Embedded

I keep seeing arguments about AI coding assistants conducted entirely in the abstract, so here is a concrete one: a real change I made recently, start to finish, including the parts that went wrong. Everything below is a real MCUboot change; the PR numbers are live and you can check my work.

Summary

An automated review of an external contributor’s pull request flagged that the author had quietly disabled a test on one simulated device. Rather than stopping at “you disabled a test”, the model worked out why the test failed — a long-standing bug in MCUboot’s own test simulator that had been invisible for as long as it had existed, and only became observable because this PR raised a configuration limit. The fix was implemented, reviewed, pushed as PR #2808, and then empirically demonstrated to resolve the original author’s problem.

Cost: roughly 30 minutes of my attention, some of it spent doing other things while test suites ran. My honest estimate for doing this by hand is one to two days — most of it spent staring at trailer arithmetic trying to work out why an “oversized” image wasn’t oversized.

The tool was Claude Code running Opus 5.

This post summarizes the relevant RFCS (and other standards) related to cryptography and, specifically, relevant to MCUboot.

I intend to update this post with more RFCs as I refer to them in my work.

Last update: 2018-05-31

Cryptography

The following documents describe protocols and encodings relevant to digital signatures.

• RFC3447: Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1

• RFC4279: Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)

• RFC5208: Public-Key Cryptography Standards (PKCS) #8: Private-Key Information Syntax Specification Version 1.2 (obsolete)

Update: After a little digging, I understand where the leading 0x00 comes from on the EC public key.

I’ve recently been working on the MCUboot project. A key feature of this bootloader is its use of digital signatures to verify images both before performing upgrades, and optionally, also before running them. The code currently supports RSA and ECDSA signatures, and we are working on adding support for EdDSA signatures (specifically Ed25519).

This post is going to focus on some particular security issues in the embedded space (the IoT space, if you will). I regularly work with microcontrollers (MCUs) based on variants of the Cortex-M architecture. These are ARM processors that typically have a small amount of ROM (½–1 MiB) and an even smaller amount of RAM (16–128 KiB).

Traditionally, security has not been much of a focus on these types of devices (think toasters, washing machines, and the likes). However, with increased connectivity, we are starting to see more and more security problems with the code in these devices.