Calculates SteamGuard code from your base64 shared secret

GetSteamGuardCode.cs

string GetSteamGuardCode(string base64SharedSecret)
{
	long time = (long)(DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1))).TotalSeconds + 10;
	byte[] timeHash = BitConverter.GetBytes(time/30).Reverse().ToArray();
	HMACSHA1 hmac = new HMACSHA1(Convert.FromBase64String(base64SharedSecret));
	hmac.Initialize();
	byte[] hash = hmac.ComputeHash(timeHash);
	int b = hash[19] & 0xF;
	int codePoint = (hash[b] & 0x7F) << 24 | (hash[b + 1] & 0xFF) << 16 | (hash[b + 2] & 0xFF) << 8 | (hash[b + 3] & 0xFF);
	string steamChars = "23456789BCDFGHJKMNPQRTVWXY";
	return new string(Enumerable.Range(0,5).Select(x => steamChars[codePoint/((int)Math.Pow(steamChars.Length,x)) % steamChars.Length]).ToArray());
}