Then after reaching the public there must be a compelling relatable story. Applied did it a bit when replying to FBI's request:

-- Weak encryption means more identity theft. This is a good one. Everyone knows someone who lost a phone, a wallet and then thieves emptied bank accounts, caused grief, time wasted and pain for years to come. This works and is great.

Others I can think of:

-- Point to large scale data breaches, even Government's own OPM office was hacked by Chinese. Having back-doors in everything means more such large scale failures.

-- Take their own rhetoric and spin it around. Look Snowden stole all those documents right under the nose of the NSA. Point to their own incompetence and say imagine future such incidents with a perpetrator walking away with "master keys". Remember to repeat phrases like "it is not a matter of if but a matter of when". That was used by talking head security experts on TV post 9/11, this one works great. I like it.

-- Point to physical analogies where master key has been lost. Was it a case for NYFD or some place like that. Where it was mandated all these places have a master key and it was copied and used for criminal activity.

-- Find something with terrorists. Nobody will ever defend terrorists. Any case where Chinese terrorists hacked Google through a backdoor created to handle government's access to data.

-- Everyone loves freedom of speech and hates when worthy dissidents in countries we don't like are martyred. Point out how strong encryption has saved lives and provided the only channel for communication freedom fighters.

The list goes on... but that's the gist basically