If Apple hypothetically agreed to iMessage backdoors, why would you trust the Telegram app updates served up by Apple's app store? Western government's can pretty much hack into any device they want - the only reason for backdooring messaging apps would be for dragnet surveillance, and I don't see big tech having the appetite for the bad publicity and lawsuits that will result when that inevitably becomes public
Apple already has a kind of "backdoor": they store the keys for encrypted cloud backups in their cloud as well. They advertise that cloud data are encrypted but prefer not to mention that they also have a key to decrypt it. Even with the highest level of security [1] your contacts list in Apple Cloud are not encrypted. Why? Probably someone asked for this.
CSV or PNG weren't created with encryption in mind, but one can easily encrypt them. Apple can always make their own proprietary protocol. This doesn't explain anything. However the version that the govt wants to be able to see who is in person't contact list explains it well.
> If Apple hypothetically agreed to iMessage backdoors, why would you trust the Telegram app updates served up by Apple's app store?
I wouldn't. I don't trust Apple hardware or software, and I don't see why anyone who cares about these issues ever would. But fortunately Telegram runs on devices and OSes from a wide range of suppliers, many of which might be less open to the influences that apply to Apple.