Prevent MITM between two microcontrollers

Hi, I recently started learning about cryptography for my small personal project and for expanding my knowledge. Basically I have several MCUs – one secure MCU (sMCU – for communication with flash memory), one less secure MCU (nMCU – for Bluetooth functionality) and one super secure MCU (xMCU) that (somehow) make sure that those two other MCUs weren’t MITM’ed. And my need is to communicate securely between sMCU and nMCU. That xMCU idea came after learning how asymmetric cryptography works and that public keys can be sniffed and communications hijacked. I also learned not to store any secrets in the MCU OTP memory as it is quite easy to read even with the correct fuse bits set. So, is there * any * way to accomplish this? I don’t have much knowledge about CA (yet), but would it be possible to somehow involve root CA stored in my xMCU? As if I pre-generate CA, generate some keys, and then store CA’s public keys in my two MCUs. Then my idea was something like: generate a new key pair on xMCU, send public key to nMCU, nMCU would verify that received public key using pre-stored CA’s public key and if it was right I would generate another key pair on nMCU, encrypt my new public key with the received public key and send it back to xMCU. I hope I explained it well, because I get a bit lost in it, there are too many keys in my idea now: D But is such a thing possible? Or am I thinking about it and is there an easier method?

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