Example code PHP / C# AES encryption - random IV

Question

I found an implementation for AES encryption/decryption that's supposed to work in PHP and C#: https://odan.github.io/2017/08/10/aes-256-encryption-and-decryption-in-php-and-csharp.html

However, in this example, the IV is always 0, which is not good. So, as the author suggests, I use the openssl_random_pseudo_bytes() function:

  $ivlen = openssl_cipher_iv_length($method);
  $iv = openssl_random_pseudo_bytes($ivlen);

However, the problem now is, that I have to store the IV somewhere so that the C# app can use it for decryption. My idea was to base64_encode it and prepend it to the encrypted message with ":" separating both, so that I can simply split the string, then base64 decode it in C# and can use the IV. However, this does not work. This is the code:

public string DecryptString(string cipherText, byte[] key, byte[] iv)
{
    // Instantiate a new Aes object to perform string symmetric encryption
    Aes encryptor = Aes.Create();

    encryptor.Mode = CipherMode.CBC;

    // Set key and IV
    byte[] aesKey = new byte[32];
    Array.Copy(key, 0, aesKey, 0, 32);
    encryptor.Key = aesKey;
    encryptor.IV = iv;

    // Instantiate a new MemoryStream object to contain the encrypted bytes
    MemoryStream memoryStream = new MemoryStream();

    // Instantiate a new encryptor from our Aes object
    ICryptoTransform aesDecryptor = encryptor.CreateDecryptor();

    // Instantiate a new CryptoStream object to process the data and write it to the 
    // memory stream
    CryptoStream cryptoStream = new CryptoStream(memoryStream, aesDecryptor, CryptoStreamMode.Write);

    // Will contain decrypted plaintext
    string plainText = String.Empty;

    try {
        // Convert the ciphertext string into a byte array
        byte[] cipherBytes = Convert.FromBase64String(cipherText);

        // Decrypt the input ciphertext string
        cryptoStream.Write(cipherBytes, 0, cipherBytes . Length);

        // Complete the decryption process
        cryptoStream.FlushFinalBlock();

        // Convert the decrypted data from a MemoryStream to a byte array
        byte[] plainBytes = memoryStream.ToArray();

        // Convert the decrypted byte array to string
        plainText = Encoding.ASCII.GetString(plainBytes, 0, plainBytes.Length);
    } finally {
        // Close both the MemoryStream and the CryptoStream
        memoryStream.Close();
        cryptoStream.Close();
    }

    // Return the decrypted data as a string
    return plainText;
}

And this is how I try to call the function with the IV prepended to the encrypted message:

private void btnDecrypt_Click(object sender, EventArgs e)
        {
            string encrypted = txtEncrypted.Text;
            string password = txtPW.Text;
            string[] temp = encrypted.Split(":".ToCharArray());
            string iv_str = temp[0];
            encrypted = temp[1];
            SHA256 mySHA256 = SHA256Managed.Create();
            byte[] key = mySHA256.ComputeHash(Encoding.ASCII.GetBytes(password));
            iv_str = Encoding.Default.GetString(Convert.FromBase64String(iv_str));
            byte[] iv = Encoding.ASCII.GetBytes(iv_str);

            txtDecrypted.Text = this.DecryptString(encrypted, key, iv); 
        }

Doesn't work. It decrypts something, but that's just gibberish and not the message I encrypted in PHP. I think it has to do with the IV somehow.

Answer 1

@Topaco: Thank you very much... now I see what my mistake was. It works now with generated IV. By the way, two more questions on the strength of the encryption:

1. Is it ok to use the following function to generate an IV (GenerateIV() for some reason won't work):

   byte[] iv = new byte[16]; Random rnd = new Random(); rnd.NextBytes(iv);

2. I added a randomly generated salt to the password of the user, so that the key is a hash from the user password and a salt. I also prepend the salt, together with the now randomly generated IV, to the message. Then for the decryption process I use the prepended salt and the key the user enters. Works so far. But does it weaken the encryption? No, right?

Thank you very much.

// Edit: Code format doesn't work, don't know why.