Microsoft appears to be sticking a finger in Google’s eye with the launch of its new YouTube app for Windows Phone. The app, ReadWrite has confirmed, strips out YouTube ads when it plays back videos and allows users to easily download video by way of a prominent “download” button.

Both behaviors violate the cardinal rules YouTube imposes on developers who use its service. To get around those restrictions, it appears that Microsoft reverse-engineered some portion of the software used to access YouTube’s basic functions, which are generally known as application programming interfaces, or APIs. If so, that could mean Microsoft can do just about whatever it wants with its YouTube app.

Google had previously declined to build a YouTube or any other Google apps for Windows Phone 8, citing the lack of users on the platform.To get an official YouTube app on Windows Phone, Microsoft decided to circumvent Google and just build the app itself.

YouTube’s restrictions, found in section II of its terms of service, are fairly explicit:

Your API Client will not, and You will not encourage or create functionality for Your users or other third parties to:

[…]

7. modify, replace, interfere with or block advertisements placed by YouTube in the YouTube Data, YouTube audiovisual content, or the YouTube player;

[…]

11. store copies of YouTube audiovisual content;

In a variety of tests, ReadWrite found that the Microsoft YouTube app indeed appears to be stripping pre-roll ads from videos that would normally appear. Ads appear on both this video from BuzzFeed on the Web and Android as well as this video from popular YouTuber Jenna Marbles. Neither of the videos had pre-roll ads when viewed via the Microsoft app.

As for downloading, that’s fairly easy to spot in Microsoft’s YouTube app — there’s an entire button dedicated to it. See the screenshots below.

Microsoft admitted to Neowin.net, a technology website that focuses on Microsoft, Apple and Linux news, that it had re-architected some of YouTube’s APIs to get the app to work. A Microsoft PR rep confirmed the statement below and offered no further comment:

Windows Phone invested additional engineering resources against existing APIs to re-architect a Windows Phone app that delivers a great YouTube experience, including support for unique Windows Phone 8 features such as Live Tiles and Kids Corner. Microsoft did not receive any additional technical support to create the Windows Phone YouTube app.

Neither Microsoft nor YouTube had responded to requests for further comment as of this morning. We’ll update the story when and if we hear back.

This wouldn’t be the first time that Microsoft has reversed-engineered Google. In February 2011, Google caught Microsoft’s Bing search engine copying Google search results. Google had set up a “honeypot” trap to catch Microsoft in the act.