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Insecure generation of random numbers - Static IV

Need​

Secure generation of random numbers and dynamic initialization vectors

Context​

  • Usage of C# for developing robust and scalable applications
  • Usage of System for managing and interacting with the operating system
  • Usage of System.Security.Cryptography for secure data encryption and decryption

Description​

Non compliant code​

using System;
using System.Security.Cryptography;

public class VulnerableClass
{
    public void InsecureRandom()
    {
        var staticIV = new byte[16] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
        var key = new byte[32];

        using (var rng = RandomNumberGenerator.Create())
        {
            rng.GetBytes(key);
        }

        using (var aes = Aes.Create())
        {
            aes.Key = key;
            aes.IV = staticIV;
        }
    }
}

In the above code, we have a class VulnerableClass with a method InsecureRandom(). This method generates a random key using a RandomNumberGenerator but uses a static, hard-coded initialization vector (IV) for the AES encryption algorithm.

The initialization vector is an input to cryptographic functions to provide randomness. It should be unique and random for each execution to ensure the security of the encryption. However, in this case, the IV is static and hard-coded (staticIV), which means it's the same for every execution.

This is a significant security risk. If an attacker knows the IV, they can perform a reverse engineering attack to predict the random values that should remain secret. This can lead to the decryption of sensitive data and potentially give the attacker access to information that should be kept confidential.

Steps​

  • Generate a random initialization vector (IV) for each encryption operation.
  • Do not use static, hardcoded values for the IV.
  • Use a secure random number generator to generate the IV.
  • Ensure that the IV is unique for each encryption operation.
  • Store the IV securely along with the encrypted data.

Compliant code​

using System;
using System.Security.Cryptography;

public class FixedClass
{
    public void SecureRandom()
    {
        var key = new byte[32];
        var iv = new byte[16];

        using (var rng = RandomNumberGenerator.Create())
        {
            rng.GetBytes(key);
            rng.GetBytes(iv);
        }

        using (var aes = Aes.Create())
        {
            aes.Key = key;
            aes.IV = iv;
        }
    }
}

The fixed code now generates a random initialization vector (IV) for each encryption operation, instead of using a static, hardcoded value. This is done by calling the GetBytes method of the RandomNumberGenerator instance on the iv byte array, which fills the array with a cryptographically strong sequence of random values.

This change mitigates the risk of an attacker being able to predict the IV and perform a reverse engineering attack. By generating a new, random IV for each encryption operation, we ensure that even if an attacker is able to compromise one IV, they cannot predict the IVs of future encryption operations.

The IV is stored securely in the iv byte array, which is only accessible within the SecureRandom method. This ensures that the IV cannot be accessed or modified by other parts of the application, further reducing the risk of an attack.

It's important to note that the IV should be stored along with the encrypted data, as it is needed for the decryption process. However, unlike the encryption key, the IV does not need to remain secret and can be safely stored in plaintext alongside the encrypted data.

References​

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