Tinycrypt key

tinycrypt — a DIY FIDO2 security key you approve from your Apple Watch

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Table of Contents

Overview

A YubiKey-style FIDO2/WebAuthn USB security key built on an ESP32-S3, where the "touch the key" step happens on your wrist instead: the key sends a push notification to an Apple Watch, you tap Approve, and the watch proves it over Bluetooth before the key signs you in.

The first target is signing into AWS IAM Identity Center with the "Security key" MFA option.

Hardware:

• ESP32-S3 (target board: Waveshare ESP32-S3-Zero) — native USB, Wi-Fi, BLE
• Microchip ATECC608 — ECC P-256 secure element (I²C) for key storage, added after the software prototype
• Apple Watch — user-presence approval

Status: feasibility spikes. Spike A passed: an ESP32-S3 running the SoloKeys solo1 CTAP2 core, with an insecure software key and a BOOT-button presence check, registered and signed into IAM Identity Center from Chrome. The earlier ATtiny85 / U2F experiments live on the spike branch.

How it fits together

1. The browser asks the key (USB HID, CTAP2) for a WebAuthn assertion.
2. The key sends an approval request: AWS IoT Core → SNS → APNs → actionable watch notification (about 2 s).
3. You tap Approve; the watch connects to the key over BLE and signs a fresh nonce with its Secure Enclave key. The watch must be in Bluetooth range: a remote approval is impossible. On a real watch the Bluetooth exchange takes about 1.4 s after you tap.
4. The key verifies the watch's signature, then signs the WebAuthn assertion and returns it to the browser.

The firmware is split into a portable CTAP2 core with swappable transport, crypto, presence and notifier back ends, so most of it also runs as a simulator on macOS.

Testing builds can use a software key instead of the ATECC608. That build is deliberately insecure, is for testing only, and must only ever be registered against a read-only test user.

Building and testing

All the firmware lives in embedded/, and the commands below run from there. The CTAP2 core is SoloKeys solo1 (Apache-2.0 OR MIT), pinned as a submodule in embedded/third_party/solo1 under its own licence. Fetch it and the libraries it builds:

cd embedded
git submodule update --init third_party/solo1
git -C third_party/solo1 submodule update --init crypto/cifra crypto/micro-ecc crypto/tiny-AES-c tinycbor

Mac sim (needs brew install libsodium):

cmake -S . -B build -DTINYCRYPT_INSECURE_SOFT_KEY=ON && cmake --build build
python3 -m venv tests/.venv && tests/.venv/bin/pip install -r tests/requirements.txt
tests/.venv/bin/python -m pytest tests
build/tinycrypt-sim --presence prompt     # run it by hand; press Enter to approve

ESP32-S3 (ESP-IDF 5.5 in ~/esp/v5.5/esp-idf):

esp32s3/build.sh                                    # build (KEYSTORE=atecc for the ATECC608B; soft by default)
esp32s3/build.sh -p /dev/cu.usbmodemXXXX flash      # flash over the UART port
esp32s3/flash-manual.sh /dev/cu.usbmodemXXXX        # if auto-reset fails: hold BOOT, tap RST, release BOOT first
tests/.venv/bin/python tests/hw_smoke.py            # register + authenticate; press BOOT when asked

The key enumerates on the S3's native USB port (not the UART one) as "tinycrypt INSECURE TEST KEY", USB ID 303a:4004.

ATECC608B (CRYPT-22). Wire a breakout to the dev board: VCC to 3V3, GND, SDA to GPIO38, SCL to GPIO21 (the PCB's pins; change them under esp-cryptoauthlib in esp32s3/build.sh menuconfig). The breakouts have their own pull-ups. A blank chip answers at 0xC0 (8-bit); the M5Stack Trust&GO unit (0x6A) is factory-locked and can't take tinycrypt's configuration. Locking is permanent: do a sacrificial chip first. The provisioner's console is the native USB port (USB-Serial-JTAG), so connect that one; flashing it and the key image straight after needs no BOOT+RST.

KEYSTORE=atecc esp32s3/build.sh provisioner                 # the provisioner image
ESPTOOL_BEFORE=default_reset esp32s3/flash-manual.sh --provisioner /dev/cu.usbmodemXXXX  # else BOOT+RST first
esp32s3/tools/atecc_provision.py /dev/cu.usbmodemXXXX info
esp32s3/tools/atecc_provision.py /dev/cu.usbmodemXXXX lock  # PERMANENT; asks for the serial
esp32s3/tools/atecc_provision.py /dev/cu.usbmodemXXXX csr tinycrypt-sandbox-key-1 /tmp/key-1.csr
infra/scripts/provision-key.sh key-1 --csr /tmp/key-1.csr   # certificate from the chip's own key
KEYSTORE=atecc esp32s3/build.sh && ESPTOOL_BEFORE=default_reset esp32s3/flash-manual.sh /dev/cu.usbmodemXXXX   # the key again

The same steps with the default soft keystore move a key's IoT private key out of prov into its (INSECURE) soft element, with no chip.

Architecture Diagram

docs/architecture/tinycrypt.drawio will be the source for the architecture diagram; the SVG is auto-regenerated on commit by the pre-commit hook in .githooks/pre-commit.

To activate the hook after cloning:

git config core.hooksPath .githooks

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If you find this useful, please consider buying me a coffee: