Signal Secure Backups

> Once you’ve enabled secure backups, your device will automatically create a fresh secure backup archive every day, replacing the previous day’s archive.

So IIUC backups will not be incremental and I will have to re-upload my 15 GB backup archive every day? Why is that? What's the security risk here? (Obviously I'm not suggesting encrypting & uploading each message & media file individually but splitting things up into same-sized chunks, like e.g. borgbackup does.)

> At the core of secure backups is a 64-character recovery key that is generated on your device. This key is yours and yours alone; it is never shared with Signal’s servers. This key is different from your Signal PIN, which serves different purposes.

Both recovery key and Signal PIN seem to serve the exact same purpose, though, namely restoring data (conversations, contacts, account, …)? Why not unify them?

> So IIUC backups will not be incremental

Nope! It's very much incremental :) At least the media is. There's one blob of containing all of your messages+metadata which does have to be re-uploaded every night, but for most people that's gonna be somewhere in the low-tens of MB. Your attachments are uploaded incrementally one at a time, typically as they're sent/received, so you usually don't even have to wait to upload them at backup-time.

> Both recovery key and Signal PIN seem to serve the exact same purpose, though, namely restoring data (conversations, contacts, account, …)? Why not unify them?

This was a hard decision and something we went back and forth on. But at the end of the day, we felt the safest thing we could do for now is to use a completely separate strong, random key. We're very aware of all the trade-offs involved, but this is where we landed.