A Chinese government web site has spilled the beans about the mobile technology patent hoard that Microsoft uses like a cudgel against Android device makers. To date, Microsoft has convinced over 20 companies to license these patents rather than face expensive and time-consuming lawsuits. But until now, what exactly they were licensing was a secret.

No more: The Microsoft patent list that the firm uses against Android device makers encompasses over 300 separate patents, including 73 that are described as "standards essential patents," or SEPs. The patent titles are somewhat vague—it's possible to patent virtually anything in the United States these days—but include such things as "Method and System for Managing Changes to a Contact Database," "Interactive Traffic Display and Trip Planner," "Soft input panel system and method," and the like.

The patent list (in Word document format)—which I found out about through Ars Tecnnica—was published by China's Ministry of Commerce (MOFCOM) and was apparently part of the interactions that Microsoft had getting China to approve of its purchase of Nokia's devices and services businesses. You may recall that China belatedly approved this purchase in early April, clearing the path for Microsoft to subsume Nokia later that month.

Until this revelation, the list of patents which Microsoft claims Android infringes on has been kept largely secret, and virtually all of the companies Microsoft threatened with lawsuits caved pretty quickly. The one very public exception was Barnes & Noble: That firm, which makes Nook-branded Android-based tablets, accused Microsoft in 2011 of "embarking on a campaign of asserting trivial and outmoded patents against manufacturers of Android devices," and asked the US Department of Justice to intervene. When that didn't pan out, B&N called Microsoft's bluff and agreed to go to court, where it figured the software giant would have to go public with the patent list.

In 2012, however, B&N abruptly settled with Microsoft and acquired a license for Microsoft's patents. Microsoft in turn invested $300 million in Nook, which B&N at the time planned to spinoff. (The firm has since changed plans several times and most recently announced a new lineup of Samsung-built Nook tablets.) So Microsoft's secret patent list was safe again.

Microsoft's strategy against Android and the firms that use it is modeled after a similar series of patents it used against Linux, which, like Android is given away for free but builds on technology developed by many others. But the situation with Android is of course more serious. While Linux never saw any traction at all on the PC desktop, Android is now the most popular mobile platform in the world. So Microsoft allegedly earns up to $2 billion a year in Android-based licensing fees, as between 50 and 70 percent of all Android devices sold are protected by its licensing.

Microsoft said it has no comment about the patent list.