# Software Feature Specification

## SF-SEC: Security & Safety

**Software Feature ID:** SF-SEC  
**Mapped System Feature:** F-SEC (Security & Safety Features)  
**Version:** 1.0  
**Date:** 2025-02-01  

## 1. Feature Overview

The Security & Safety software feature implements comprehensive security mechanisms to protect the system against unauthorized access, malicious firmware, and data breaches. This feature provides the software implementation of secure boot, secure flash storage, encrypted communication, and security violation handling.

### 1.1 Mapped System Features

- **F-SEC-01**: Secure Boot
- **F-SEC-02**: Secure Flash Storage
- **F-SEC-03**: Encrypted Communication
- **F-SEC-04**: Security Violation Handling

## 2. Static View - Component Architecture

```mermaid
graph TB
    subgraph "Security Management Layer"
        SM[Security Manager]
        SVH[Security Violation Handler]
        KM[Key Manager]
    end
    
    subgraph "Boot Security"
        SB[Secure Boot Controller]
        BV[Boot Validator]
        ARB[Anti-Rollback Manager]
    end
    
    subgraph "Communication Security"
        TLS[TLS Manager]
        CA[Certificate Authority]
        CM[Crypto Manager]
    end
    
    subgraph "Storage Security"
        FE[Flash Encryption Manager]
        SE[Storage Encryption]
        AI[Access Inspector]
    end
    
    subgraph "Hardware Security"
        EFUSE[eFuse Manager]
        HSM[Hardware Security Module]
        RNG[Random Number Generator]
    end
    
    SM --> SB
    SM --> TLS
    SM --> FE
    SM --> SVH
    SB --> BV
    SB --> ARB
    TLS --> CA
    TLS --> CM
    FE --> SE
    FE --> AI
    KM --> EFUSE
    KM --> HSM
    CM --> RNG
```

### 2.1 Component Interfaces

#### 2.1.1 Security Manager Interfaces

**Provided Interfaces:**
- `ISecurityManager`: Main security management interface
- `ISecurityPolicy`: Security policy enforcement interface
- `ISecurityStatus`: Security status monitoring interface

**Required Interfaces:**
- `ISecureBootController`: Secure boot interface
- `ITLSManager`: TLS communication interface
- `IFlashEncryptionManager`: Flash encryption interface
- `IDiagnosticsManager`: Diagnostic reporting interface

#### 2.1.2 Key Manager Interfaces

**Provided Interfaces:**
- `IKeyManager`: Cryptographic key management interface
- `ICertificateManager`: Certificate management interface
- `IKeyStorage`: Secure key storage interface

**Required Interfaces:**
- `IeFuseManager`: eFuse hardware interface
- `IHardwareSecurityModule`: HSM interface
- `IRandomNumberGenerator`: RNG interface

## 3. Dynamic View - Security Sequences

### 3.1 Secure Boot Sequence

```mermaid
sequenceDiagram
    participant PWR as Power On
    participant ROM as ROM Bootloader
    participant SB as Secure Boot Controller
    participant BV as Boot Validator
    participant ARB as Anti-Rollback Manager
    participant APP as Application
    participant DIAG as Diagnostics
    
    PWR->>ROM: System Reset
    ROM->>SB: initializeSecureBoot()
    SB->>BV: validateFirmwareSignature()
    BV->>BV: verifyRSA3072Signature()
    BV->>ARB: checkAntiRollback()
    ARB-->>BV: version_valid
    
    alt Signature and Version Valid
        BV-->>SB: validation_success
        SB->>APP: jumpToApplication()
    else Validation Failed
        BV-->>SB: validation_failed
        SB->>DIAG: reportSecurityViolation(BOOT_FAILURE)
        SB->>SB: enterBootFailureState()
    end
```

### 3.2 TLS Communication Establishment Sequence

```mermaid
sequenceDiagram
    participant APP as Application
    participant TLS as TLS Manager
    participant CA as Certificate Authority
    participant CM as Crypto Manager
    participant MH as Main Hub
    participant KM as Key Manager
    
    APP->>TLS: establishSecureConnection(main_hub_address)
    TLS->>KM: getDeviceCertificate()
    KM-->>TLS: device_certificate
    TLS->>MH: TLS_ClientHello + Certificate
    MH->>TLS: TLS_ServerHello + Certificate
    TLS->>CA: validateServerCertificate(server_cert)
    CA-->>TLS: certificate_valid
    TLS->>CM: generateSessionKeys()
    CM-->>TLS: session_keys
    
    alt Mutual Authentication Success
        TLS-->>APP: secure_connection_established
    else Authentication Failed
        TLS->>DIAG: reportSecurityViolation(AUTH_FAILURE)
        TLS-->>APP: connection_failed
    end
```

### 3.3 Flash Encryption Access Sequence

```mermaid
sequenceDiagram
    participant APP as Application Component
    participant FE as Flash Encryption Manager
    participant AI as Access Inspector
    participant SE as Storage Encryption
    participant FLASH as Flash Hardware
    participant DIAG as Diagnostics
    
    APP->>FE: readSecureData(address, size)
    FE->>AI: validateAccess(component_id, address)
    
    alt Access Authorized
        AI-->>FE: access_granted
        FE->>SE: decryptData(address, size)
        SE->>FLASH: readEncryptedData(address, size)
        FLASH-->>SE: encrypted_data
        SE->>SE: aes256Decrypt(encrypted_data)
        SE-->>FE: decrypted_data
        FE-->>APP: secure_data
    else Access Denied
        AI-->>FE: access_denied
        FE->>DIAG: reportSecurityViolation(UNAUTHORIZED_ACCESS)
        FE-->>APP: access_error
    end
```

