It is not a question of writing the software. Literally with a linux desktop it comes with the
software to do it built in. I bet a mac is the same, I would be surprised if windows does not have the algorithm in it somewhere if not exposed to the user.
Sure, you may get some stupid criminals who even after all the publicity of whatsapp and co. leaving the uk still talk about crimes over the government sponsored application. That is not going to be the organised crime and terrorists that you were using to justify the law.
Remember, to get these stupid criminals you are backdooring everyone's communications. So these same organised criminals and terrorists when they
get hold of
the comms companies
data they now have access to all our communications to date. As someone whose job makes them a target for terrorists I can tell you I feel much more at risk from that than any gain I may get from stupid criminals.