Platform Security Architecture (PSA)

The PSA Threat Models and Security Analysis (TMSA) methodology provides a structured, asset-centric approach to security design. It guides developers through a systematic process to:

• Identify critical assets (e.g., cryptographic keys, sensor data).
• Enumerate potential threats and attack vectors (software, network, physical).
• Perform a risk assessment to prioritize security requirements.
• Define security objectives and functional requirements. This analysis forms the essential blueprint, ensuring security controls are implemented where they are most needed and cost-effective.

PSA Certified provides a multi-level certification scheme and a suite of hardware and firmware specifications that translate security analysis into concrete, testable requirements. Key specifications include:

• PSA-RoT: Defines the requirements for a Hardware Root of Trust, the immutable foundation for secure boot, attestation, and cryptographic services.
• PSA Functional APIs (PSA FF-M): A standardized set of cryptographic and storage APIs for secure services, enabling portable security code across different silicon vendors.
• PSA Firmware Framework (PSA FF-M): The specification for the Trusted Firmware-M (TF-M) secure runtime environment, defining secure partition management and inter-process communication. These specs ensure interoperability and a consistent security baseline.

Trusted Firmware-M (TF-M) is the Arm-led, open-source reference implementation of the PSA Firmware Framework for Armv8-M and Armv7-M architectures. It provides the critical secure runtime environment, including:

• A Secure Partition Manager (SPM) that isolates trusted applications from the non-secure processing environment.
• Pre-integrated, PSA-compliant cryptographic, attestation, and secure storage services.
• Support for ARM TrustZone-M hardware isolation.
• A Secure Bootloader for verifying and loading firmware images. TF-M accelerates development by providing a production-ready, PSA-aligned secure codebase that device makers can integrate directly or use as a reference.

The PSA Certified program offers independent, laboratory-based evaluation to verify that a device's hardware, firmware, and associated development process meet the PSA specifications. The multi-level scheme includes:

• Level 1: Automated lab testing of the PSA-RoT APIs, providing basic assurance.
• Level 2: In-depth lab evaluation of the PSA-RoT implementation against 25+ security requirements.
• Level 3: Evaluation of the entire device against a PSA-defined Protection Profile, requiring resistance to sophisticated software and physical attacks. This independent validation provides a trusted, industry-recognized security credential for products.

The PSA Functional APIs are a standardized set of C programming interfaces for accessing security services, abstracting the underlying hardware. This enables portable security code across different microcontroller vendors (e.g., STMicroelectronics, NXP, Microchip). Core API groups include:

• Cryptography: Unified interfaces for symmetric/asymmetric crypto, hashing, MAC, and key management.
• Secure Storage: APIs for creating and accessing encrypted, integrity-protected data stores.
• Attestation: Services for generating a cryptographically signed report of the device's software state. By using these APIs, developers can write security-critical software once and deploy it across a heterogeneous device fleet.

To accelerate adoption, PSA provides comprehensive resources and integrations with major microcontroller vendor SDKs and development tools. This ecosystem includes:

• PSA Developer Tools: Such as the Trusted Firmware-M integration into Arm Mbed OS and Zephyr RTOS.
• Vendor SDK Support: Pre-integrated PSA services in SDKs from silicon partners.
• Open-Source Reference Implementations: For attestation tokens, secure storage, and more.
• Documentation & Training: Detailed API guides, porting guides, and threat model examples. These resources lower the barrier to entry, allowing developers to focus on application logic rather than low-level security plumbing.

PSA provides mechanisms to protect proprietary machine learning models deployed on edge devices from theft, reverse engineering, or tampering. This is critical for safeguarding intellectual property in commercial products.

• Secure Storage APIs allow encrypted storage of model weights and parameters within a Trusted Execution Environment (TEE) or Secure Element.
• Hardware Root of Trust ensures model decryption keys are never exposed to the main application core.
• Prevents attackers from extracting a trained model for offline analysis or cloning onto unauthorized devices.

The open-source, PSA-aligned reference secure firmware implementation for Arm Cortex-M microcontrollers. It provides the foundational Secure World software, including:

• Secure Boot second stage
• A Trusted Execution Environment kernel
• PSA Cryptography, Storage, and Attestation services
• Isolation between secure and non-secure applications

The hardware security extension for Arm Cortex-M processors that creates the two isolated worlds (Secure and Normal) required for a Trusted Execution Environment. It is the primary hardware enabler for PSA on microcontrollers, providing:

• Hardware-enforced memory and peripheral isolation
• Secure entry points (gates) for service calls
• A secure debug authentication interface PSA's threat models and TF-M are designed to leverage TrustZone-M capabilities.