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Boot-Up Trust Chain

The ZeroKeyUSB Bootloader executes a fast, cryptographic verification process before yielding control to the application firmware. This process ensures that the firmware has not been altered (Integrity) and originates from an official source (Authenticity).

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⚡ Fast Integrity Check (Hardware CRC32)

Verification is performed using the DSU (Data Scrambling Unit) hardware of the SAMD21 microcontroller for CRC32 calculation. This allows scanning the entire Flash memory at maximum bus speed:
  • Cumulative CRC32: The CRC32 is calculated efficiently chunk-by-chunk.
  • Speed: Minimizes boot time, ensuring the full verification takes only a few milliseconds.

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🔐 Cryptographic Authentication (BLAKE2s MAC)

To ensure the firmware was signed by the secret key, the BLAKE2s-128 MAC (Message Authentication Code) algorithm is used.
  1. MAC in the Footer: The final application firmware ends with a 28-byte Security Footer, which contains the final CRC32 and the pre-calculated BLAKE2s MAC.
  2. Recalculation: The Bootloader recalculates the MAC over the entire application code using the embedded secret key (ZK_SECRET_KEY).
  3. Approval: If the calculated MAC matches the MAC in the footer, authentication is successful.

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🛡️ Sanity and Range Checks

Before cryptographic verification, pointer checks are executed to prevent redirection attacks:
  • Stack Pointer (SP): The initial address of the Stack Pointer is verified to be within the valid SRAM range.
  • Reset Handler: The application’s start function address is checked to be within the Flash region reserved for firmware.

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🚨 Penalty for Unofficial Software (15 Seconds)

For cases where the firmware has been altered or comes from an unsigned source, the Bootloader enforces a strict penalty policy:
  • Verification Failure: If the CRC32 or BLAKE2s MAC does not match, a 15,000 millisecond delay (PENALTY_DELAY_MS) is applied using the SysTick Timer.
  • Effect: This delay discourages the use of unauthorized firmware and prevents fast reboot loops, offering a time window for the user to enter the flashing Bootloader mode.