Last year Steam introduces their Steam Guard Mobile Authenticator, which is an app for iOS and Android which provides 2 factor authentication for trades on their Steam Community Market. They have since made it more or less mandatory to use the app, with users who are not able to use it being forced to wait 15 days to complete a sale.
This of course left Windows Phone users out in the cold, though some 3rd party apps have arrived recently that do allow us to take part in the trading system more easily such as Winauth or Unofficial Steam Authenticator.
It appears from the source code of the Steam Guard Mobile Authenticator app that Steam is looking to provide a first party solution to Windows Phone users however.
The code makes specific references to Windows Phone and WinRT, suggesting a Universal Windows App may be on the way.
It is not known if the app will be phone only or desktop, but we hope a full UWP version will become available, which would serve not only Windows Phone users, but also those who prefer not to use a smartphone.
The code can be perused at Github here.