Another problem: legislation like this cements the status quo. It's easy enough for large incumbents to add features like this, but to a handful of programmers trying to launch an app from their garage, this adds another hurdle into the process. Remember: Signal and Telegram are only about a decade old, we've seen new (and better) apps launch recently. Is that going to stop?
It's easy to say "this is just a simple hash lookup, it's not that big a deal!", but (1) it opens the door to client-side requirements in legislation, it's unlikely to stop here, (2) if other countries follow suit, devs will need to implement a bunch of geo-dependant (?) lookups, and (3) someone is going to have to monitor compliance, and make sure images are actually being verified--which also opens small companies up to difficult legal actions. How do you prove your client is complying? How can you monitor to make sure it's working without violating user privacy?
Also: doesn't this close the door on open software? How can you allow users to install open source message apps, or (if the lookup is OS-level) Linux or a free version of Android that they're able to build themselves? If they can, what's to stop pedophiles from just doing that--and disabling the checks?
If you don't ban user-modifiable software on phones, you've just added an extra hurdle for creeps: they just need to install a new version. If you do, you've handed total control of phones to corporations, and especially big established corporations.
