It does not need to be particularly secure—the messages are still E2E encrypted so long as nothing trips the client-side scanner.
I think you are letting your ideological alignment (against surveillance state) push you into irrational standpoints ("more surveillance would not catch additional criminals").
I'm 100% with you on opposing legislation like this, but it is very important to not delude oneself about its likely effects, and to pick the right hills to die on, figuratively speaking.
This is not some wild hypothetical, the recent explosion in VPN use by every country that has implemented an age restriction law should be sufficient to display this effect in place. In a world without weird country restrictions (whether that be intellectual property restrictions or content restrictions), VPNs would be a niche technology for business. Instead unbelievably large amounts of the general population are now not only using it, but paying for it.
I think the assumption that criminals would not learn how to use one of the many free E2E encrypted messengers is the deluded and naive position.
This just isn't the case. Many criminals use non-encrypted phone calls, leave voice mails, etc. all the time. For example this recent theft of a gold toilet:
https://www.bbc.co.uk/news/articles/cgeg39vr3j3o
> A photograph found by police on his phone showed a carrier bag stuffed with cash, which was sent on WhatsApp with the message "520,000 ha ha ha".
The only reason that was E2E encrypted is because everyone in the UK uses WhatsApp and they enable E2E encryption by default.
> I think the assumption that criminals would not learn how to use one of the many free E2E encrypted messengers is the deluded and naive position.
It absolutely isn't. Some would, but the vast majority of criminals are not security experts.
It's still a dumb law. Also the criminals that it claims to target (paedophiles) are probably the least likely to get caught because they're already used to lots of electronic scanning things. Though even there it's not like they're all criminal masterminds. I can't find it now but there was recently a story about a someone who tried to hide child porn just in a deep folder structure like .../secret/do_not_open/i_warned_you/...
Dumb law, but lets use real reasons to argue that.
Given that we're talking about cybercrime here, what are the odds that the criminals in question are too dumb to Google "how can i get around whatsapp image scanning"?
The bomb was handed off across a political boundary and detonated at some arbitrary point on the way to its target, an hour earlier than expected.
(And then there was the one in France, where a cellphone-triggered bomb detonated prematurely and eliminated its builders because the mobile carrier they were using sent a "HAPPY NEW YEAR" SMS to every customer).
It's a good model for identifying and closing gaps (especially if one is not, oneself, prone to think like a criminal), but like all other human population groups, half of all criminals are below average.
You see this all the time with all sorts of areas of law, not just guns. The real evil-doers are the enablers cheering it on. "Well they didn't have a permit so they deserved it" and the like.