# Security & Safety Features

**Feature Group ID:** FG-SEC  
**Version:** 2.0  
**Date:** 2025-01-19  
**Scope:** Sensor Hub (Sub-Hub only)  
**Target Platform:** ESP32-S3–based Sensor Hub, ESP-IDF v5.4

## 1 Feature Overview

The **Security & Safety Features** ensure that the Sensor Hub operates only with trusted firmware, protects sensitive data at rest, and guarantees confidentiality and integrity of all communications. These features are foundational and cross-cutting, supporting all other functional features (DAQ, DQC, COM, DIAG, DATA, OTA).

This feature set is designed to:

*   Prevent execution of unauthorized or malicious firmware
*   Protect firmware, configuration, and machine constants stored in memory
*   Secure all communications with cryptographic mechanisms
*   Provide deterministic and auditable behavior in case of security violations

**Technology Stack:**
- **Secure Boot:** Secure Boot V2 (hardware-enforced)
- **Flash Encryption:** AES-256 (hardware-accelerated)
- **Communication Security:** Mutual TLS (mTLS) with X.509 certificates
- **Anti-Rollback:** eFuse-based version protection

## 2 Scope and Assumptions

**In Scope**

*   Sensor Hub (ESP32-S3–based)
*   Boot process security
*   Flash and external storage protection
*   Communication security with Main Hub and peer Sensor Hubs
*   Device identity and authentication

**Out of Scope**

*   Cloud server security policies
*   User identity management
*   Physical tamper detection hardware (optional future feature)

## 3 Sub-Feature Breakdown

### 3.1 F-SEC-01: Secure Boot

#### Description

Secure Boot ensures that only authenticated and authorized firmware images are executed on the Sensor Hub using **Secure Boot V2**. During system startup, the bootloader verifies the cryptographic signature of the firmware image before handing over execution.

If verification fails, the system enters **BOOT_FAILURE** state and prevents normal operation.

#### Implementation

**Secure Boot V2 Configuration:**
- **Signature Algorithm:** RSA-3072 or ECDSA-P256
- **Verification:** Hardware-enforced (ROM bootloader)
- **Key Storage:** Root-of-trust key in eFuse (one-time programmable)
- **Enforcement:** Every boot (cold or warm)

#### Responsibilities

*   Firmware authenticity verification
*   Root-of-trust enforcement
*   Prevention of downgrade or rollback attacks (via eFuse anti-rollback)

#### Constraints

*   Must complete before any application code execution
*   Must be enforced on every boot (cold or warm)
*   Root-of-trust key is one-time programmable (eFuse)

### 3.2 F-SEC-02: Secure Flash Storage

#### Description

Secure Flash Storage protects sensitive data stored in internal flash and external storage (e.g., SD card) from unauthorized access or modification using **Flash Encryption**.

**Encryption Method:** AES-256 (hardware-accelerated)

Sensitive data includes:

*   Firmware images
*   Machine constants
*   Calibration data
*   Cryptographic material
*   Persistent diagnostics and logs

#### Implementation

**Flash Encryption Configuration:**
- **Algorithm:** AES-256
- **Mode:** Release mode (recommended for production)
- **Key Derivation:** Hardware-based (eFuse)
- **Transparency:** Automatic decryption on read (transparent to application)

**External Storage Encryption:**
- **SD Card:** Optional encryption for sensitive files (SWR-SEC-006)
- **Encryption Method:** AES-256 (software-based for SD card)

#### Responsibilities

*   Encrypted storage of sensitive regions
*   Access control enforcement
*   Prevention of unauthorized read/write operations

#### Constraints

*   Encryption must not compromise system boot reliability
*   Storage access must be mediated through controlled software components (e.g., DP component)

### 3.3 F-SEC-03: Encrypted Communication

#### Description

Encrypted Communication ensures that all data exchanged between the Sensor Hub and other entities (Main Hub, peer Sensor Hubs) is protected against eavesdropping, tampering, and impersonation using **Mutual TLS (mTLS)**.

This includes:

*   Sensor data transmission
*   Diagnostics reporting
*   OTA negotiation and data transfer
*   Configuration and machine constant updates

#### Implementation

**Device Identity & Authentication:**
- **Identity:** Device-unique X.509 certificate
- **Private Key:** Stored securely in eFuse or encrypted flash
- **Authentication:** Mutual TLS (mTLS) for all broker communications
- **Provisioning:** Handled via secure factory or onboarding mode

**Key Lifecycle Management:**

| Phase | Mechanism |
|-------|-----------|
| **Manufacturing** | Injection of unique device certificate and private key |
| **Operation** | Use of TLS session keys for encrypted communication |
| **Rotation** | Certificate rotation managed on broker/server side |
| **Revocation** | Certificate Revocation Lists (CRL) or broker-side denylists |

**Key Insight:**  
The ESP32-S3 is optimized to handle a single device certificate efficiently. It is recommended to avoid managing large certificate chains on the device itself to conserve resources.

**Certificate Constraints:**
- **Maximum Certificate Size:** <2KB
- **Certificate Format:** X.509 v3
- **Key Algorithm:** RSA-2048 or ECDSA-P256

#### Responsibilities

*   Confidentiality of transmitted data
*   Integrity and authenticity verification
*   Secure session establishment

#### Constraints

*   Must be compatible with ESP32-S3 cryptographic capabilities
*   Must support reconnection and key renewal mechanisms
*   Certificate size must be optimized for ESP32-S3

### 3.4 F-SEC-04: Security Violation Handling [NEW]

#### Description

The system detects and reports security violations with appropriate responses.

