Encrypted backups can't be deduplicated unless the encryption is flawed. There shouldn't be a way to tell that one Signal backup is somewhat related to another, unless you have the passphrase.
That also means that Syncthing can't do better than sending the full backup. But if you're syncing via wifi (e.g. at home) it's not really a problem anyway.
> Encrypted backups can't be deduplicated unless the encryption is flawed.
Would you mind elaborating on why this would be an issue? 1) Tools like borgbackup provide the exact functionality you're describing and considered secure. 2) Encrypted file systems also don't re-encrypt your entire HDD whenever you change a single file.
> Encrypted backups can't be deduplicated unless the encryption is flawed
This isn't an encryption problem; each device can only have one instance of Signal installed, and the latest backup (assuming it has terminated successfully) is a superset of the previous ones (aside from any messages that have dropped from retention, which you presumably don't want to be preserving, by definition).
"Deduplicate" in this context means ensuring that you only have N backups in your remote storage, rather than cumulatively storing every day.
That also means that Syncthing can't do better than sending the full backup. But if you're syncing via wifi (e.g. at home) it's not really a problem anyway.