ASF_01_sys_sw_arch/0 system_design/features/[SEC] Security & Safety Features.md

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

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)

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)