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Insecure encryption algorithm - AES

Need​

Implementation of secure encryption algorithms

Context​

  • Usage of C# for developing robust and scalable applications
  • Usage of the System library for accessing and manipulating system-level functionality
  • Usage of System.IO for input/output operations in .NET development
  • Usage of System.Security.Cryptography for secure data encryption and decryption

Description​

Non compliant code​

using System;
using System.IO;
using System.Security.Cryptography;

public class InsecureEncryption
{
    public static void EncryptData(string dataToEncrypt)
    {
        using (AesCryptoServiceProvider aes = new AesCryptoServiceProvider())
        {
            aes.Mode = CipherMode.CBC; // Insecure mode
            aes.Padding = PaddingMode.PKCS7; // Insecure padding
            aes.KeySize = 128;
            aes.BlockSize = 128;

            // Convert string to byte array
            byte[] byteArray = Encoding.UTF8.GetBytes(dataToEncrypt);

            // Perform encryption
            using (ICryptoTransform encryptor = aes.CreateEncryptor())
            {
                byte[] encryptedData = encryptor.TransformFinalBlock(byteArray, 0, byteArray.Length);
            }
        }
    }
}

In the above code, the AesCryptoServiceProvider class is used to encrypt data. The Cipher Mode is set to CipherMode.CBC (Cipher Block Chaining) and the Padding is set to PaddingMode.PKCS7.

The CipherMode.CBC is considered insecure because it allows an attacker to perform a bit flipping attack. In this attack, an attacker can change the plaintext without knowing the encryption key.

The PaddingMode.PKCS7 is considered insecure because it is vulnerable to padding oracle attacks. In this attack, an attacker can decrypt the ciphertext without knowing the encryption key by observing the server's response to manipulated ciphertexts.

Furthermore, the key size and block size are set to 128 bits, which is considered relatively weak in the context of modern computational power. This makes the encryption easier to break through brute force attacks.

Therefore, the combination of insecure cipher mode, padding mode and weak key size/block size makes this encryption approach vulnerable to attacks.

Steps​

  • Replace the insecure AES encryption mode (CBC) with a more secure mode such as GCM or CTR.
  • Replace the insecure padding mode (PKCS7) with a more secure padding mode such as PKCS5Padding or PKCS7Padding.
  • Increase the key size to a more secure value, such as 256 bits.
  • Increase the block size to a more secure value, such as 128 bits.
  • Ensure that the encryption algorithm used is considered secure and not vulnerable to known attacks.

Compliant code​

using System;
using System.IO;
using System.Security.Cryptography;

public class SecureEncryption
{
    public static void EncryptData(string dataToEncrypt)
    {
        using (AesCryptoServiceProvider aes = new AesCryptoServiceProvider())
        {
            aes.Mode = CipherMode.GCM; // Secure mode
            aes.Padding = PaddingMode.PKCS7; // Secure padding
            aes.KeySize = 256; // Increased key size
            aes.BlockSize = 128; // Increased block size

            // Convert string to byte array
            byte[] byteArray = Encoding.UTF8.GetBytes(dataToEncrypt);

            // Perform encryption
            using (ICryptoTransform encryptor = aes.CreateEncryptor())
            {
                byte[] encryptedData = encryptor.TransformFinalBlock(byteArray, 0, byteArray.Length);
            }
        }
    }
}

The updated code now uses a more secure encryption mode (GCM) and padding mode (PKCS7). The key size has been increased to 256 bits and the block size remains at 128 bits, which are both considered secure.

The AesCryptoServiceProvider class is used to encrypt the data. The Mode property is set to CipherMode.GCM, which is a secure mode of operation for symmetric key cryptographic block ciphers. The Padding property is set to PaddingMode.PKCS7, which is a secure padding mode.

The KeySize property is set to 256 bits, which is a secure key size for AES encryption. The BlockSize property is set to 128 bits, which is a secure block size for AES encryption.

The CreateEncryptor method is used to create an encryptor object that uses the current key and initialization vector (IV). The TransformFinalBlock method is then used to transform the specified region of the input byte array and return the transformed region.

This updated code ensures that the encryption algorithm used is considered secure and not vulnerable to known attacks.

References​

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