C# cryptoStream.Read error

Question

Before you create your file, you convert the "encrypted" bytes to a base64 string. When you read this file, you don't convert back the base64 string to the "encrypted" bytes.

To solve your problem you could update your WriteVBScriptDecrypt method like this:

public string writeVBScriptDecrypt(string nameFile)
{
  var base64EncryptedBytes = File.ReadAllText(nameFile);
  byte[] bytes = Convert.FromBase64String(base64EncryptedBytes);

  ...
}

A better way is to get rid of the Base64 conversion, you can write the bytes directly to a file instead of converting it to a base64 string first.

You should also consider the using {} statement to make sure that everything is disposed nicely.

 public class DeltaGeIO
{
    // This constant string is used as a "salt" value for the PasswordDeriveBytes function calls.
    // This size of the IV (in bytes) must = (keysize / 8).  Default keysize is 256, so the IV must be
    // 32 bytes long.  Using a 16 character string here gives us 32 bytes when converted to a byte array.
    private const string initVector = "aa00bb11cc22dd33";

    // This constant is used to determine the keysize of the encryption algorithm.
    private const int keysize = 256;

    private string debug;

    public string cryptDecryptScript(string nameFileToCrypt)
    {
        try
        {
            string prova = writeVBScriptEncrypt(nameFileToCrypt, "");
            prova = writeVBScriptDecrypt("testCrypt.txt");
            return prova;
        }
        catch
        {
            return debug;
        }
    }

    public string writeVBScriptEncrypt(string nameFile, string nameScript)
    {
        byte[] bytes = System.IO.File.ReadAllBytes(nameFile);
        byte[] encryptedBytes = this.EncryptBytes(bytes, "test");
        try
        {
            File.Delete("testCrypt.txt");
        }
        catch
        {
            debug = "WVBSSE - i cannot delete testCrypt.txt file";
        }

        File.WriteAllBytes("testCrypt.txt", encryptedBytes);
        return "OK file Encrypted";
    }

    public string writeVBScriptDecrypt(string nameFile)
    {
       var encryptedBytes = File.ReadAllBytes(nameFile);
       byte[] decryptedBytes = this.DecryptBytes(encryptedBytes, "test");
       System.IO.File.WriteAllBytes("testDecrypt.vbs", decryptedBytes);
        try
        {
            System.IO.File.Delete("testCrypt.txt");
        }
        catch
        {
            debug = "WVBSSD - i cannot delete testCrypt.txt file";
        }
        return "OK file Decrypted";
    }

    private byte[] EncryptBytes(byte[] plainText, string passPhrase)
    {
       byte[] initVectorBytes = Encoding.UTF8.GetBytes(initVector);
       byte[] plainTextBytes = plainText;
       PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null);
       byte[] keyBytes = password.GetBytes(keysize / 8);
       using (RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC})
       {
          using (ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes))
          using (MemoryStream memoryStream = new MemoryStream())
          using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
          {
             cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
             cryptoStream.FlushFinalBlock();
             byte[] cipherTextBytes = memoryStream.ToArray();
             return cipherTextBytes;
          }
       }
    }

    private byte[] DecryptBytes(byte[] cipherText, string passPhrase)
    {
        byte[] initVectorBytes = Encoding.UTF8.GetBytes(initVector);
        byte[] cipherTextBytes = cipherText;
        PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null);
        byte[] keyBytes = password.GetBytes(keysize / 8);
        using (RijndaelManaged symmetricKey = new RijndaelManaged(){Mode = CipherMode.CBC})
        using (ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes))
        using (MemoryStream memoryStream = new MemoryStream(cipherTextBytes))
        using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
        {
           byte[] buffer = new byte[cipherTextBytes.Length];
           int decryptedByteCount = cryptoStream.Read(buffer, 0, buffer.Length);
           byte[] copy = new byte[decryptedByteCount];
           Array.Copy(buffer, copy, decryptedByteCount);

           return copy;
        }
    }
}