Trusted Boot without TPM: Is it Possible?

TL;DR

Yes, an OS can implement Trusted Boot without a TPM if Secure Boot is already enabled. However, it’s less secure and relies heavily on the UEFI firmware for integrity checks. A TPM provides a hardware root of trust which is much stronger than relying solely on the UEFI.

Understanding the Concepts

Before we dive in, let’s quickly define these terms:

• Secure Boot: Ensures that only digitally signed bootloaders and OS kernels are allowed to run. It verifies code signatures during startup.
• Trusted Boot: A more comprehensive process than Secure Boot. It aims to verify the entire boot chain – from firmware through the OS kernel and initial drivers – ensuring no malicious software is loaded before control is handed over to the operating system.
• TPM (Trusted Platform Module): A hardware security module that provides a secure root of trust for cryptographic operations, including storing keys and verifying platform integrity.

Implementing Trusted Boot without a TPM

If you don’t have a TPM, achieving Trusted Boot relies on the UEFI firmware being trustworthy and correctly configured.

1. Verify UEFI Firmware Integrity: This is the most crucial step. You need to ensure your UEFI firmware hasn’t been tampered with.
   • Check for Updates: Regularly update your UEFI firmware from the manufacturer’s official website.
   • Secure Boot Configuration: Confirm Secure Boot is enabled in your UEFI settings. Look for options like “Key Management” or similar to see which keys are trusted.
   • Hash Verification (if available): Some manufacturers provide a way to verify the hash of the current firmware against a known good value. This is ideal but not always possible.
2. Bootloader Integrity Checks: Secure Boot already handles this, but it’s important to understand how.
   • The UEFI firmware will only load bootloaders signed with a trusted key.
   • Ensure your OS bootloader (e.g., GRUB for Linux) is also digitally signed.
3. Kernel Integrity Checks: Again, Secure Boot plays a role here.
   • The UEFI firmware verifies the signature of the OS kernel before loading it.
   • Ensure your kernel is properly signed with a trusted key.
4. Early Launch Anti-Malware (ELAM): This feature, available in Windows, helps protect against bootkits.
   • ELAM loads early-boot drivers into a secure environment before they are fully initialized, allowing them to scan for malware.
   • Configure ELAM with trusted anti-malware solutions. You can use PowerShell to manage ELAM:

     Enable-AntimalwareDriverInterface

5. Measured Boot (if supported): Some UEFI implementations offer Measured Boot, which logs the boot process.
   • This log can be used to detect changes in the boot chain. However, without a TPM, this log is stored on the hard drive and is vulnerable to tampering.

Limitations of Trusted Boot without a TPM

• Lack of Hardware Root of Trust: The biggest drawback. UEFI firmware can be compromised, making it difficult to detect malicious changes. A TPM provides a much stronger foundation.
• Vulnerability to Firmware Attacks: If an attacker gains control of the UEFI firmware, they can bypass all security measures.
• Log Tampering: Measured Boot logs stored on the hard drive are susceptible to modification by malware.

Conclusion

While it’s possible to implement Trusted Boot without a TPM using Secure Boot and UEFI firmware integrity checks, it’s significantly less secure than having a TPM. A TPM provides a hardware-based root of trust that is much more resistant to attacks. If security is critical, investing in a system with a TPM is highly recommended.