Janus (DRM)

Janus is the codename for portable version of Windows Media DRM for portable devices, whose marketing name is Windows Media DRM for Portable Devices (or in short form WMDRM-PD) introduced by Microsoft in 2004 for use on portable media devices which store and access content offline. Napster To Go was the first online music store to require the Janus technology. Supporting Janus often implies that the device also make use of the (MTP).

Janus initially required supporting devices to not support non-Microsoft audio formats such as Ogg Vorbis, but this requirement has since been rescinded.

To support Janus devices must support:

All these are supported by way of challenge-response authentication commands.

...
Wikipedia