you’re describing is a different thing. It’s an “evil maid” attack, when you tamper with a wallet that is already set up by the user. Also, it would be ultra difficult and expensive.
What you’re saying is possible. But:
It would probably be cheaper to build a total fake Ledger Wallet, then just replace the new real wallet and deliver a fake with a weak entropy generator. Any case, that’s not what has happened this time.
(The MITM supply chain attack which has happened succeeds because it preserves the original secure hardware but intercepts the secret data at the display interface. But a low-effort mimic of the entire device like I suggested? That wouldn’t work).
And as this workaround leaves the original Secure Element intact, good luck detecting the supply chain attack.