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BOOTING.md

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Booting

NOTE: This is a planning document on ideas for the secure bootloader. It is not currently implemented.

The first-stage bootloader is located in the ROM and boots the second-stage bootloader.

The second-stage bootloader:

  1. Initializes: @TODO list things
  2. Initialize flash encryption and secure booting
  3. If the DFU key sequence is held down for 5s; boot into factory
  4. If no OTA apps installed; boot into factory
  5. Select the boot partition based on ota_data
  6. Load the image into RAM (IRAM + DRAM) and transfer managment to that image

Factory App

  1. If no existing configuration, perform provisioning over BLE
  2. Enter DFU mode; searching for a BLE source for Firmware

Provisioning Manager

Each provisioning pod has a private key, whose public key is embedded in the DFU firmware. The provisioning manager communicates over BLE.

  • queryEntropy(sessionPubkey)

    • generate ephemeral key, compute shared secret K
    • return encrpyt(randomBytes(16), K)

  • register(clientMac, clientPubkey)

    • assign a serial number S
    • sign(config,

  • device:

    • creates random attestation private key:
      • using local and manager entropy
      • discard if compressed key would not begin with a 0x02
      • write it to flash (encrypted)
      • flush and verify
    • sends:
      • MAC address
      • attestion public key

  • provisioning manager:

    • replies:
    • device config (serial number, model number, zeros)
    • sign(v=1&config=${ hex(config, 8) }&pubkey=${ hex(pubKey, 32) })

  • device:

    • writes attestation to flash
    • flush and verify
    • write config to block 3 of eFuses

Notes

  • This process (from the devices point of view) is idempotent and can be restarted at any time and continued from any step
  • The Provisioner must store the serial number and MAC to ensire the same device always receives the same serial number