Boot integrity, also known as secure boot, is a security mechanism that ensures the integrity and authenticity of the boot process of a computer or device. It is designed to prevent unauthorized or malicious code from executing during the boot-up phase, thereby protecting the system from various forms of malware attacks that attempt to compromise the boot process. Here's how boot integrity works:
Boot integrity starts with establishing a root of trust, which is a secure and tamper-resistant component or process in the system. This root of trust typically involves hardware-based features like Trusted Platform Module (TPM) or a secure boot ROM on the device's motherboard.
During the boot process, the system verifies the integrity of each component involved in the boot sequence. This includes the firmware, bootloader, and operating system. The system checks for digital signatures and cryptographic hashes to ensure that these components have not been tampered with or modified by unauthorized parties.
Secure boot relies on digital signatures to verify the authenticity of the firmware and software components. Each component is signed with a cryptographic key, and the system checks the validity of these signatures against a set of trusted public keys stored in the root of trust. If a component's signature does not match or is missing, the boot process is interrupted, and the system may display an error or enter a recovery mode.
Secure boot establishes a chain of trust, where each component in the boot process verifies the next component before loading it. This chain of trust ensures that only trusted and authorized code is executed at each step of the boot process.
Platform Configuration Registers (PCR) are used to store measurements of various components during the boot process. These measurements are cryptographically bound to each other, creating a "PCR composite" that represents the integrity of the entire boot sequence. Any changes to the boot process or components will result in a different PCR composite, indicating potential tampering.
Secure boot relies on policies set by the system administrator or device manufacturer to determine what is considered a valid and trusted boot configuration. If the boot process does not adhere to the established policies, the system may prevent the OS from booting or enter a recovery mode.
Boot integrity or secure boot is a fundamental security mechanism that protects computers and devices from unauthorized modifications to the boot process. By establishing a root of trust, using digital signatures, and enforcing a chain of trust, secure boot ensures that only trusted and authenticated firmware and software components are executed during the boot-up phase. The use of Platform Configuration Registers (PCR) further enhances the verification process, making it difficult for attackers to tamper with the boot sequence without detection. By enforcing policies, secure boot helps to maintain the integrity and security of the system, providing a robust defense against various boot-time attacks and malware threats.