using System.Linq;
using System.Security.Cryptography;
using System.Text;

public static class CryptoUtils
{
    public static string GetUniqueKey(int size = 64)
    {
        const int minSize = 5;
        const int maxSize = 64;
        char[] chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();
        byte[] data = new byte[1];
        if (size < minSize)
            size = minSize;
        if (size > maxSize)
            size = maxSize;
        StringBuilder result = new StringBuilder(size);
        using (RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider())
        {
            crypto.GetNonZeroBytes(data);
            data = new byte[size];
            crypto.GetNonZeroBytes(data);
            foreach (byte b in data)
                result.Append(chars[b % (chars.Length - 1)]);
        }
        return result.ToString();
    }
}