## 4. Software Constraints

### 4.1 Performance Constraints
- **SWC-SEC-001**: Secure boot validation must complete within 5 seconds
- **SWC-SEC-002**: TLS handshake must complete within 10 seconds
- **SWC-SEC-003**: Flash encryption/decryption must not add >10% overhead

### 4.2 Resource Constraints
- **SWC-SEC-004**: Certificate storage limited to 2KB per certificate
- **SWC-SEC-005**: Cryptographic key storage limited to eFuse capacity
- **SWC-SEC-006**: TLS session memory limited to 16KB

### 4.3 Security Constraints
- **SWC-SEC-007**: All cryptographic keys must be hardware-protected
- **SWC-SEC-008**: Private keys must never be accessible in plaintext
- **SWC-SEC-009**: Security violations must trigger immediate protective actions

### 4.4 Reliability Constraints
- **SWC-SEC-010**: Security system must remain operational during system faults
- **SWC-SEC-011**: Security violations must be logged persistently
- **SWC-SEC-012**: Boot failure must not compromise security state

## 5. Traceability Matrix - Software Requirements

| Software Requirement ID | Feature Mapping | Component | Verification Method |
|-------------------------|-----------------|-----------|-------------------|
| SWR-SEC-001 | F-SEC-01 | Secure Boot Controller | Hardware Test |
| SWR-SEC-002 | F-SEC-01 | Boot Validator | Security Test |
| SWR-SEC-003 | F-SEC-01 | Security Manager | Unit Test |
| SWR-SEC-004 | F-SEC-01 | Anti-Rollback Manager | Hardware Test |
| SWR-SEC-005 | F-SEC-02 | Flash Encryption Manager | Hardware Test |
| SWR-SEC-006 | F-SEC-02 | Storage Encryption | Unit Test |
| SWR-SEC-007 | F-SEC-02 | Key Manager | Security Test |
| SWR-SEC-008 | F-SEC-02 | Access Inspector | Unit Test |
| SWR-SEC-009 | F-SEC-03 | TLS Manager | Integration Test |
| SWR-SEC-010 | F-SEC-03 | TLS Manager | Security Test |
| SWR-SEC-011 | F-SEC-03 | TLS Manager | Integration Test |
| SWR-SEC-012 | F-SEC-03 | Security Violation Handler | Unit Test |
| SWR-SEC-013 | F-SEC-04 | Security Violation Handler | Unit Test |
| SWR-SEC-014 | F-SEC-04 | Anti-Rollback Manager | Hardware Test |
| SWR-SEC-015 | F-SEC-04 | Key Manager | Security Test |

## 6. Implementation Notes

### 6.1 Secure Boot Implementation
- **Secure Boot V2**: Hardware-enforced signature verification
- **Signature Algorithm**: RSA-3072 or ECDSA-P256
- **Root of Trust**: One-time programmable eFuse key
- **Anti-Rollback**: eFuse-based version tracking
- **Boot Failure State**: Secure state with diagnostic reporting only

### 6.2 Flash Encryption Architecture
- **Algorithm**: AES-256 hardware-accelerated encryption
- **Mode**: Release mode for production deployment
- **Key Derivation**: Hardware-based from eFuse
- **Transparency**: Automatic encryption/decryption on flash access
- **External Storage**: Optional AES-256 encryption for SD card

### 6.3 TLS Communication Security
- **Protocol**: TLS 1.2 with mutual authentication (mTLS)
- **Device Identity**: Unique X.509 certificate per device
- **Private Key Storage**: eFuse or encrypted flash
- **Certificate Size**: Optimized <2KB for ESP32-S3
- **Session Management**: Automatic reconnection and key renewal

### 6.4 Key Management System
- **Root Keys**: Stored in one-time programmable eFuse
- **Device Keys**: Derived from root keys using hardware KDF
- **Session Keys**: Generated using hardware RNG
- **Key Rotation**: Supported for communication keys
- **Key Backup**: Not supported for security reasons

### 6.5 Security Violation Handling
- **Detection Mechanisms**:
  - Secure boot signature failures
  - TLS authentication failures
  - Unauthorized flash access attempts
  - Certificate validation failures
  - Message tampering detection
- **Response Actions**:
  - Immediate diagnostic event generation
  - Connection termination for communication violations
  - System state transition to secure mode
  - Persistent logging of security events

### 6.6 Certificate Management
- **Device Certificate**: Factory-provisioned unique identity
- **CA Certificate**: Root certificate for server validation
- **Certificate Chain**: Minimal chain to conserve resources
- **Validation**: Full chain validation with CRL support
- **Provisioning**: Secure factory or field provisioning process

### 6.7 Hardware Security Features
- **eFuse**: One-time programmable secure storage
- **Hardware RNG**: True random number generation
- **Crypto Accelerators**: AES, SHA, RSA hardware acceleration
- **Secure Boot ROM**: Immutable boot validation code
- **Flash Encryption**: Hardware-transparent encryption

### 6.8 Error Handling and Recovery
- **Boot Failures**: Enter secure diagnostic mode
- **Authentication Failures**: Reject connections, log events
- **Key Corruption**: Attempt recovery, escalate if failed
- **Certificate Expiry**: Graceful degradation with alerts
- **Security Violations**: Immediate protective response

### 6.9 Compliance and Standards
- **Secure Boot**: Industry standard for embedded systems
- **AES-256**: FIPS 140-2 approved encryption
- **TLS 1.2**: Industry standard secure communication
- **X.509**: Standard certificate format (RFC 5280)
- **RSA/ECDSA**: Standard digital signature algorithms