RSA Password vs. LUKS: Security Comparison

TL;DR

Brute-forcing an RSA private key password is generally much faster and easier than cracking a strong LUKS volume, even with the same password length. This is because RSA decryption operations are relatively quick, allowing for many attempts per second, while LUKS relies on slower cryptographic hashes and potentially key stretching. Protect your RSA keys with very long passphrases or hardware security modules (HSMs).

Understanding the Problem

Both RSA private keys and LUKS volumes use passwords to protect data. However, the way they do it differs significantly, impacting their resistance to brute-force attacks.

Why Brute-forcing an RSA Key is Easier

1. Faster Decryption: RSA decryption (which requires the password) is a computationally fast operation compared to LUKS unlocking. This means you can try many more passwords per second with RSA.
2. Direct Feedback: When attempting to decrypt an RSA key, you get immediate feedback – either it works or it doesn’t. With LUKS, there’s often less direct feedback unless you are trying to mount the volume.
3. Less Overhead: The process of loading and using an RSA key for decryption is generally simpler than the full unlocking process of a LUKS volume.

Why LUKS is More Secure (When Done Right)

LUKS uses several techniques to slow down brute-force attacks:

• Key Derivation Functions (KDFs): LUKS employs KDFs like PBKDF2 or Argon2. These functions intentionally take a long time to compute, even with a fast password. This makes each attempt much slower.
• Salting: A random salt is added to the password before hashing, preventing pre-computed rainbow table attacks.
• Key Stretching: KDFs repeatedly hash the password and salt, further increasing the computational cost of each attempt.

Step-by-Step Comparison & Mitigation

1. RSA Password Brute-Force (Simplified):
   • Load the encrypted RSA key file.
   • Attempt to decrypt with a password.
   • Repeat for each possible password.

   Tools like OpenSSL can be used, but dedicated cracking tools are more efficient.

2. LUKS Volume Cracking (Simplified):
   • Identify the LUKS device (e.g., /dev/sda2).
   • Attempt to unlock with a password using cryptsetup.
   • Repeat for each possible password.

   Tools like Hashcat or John the Ripper can be used, leveraging KDFs and salts.

3. Mitigation: RSA Key Protection
   • Use Long Passphrases: A passphrase of 20+ characters is highly recommended. The longer the passphrase, the exponentially more difficult it becomes to brute-force.
   • Hardware Security Modules (HSMs): Store your private key on an HSM. HSMs provide a secure environment and prevent the key from being exposed for offline cracking.
   • Key Stretching: While not standard in RSA itself, consider wrapping the key with additional layers of encryption that use strong KDFs before storing it.
4. Mitigation: LUKS Volume Protection
   • Strong Passwords: Use a long and complex password (16+ characters).
   • Keyfiles: Combine passwords with keyfiles for added security.
   • Argon2id: Use Argon2id as the KDF in LUKS, as it’s more resistant to GPU-based attacks than PBKDF2. You can specify this during volume creation:

     cryptsetup luksFormat /dev/sda2 --type luks2 --cipher aes-xts-plain64 --key-size 512 --hash argon2id

Conclusion

While both RSA and LUKS can be secure, the inherent speed differences in their operations make brute-forcing an RSA private key password significantly easier. Prioritize strong passphrase protection for your RSA keys or use HSMs to mitigate this risk. For LUKS volumes, ensure you are using a robust KDF like Argon2id and long, complex passwords.