Run mp4info output.mp4 . If the "Encrypted" flag is missing from the tracks, you have succeeded.

Most DRM systems provide this in an XML or JSON metadata file. You need two hexadecimal strings:

This command tells Mp4decrypt.exe to decrypt the encrypted_video.mp4 file using the specified key and save the decrypted file as decrypted_video.mp4 .

You downloaded a very old version of the tool (pre-2015). Fix: Download the latest Bento4 SDK (Version 1.6.0 or higher).