AES Key Break Time

TL;DR

Breaking an AES key depends on the key length (128, 192 or 256 bits) and available computing power. A brute-force attack is theoretically possible but practically infeasible with current technology for strong keys. This guide explains how to estimate break times.

Understanding AES Key Strength

AES (Advanced Encryption Standard) uses symmetric key cryptography, meaning the same key encrypts and decrypts data. The security of AES relies entirely on keeping the key secret. Longer keys are exponentially harder to crack.

Estimating Break Time

1. Key Length: Determine the length of the AES key being used (128, 192 or 256 bits).
2. Brute-Force Calculation: The theoretical minimum time to break an AES key using brute force is based on trying every possible combination. There are 2^key_length possible keys.
   • For a 128-bit key, there are 2¹²⁸ (approximately 3.4 x 10³⁸) possibilities.
   • For a 192-bit key, there are 2¹⁹² (approximately 6.2 x 10⁵⁷) possibilities.
   • For a 256-bit key, there are 2²⁵⁶ (approximately 1.16 x 10⁷⁷) possibilities.
3. Hashing Speed: Estimate the hashing speed of your attack platform. This is how many keys can be tested per second. Modern GPUs can perform trillions of hashes per second (TH/s). CPUs are significantly slower.
   • Example: A GPU might achieve 10 TH/s = 10 x 10¹² hashes/second.
4. Calculate Time in Seconds: Divide the total number of possible keys by the hashing speed.

   time_in_seconds = (2 ^ key_length) / hashing_speed

   • Example (128-bit key, 10 TH/s): time_in_seconds = (2¹²⁸) / (10 x 10¹²) ≈ 3.4 x 10²⁶ seconds
5. Convert to More Understandable Units: Convert the time in seconds into years.

   time_in_years = time_in_seconds / (60 * 60 * 24 * 365.25)

   • Example (128-bit key, 10 TH/s): time_in_years ≈ 1.08 x 10¹⁹ years

Practical Considerations

The above calculation assumes a perfect brute-force attack with no optimizations. In reality:

• Parallelism: Using multiple GPUs or CPUs in parallel significantly reduces the break time, but is still limited by hardware and cost.
• Rainbow Tables & Precomputation: These techniques can speed up attacks on weak passwords used as AES keys, but are ineffective against truly random keys.
• Side-Channel Attacks: These attacks exploit vulnerabilities in the implementation of AES (e.g., power consumption, timing variations) rather than attempting to brute-force the key directly. They require physical access to the system running AES.

Key Takeaways

• Breaking a 128-bit AES key with current technology is practically impossible and would take billions of years, even with significant computing resources.
• A 192-bit or 256-bit key offers even greater security.
• The biggest risk to cyber security when using AES isn’t the algorithm itself, but poor key management (e.g., weak passwords, insecure storage).