MCU Secure Boot Key Handling for Trusted Firmware Deployment
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Solution Overview
Problem
Existing bootloader technologies for compute devices lack secure methods for deploying firmware, particularly in surveillance cameras, which can lead to unauthorized access and inefficient firmware updates.
Innovation Solution
Implementing a secure boot process using asymmetric encryption keys to encrypt and decrypt firmware images, ensuring only authorized firmware is deployed by verifying digital signatures and using a bootloader stored in read-only memory to prevent modification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional bootloader technologies are used for firmware deployment, then the system can boot and execute firmware, but the system is vulnerable to unauthorized access and firmware tampering
Solution Approach 1:
The firmware image is digitally signed by the manufacturer before deployment, and the bootloader verifies this signature before execution. This preliminary verification ensures that only authenticated firmware can be loaded, preventing unauthorized access while maintaining a relatively simple boot process.
Solution Approach 2:
A trusted execution environment (TEE) is introduced as an intermediary between the bootloader and firmware execution. The TEE securely stores cryptographic keys and performs verification operations, isolating security-critical functions from the main bootloader and reducing the attack surface while enabling robust security verification.
2Reliability
If secure encryption methods are implemented for firmware deployment, then unauthorized access is prevented, but the firmware deployment process becomes more complex and time-consuming
Solution Approach 1:
The firmware image is pre-signed with the manufacturer's private key during the build process. This preliminary authentication allows the bootloader to verify firmware integrity quickly using the public key, avoiding time-consuming cryptographic operations during the actual firmware update and deployment phase.
Solution Approach 2:
The bootloader itself performs the firmware verification using embedded cryptographic primitives. By making the verification process self-contained within the bootloader, the system eliminates the need for external verification services, reducing deployment time while maintaining security.
3Reliability
If digital signature verification is performed on firmware images, then firmware integrity is ensured, but the bootloader complexity increases
Solution Approach 1:
The trusted execution environment (TEE) serves as an intermediary that handles complex cryptographic verification operations. The main bootloader can delegate signature verification to the TEE, which has dedicated security modules, thereby reducing the complexity of the main bootloader while ensuring firmware integrity through robust cryptographic verification.
4Reliability
If symmetric and asymmetric encryption keys are used for firmware protection, then security is enhanced, but the key management process becomes more complex
Solution Approach 1:
The trusted execution environment (TEE) acts as a secure key management intermediary. Private keys are stored securely within the TEE and never exposed to the main system. The TEE manages the symmetric and asymmetric key pairs, performing encryption and decryption operations while isolating key material from potential attacks, thereby simplifying key management for the rest of the system while maintaining enhanced security.
Data Source
AI summary
A method includes building a firmware image to execute on a bootloader of a system on chip (SoC), the firmware image including first encryption public and private keys, and digitally signing the firmware image with a second encryption private key. The signed firmware image is encrypted with a symmetric encryption key, which in turn is encrypted with a second encryption public key. The encrypted signed firmware image and the encrypted symmetric encryption key are sent to the SoC to cause the SoC to (1) decrypt the encrypted symmetric encryption key to produce the symmetric encryption key using a third encryption private key from a first asymmetric key pair, (2) decrypt the encrypted signed firmware image to produce the signed firmware image using the symmetric encryption key, and (3) verify a digital signature of the signed firmware image using a third encryption public key from a second asymmetric key pair.