Replace private.pem and privateaes.bin with your own keys - your signing key must be for the secp256k1 curve, in PEM format. You can create a .PEM file with:
openssl ecparam -name secp256k1 -genkey -out private.pemThe AES key is just be a 32 byte binary file - you can create one with
dd if=/dev/urandom of=privateaes.bin bs=1 count=32Then either drag & drop the UF2 files to the device in order (enc_bootloader first, then hello_serial_enc) waiting for a reboot in-between, or run
picotool load enc_bootloader.uf2
picotool reboot -u
picotool load -x hello_serial_enc.uf2