[shmichael]

It is, as far as I know, an unsolved problem how to implement full metadata transparency on a mobile device.

For example, Aztec, a privacy focused blockchain, requires recipients to download the entire block to determine if any private message is addressed to them (and BTW use techniques resembling Signal's double ratcheting in creating these identifiers) [1]

This is infeasible on mobile devices. At best, it allows the user to select a proxy server they trust to identify messages intended for them and forward a notification.

1 - https://www.taurushq.com/blog/enhancing-token-transaction-pr... (search for "synchronizer")