0x0026a Custom AES-128-CMAC Secure Bootloader

This project builds a custom secure-boot chain for the Pico 2 (RP2350) that sits entirely above the ROM bootloader. A first-stage bootloader in XIP flash authenticates a signed application with AES-128-CMAC (RFC 4493) before jumping to it, and the signed application decrypts and prints a secret banner using the SDK Mbed TLS AES API.

What You'll Learn

Why a bootloader-only CRC/manifest is insufficient and what a MAC adds
How AES-128-CMAC works (K1/K2 subkeys) and the RFC 4493 test vectors
Custom linker-script editing (pico_flash_region.ld, memmap_app.ld) to relocate the app
Flashing three images (bootloader, manifest, app) with OpenOCD
Reverse engineering both images with GDB and Ghidra (find the key, IV, ciphertext, banner)

See the full tutorial: WEEK11a.md / WEEK11a.pdf

Flash Map

Region	Address	Content
Bootloader	`0x10000000`	0x0026a bootloader (27772 B)
Manifest	`0x1001F000`	header + AES-128-CMAC tag
App	`0x10020000`	0x0026a signed app (21656 B)

The CMSIS-RM9 flash-safe range starts at 0x10000000 on the RP2350, so adding 2 MB relocates the app to 0x10020000.

Artifacts

File	Description
`0x0026a_bootloader.bin`	Bootloader image (flash at `0x10000000`)
`0x0026a_bootloader.uf2`	Bootloader-only UF2 for the BOOTSEL drag-and-drop
`0x0026a_app.bin`	Signed app image (flash at `0x10020000`)
`manifest.bin`	Manifest (flash at `0x1001F000`)
`0x0026a_full.uf2`	All three images in one BOOTSEL drag
`WEEK11a.md` / `WEEK11a.pdf`	Full lab notebook

Build

cmake -B build -DPICO_SDK_PATH=~/.pico-sdk/sdk/2.2.0
cmake --build build
python3 tools/sign_firmware.py build/0x0026a_app.bin --out manifest.bin --key "IronNetBootKey!!" --app-base 0x10020000
PICOTOOL=~/.pico-sdk/picotool/2.3.1/picotool/picotool python3 tools/make_full_uf2.py

This produces 0x0026a_bootloader.bin/.uf2 and 0x0026a_app.bin; the signer writes manifest.bin; make_full_uf2.py packs all three into 0x0026a_full.uf2.

Flash

Easiest - copy 0x0026a_full.uf2 to the RP2350 BOOTSEL drive; one drag writes the whole chain (bootloader + manifest + app).

Or with OpenOCD:

openocd -f interface/picoprobe.cfg -f target/rp2350.cfg -c \
  "program 0x0026a_bootloader.bin 0x10000000 verify; \
   program manifest.bin 0x1001F000 verify; \
   program 0x0026a_app.bin 0x10020000 verify; reset run; shutdown"

Verify at Runtime

Halt the core after boot and read the program counter. A valid signed image leaves PC inside the app (0x100201xx); a tampered or unsigned image leaves PC in the bootloader boot_halt idle loop near 0x10000eb2.

Disclaimers

This is an educational project for the Embedded-Hacking course. The key IronNetBootKey!! ships in the repo and is intentionally embedded in both images so it can be recovered with GDB/Ghidra. Do not use this scheme to protect real products.

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