Aside from questions like how the new Messages in iCloud feature works, a lot of people are wonder how secure it is. Apple just updated its iCloud security overview page to give us an idea.
Messages in iCloud
As it turns out, Messages in iCloud is pretty secure. It uses end-to-end encryption just like certain other services like iCloud Keychain, Siri data, payment information, etc. From the page:
Messages in iCloud also uses end-to-end encryption. If you have iCloud Backup turned on, a copy of the key protecting your Messages is included in your backup. This ensures you can recover your Messages if you’ve lost access to iCloud Keychain and your trusted devices. When you turn off iCloud Backup, a new key is generated on your device to protect future messages and it is not stored by Apple.
What this means is that all of your messages are encrypted by a key generated using your device’s passcode. This makes it inaccessible to Apple and other third parties. But there’s a caveat.
If you enable iCloud Backup, that encryption key is included. It sounds like a copy of might also be stored in iCloud Keychain. That means if Apple is served a warrant by law enforcement, your iCloud Backup, along with all of its data, can be accessed. But this has always been true of iCloud Backup; the inclusion of Messages in iCloud hasn’t changed this fact.
Essentially, if you’ve trusted Apple with your personal data so far, you can continue to do so. If you’re a person who’s worried about law enforcement/government access, chances are you’re using a different messaging app like Signal.