There are 'tools' often sold here to download streams from popular streaming sites. I know those ones can print the keys for you with just a few minor modifications to the scripts.
I think you are confusing couple of things here. You can print those keys without any modifications to those tools because they have option to print only decrypting keys. Those keys are NOT device keys. To get decryption key for audio/video streams you first need to have device private key or know where to hook in dll (for L3 widevine for example). There are only two ways to obtain that and both require reverse engineering.