As a fun evening read I'd like to remind everyone of Pavel Durov's gaslighting on how their approach of everything-leaks-to-server was the right way to implement "cloud backups" for Telegram.
https://web.archive.org/web/20200226124508/https://tgraph.io...
Nice to finally see someone competent show how it's actually done :)
"On the other hand, symmetric algorithms such as AES are believed to be immune to Shor. In most cases, the best-known quantum key recovery attack uses Grover’s algorithm which provides a generic square-root speed-up over classical exhaustion in terms of the number of queries to the symmetric algorithm. In other words, Grover would recover the 256-bit key for AES-256 with around 2^128 quantum queries to AES compared to around 2^256 classical queries for exhaustion. "
- https://csrc.nist.gov/csrc/media/Events/2024/fifth-pqc-stand...
</pedantry>
the paper itself concludes "the practical security impact of Grover with existing techniques on plausible near-term quantum hardware is limited."
Wrap it in whatever security deemed necessary (or make migration/backup opt-in), but just let the blob copy over like every other app on the planet.
This cumbersome backup nonsense is a senseless no more secure bandaid for a problem that shouldn’t exist in the first place.
Those backups are stored locally, are platform-specific (Android-only), and there is no feasible way to automate their transfer to any other device, which means that either you have to manually manage them regularly, or you risk losing your entire message history if your phone suddenly dies (or is stolen, or broken beyond repair, etc.).
This is a true automated, off-site backup feature.