Random seed might be predictable

[giwiro]

In this function:

//creates random password for encryption
public string CreateRandomString(int length, String str)
{
            string valid = str;
            StringBuilder res = new StringBuilder();
            Random rnd = new Random();
            while (0 < length--) {
                res.Append(valid[rnd.Next(valid.Length)]);
            }
            return res.ToString();
}

Your are using System.Random() to generate random position index in the str string. The problem is that the default seed is generated by Environment.TickCount (https://referencesource.microsoft.com/#mscorlib/system/random.cs) and it can be predictable as it's said in this article: https://utkusen.com/blog/destroying-the-encryption-of-hidden-tear-ransomware.html.

Solution

Use RNGCryptoServiceProvider to generate the private key (https://dotnetcodr.com/2016/10/05/generate-truly-random-cryptographic-keys-using-a-random-number-generator-in-net/).