> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zerokeyusb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Integrity and Authenticity Verification

> The application verification process using hardware CRC32 and BLAKE2s MAC, including the penalty logic.

## 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)**.

### ⚡ 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.

### 🔐 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.

### 🛡️ 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.

### 🚨 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.

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