Col. Florian Manet, who heads France’s Home Affairs Ministry National Cyber Command Technical Department, said in a statement issued by Australian police that his officers provided technical resources to the task force over several years that helped decrypt the communications.

McCartney said the French had “provided a foot in the door” for Australian police to decrypt Ghost communications.

Australian police technicians were able to modify software updates regularly pushed out by the administrator, McCartney said.

“In effect, we infected the devices, enabling us to access the content on Australian devices,” McCartney said, adding that the alleged administrator lived in his parents’ Sydney home and had no police record.

It's hard to parse what happened here. Sounds like a MITM attack where they gained access to the device OS which allowed them to view messages once decrypted by the device?