TL;DR
While theoretically possible with enough resources and expertise to design a custom CPU specifically targeting encryption algorithms, it’s extremely difficult and expensive. Existing CPUs are already designed to be secure against most attacks. A custom build wouldn’t necessarily guarantee success and would likely require significant vulnerabilities in the existing encryption implementation.
Understanding Hard Drive Encryption
Most modern hard drives use full-disk encryption (FDE) like BitLocker (Windows), FileVault (macOS), or LUKS (Linux). These systems rely on strong cryptographic algorithms, typically AES with a 128-, 256-bit key. The key itself is protected using various methods, including:
- Trusted Platform Module (TPM): A hardware chip storing the encryption key securely.
- User Password/PIN: Used to unlock the key stored in the TPM or other secure storage.
- Key Derivation Functions (KDFs): Stretching the user password into a strong encryption key.
Why a Custom CPU?
A standard CPU isn’t designed to break encryption; it’s designed to perform calculations, including those used in encryption and decryption. However, a custom CPU could be built with the following goals:
- Side-Channel Attacks: Analysing power consumption, electromagnetic radiation, or timing variations during encryption/decryption processes to infer the key.
- Algorithm Weaknesses: Exploiting potential flaws in the encryption algorithm itself (though AES is very well-studied).
- Direct Memory Access (DMA) Attacks: Bypassing normal CPU security features to access encrypted data directly.
Steps to Attempt Breaking Encryption with a Custom CPU
- Reverse Engineer the Target System: Identify the specific encryption algorithm, key length, and key storage mechanism used by the target hard drive (e.g., BitLocker version, TPM model).
- Hardware Design & Fabrication: This is the most challenging part.
- Instruction Set Architecture (ISA): Design a custom ISA optimised for side-channel attacks or specific algorithm weaknesses.
- Physical Implementation: Fabricate the CPU using specialised foundries. This requires significant expertise in VLSI design and manufacturing.
- Debugging Tools: Develop tools to monitor and analyse the CPU’s behaviour during encryption/decryption.
- Side-Channel Attack Implementation (Example – Simple Power Analysis):
This is a simplified example; real attacks are far more complex.
- Monitor Power Consumption: Use an oscilloscope to measure the CPU’s power consumption during encryption.
- Data Acquisition: Collect a large number of power traces while encrypting known data.
- Statistical Analysis: Apply statistical techniques (e.g., Differential Power Analysis – DPA) to identify correlations between power consumption and key bits. Tools like ChipWhisperer can help with this.
# Example using a hypothetical tool for power analysis
- DMA Attack Implementation (Highly Complex):
- Bypass Security Features: Design the CPU to bypass TPM or other security mechanisms. This often involves exploiting firmware vulnerabilities.
- Direct Memory Access: Implement DMA controllers capable of reading encrypted data directly from the hard drive’s storage.
- Key Extraction: Attempt to extract the encryption key from memory.
- Algorithm Exploitation (If Applicable):
- Identify Weaknesses: Research and identify potential weaknesses in the encryption algorithm.
- Implement Attack Logic: Design CPU instructions to exploit these weaknesses.
Challenges & Considerations
- Cost: Designing and fabricating a custom CPU is incredibly expensive (millions of pounds).
- Complexity: Requires deep expertise in cryptography, hardware design, VLSI manufacturing, and security.
- Time: The development process can take years.
- Existing Protections: Modern encryption systems have multiple layers of protection (TPM, KDFs) making attacks difficult.
- Algorithm Strength: AES is a very strong algorithm; finding exploitable weaknesses is unlikely.
- Legal Implications: Attempting to bypass encryption may be illegal in some jurisdictions.
Conclusion
Building a custom CPU to break hard drive encryption is a monumental task with a low probability of success. It’s far more practical to focus on securing the system properly (strong passwords, TPM enabled, regular updates) than attempting such a complex attack.