I see there's a lot written about this...and I didn't see this mentioned. This has been available since .NET Framework 4.5
The ValidateRequestMode setting for a control is a great option. This way the other controls on the page are still protected. No web.config changes needed.
protected void Page_Load(object sender, EventArgs e)
{
txtMachKey.ValidateRequestMode = ValidateRequestMode.Disabled;
}