On the Pico there's also the Crypto library that should do AES encryption - you could use that with the code that you already have for key exchange?
Crypto
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
On the Pico there's also the
Crypto
library that should do AES encryption - you could use that with the code that you already have for key exchange?