**Security Violation Types:**
- Secure boot failures
- Authentication failures
- Message tampering
- Unauthorized access attempts
- Certificate validation failures

**Response Actions:**
- Secure boot failure → BOOT_FAILURE state
- Authentication failure → FATAL diagnostic event, connection rejection
- Message tampering → ERROR diagnostic event (escalates to FATAL if persistent)
- Unauthorized access → FATAL diagnostic event, access denial

## 4 Functional Flow Overview

### Secure Boot Flow

```mermaid
sequenceDiagram
    participant Power as Power On
    participant ROM as ROM Bootloader
    participant SecureBoot as Secure Boot V2
    participant App as Application
    
    Power->>ROM: System Reset
    ROM->>SecureBoot: Load Firmware
    SecureBoot->>SecureBoot: Verify Signature
    alt Signature Valid
        SecureBoot->>App: Jump to Application
    else Signature Invalid
        SecureBoot->>SecureBoot: Enter BOOT_FAILURE State
    end
```

### Secure Communication Flow (mTLS)

```mermaid
sequenceDiagram
    participant SH as Sensor Hub
    participant Broker as Main Hub Broker
    
    SH->>Broker: TLS Client Hello + Certificate
    Broker->>SH: TLS Server Hello + Certificate
    SH->>SH: Verify Server Certificate
    Broker->>Broker: Verify Client Certificate
    alt Both Valid
        SH->>Broker: TLS Session Established
        SH->>Broker: Encrypted Data Exchange
    else Invalid Certificate
        SH->>SH: Reject Connection, Log FATAL
    end
```

## 5 System SHALL Requirements (Formal)

### Secure Boot Requirements

*   **SR-SEC-001**: The system shall verify the authenticity of the firmware image before execution during every boot cycle using Secure Boot V2.
*   **SR-SEC-002**: The system shall prevent execution of firmware images that fail cryptographic verification.
*   **SR-SEC-003**: The system shall enter BOOT_FAILURE state upon secure boot verification failure.
*   **SR-SEC-004**: The system shall protect the root-of-trust against modification (eFuse protection).

### Secure Flash Storage Requirements

*   **SR-SEC-005**: The system shall protect sensitive data stored in internal flash memory from unauthorized access using Flash Encryption (AES-256).
*   **SR-SEC-006**: The system shall support encryption of sensitive data stored in external storage devices (SD card).
*   **SR-SEC-007**: The system shall restrict access to cryptographic keys to authorized system components only.
*   **SR-SEC-008**: The system shall ensure data integrity for stored configuration and machine constant files.

### Encrypted Communication Requirements

*   **SR-SEC-009**: The system shall encrypt all communication with the Main Hub using TLS 1.2 with mutual authentication (mTLS).
*   **SR-SEC-010**: The system shall ensure integrity and authenticity of all received and transmitted messages.
*   **SR-SEC-011**: The system shall use secure communication channels for OTA firmware updates.
*   **SR-SEC-012**: The system shall detect and report communication security violations.

### Security Violation Handling Requirements [NEW]

*   **SR-SEC-013**: The system shall report security violations as FATAL diagnostic events.
*   **SR-SEC-014**: The system shall implement eFuse-based anti-rollback to prevent downgrade attacks.
*   **SR-SEC-015**: The system shall protect cryptographic keys during power loss and system resets.

## 6 Traceability Matrix (Feature → System Requirements)

| Feature ID | Related System Requirements |
|------------|----------------------------|
| F-SEC-01 | SR-SEC-001, SR-SEC-002, SR-SEC-003, SR-SEC-004 |
| F-SEC-02 | SR-SEC-005, SR-SEC-006, SR-SEC-007, SR-SEC-008 |
| F-SEC-03 | SR-SEC-009, SR-SEC-010, SR-SEC-011, SR-SEC-012 |
| F-SEC-04 | SR-SEC-013, SR-SEC-014, SR-SEC-015 |

## 7 Design & Implementation Notes (Non-Normative)

*   **Secure Boot V2:** Mandatory for production deployment. Key management and signing infrastructure must be documented.
*   **Flash Encryption:** Release mode recommended for production. Development mode available for debugging.
*   **Key Provisioning:** Should occur during manufacturing or secure onboarding. HSM or secure signing server required.
*   **Certificate Lifecycle:** Certificate rotation and revocation must be managed on broker/server side.
*   **eFuse Anti-Rollback:** Version numbering strategy must be carefully defined (eFuse is one-time programmable).
*   **Security Failures:** Should integrate with the Diagnostics & Health Monitoring feature set.
*   **Security Features:** Must be active before any sensor data acquisition or communication begins (CFC-SEC-01).

## 8 Dependencies

*   **OTA Features** (for secure firmware updates)
*   **Communication Features** (transport layer for mTLS)
*   **Diagnostics Features** (security fault reporting)
*   **Persistence & DP Component** (secure storage abstraction)
*   **System Management** (BOOT_FAILURE state handling)

## 9 Industrial Standards Compliance

*   **Secure Boot V2:** Industry standard for production-ready industrial embedded systems
*   **Flash Encryption:** Hardware-accelerated AES-256 (industry standard)
*   **mTLS:** Industry standard for device authentication
*   **X.509 Certificates:** Standard certificate format (RFC 5280)