Weak Symmetric Key Generation: Risks & Fixes

TL;DR

Generating symmetric keys using simple methods like timestamps or predictable sequences is very insecure. Attackers can easily guess these keys, compromising your data. Use a cryptographically secure random number generator (CSPRNG) provided by your operating system or a well-vetted cryptography library.

Understanding the Problem

Symmetric key encryption relies on keeping the key secret. If an attacker knows the key, they can decrypt everything encrypted with it. Methods that aren’t truly random make keys predictable and therefore vulnerable.

Why Common Techniques Fail

Let’s look at why some approaches are bad:

• Timestamps: Timestamps change predictably. An attacker knowing when data was encrypted narrows down the possible key space dramatically.
• Sequential Numbers: If you generate keys 1, 2, 3… an attacker can easily guess subsequent keys.
• User Input (without salting): User input is rarely random enough and can be guessed or cracked through dictionary attacks.
• Simple Hash Functions (MD5, SHA1) on predictable data: These are no longer considered secure for key generation due to collision vulnerabilities and the availability of precomputed tables.

How to Generate Strong Symmetric Keys

The goal is to create a key that’s indistinguishable from random noise.

1. Use Your Operating System’s CSPRNG: Most operating systems provide tools for generating cryptographically secure random numbers.
• Linux/macOS: Use /dev/urandom or openssl rand -base64 32 (for a 256-bit key).

  openssl rand -base64 32

• Windows: Use the CryptGenRandom API. PowerShell can use [System.Security.Cryptography.RNGCryptoServiceProvider]::Create().
2. Use a Cryptography Library: Libraries like OpenSSL, PyCryptodome (Python), or Bouncy Castle (Java) provide functions for secure key generation. This is generally the best approach as they handle many of the complexities for you.
• Python Example (PyCryptodome):

  from Cryptodome.Random import get_random_bytes
  key = get_random_bytes(32) # 32 bytes = 256 bits

3. Key Length: Choose an appropriate key length. For modern encryption, 128-bit keys are the minimum; 256-bit keys are recommended for higher security.
4. Store Keys Securely: Generating a strong key is only half the battle! Protect it from unauthorized access (see section below).

Storing Symmetric Keys

Never store keys in plain text!

• Key Management Systems (KMS): Dedicated systems designed for secure key storage and rotation.
• Hardware Security Modules (HSM): Physical devices that protect keys from compromise.
• Encrypted Configuration Files: If you must store keys in files, encrypt the entire file using a separate master key managed securely.

Checking Key Randomness

While trusting your CSPRNG is important, it’s good practice to verify randomness occasionally.

• Statistical Tests: Tools like ent (Linux) can analyze the entropy of a key. A good key should have high entropy.