Encrypt backups written into file

[link2xt]

Backups are currently unencrypted. Secure way to transfer backups is over the network using the backup transfer function ("Add second device" in the UIs). When it does not work, users export the backups which stores to Downloads folder on Android, see forum topic. See also https://support.delta.chat/t/researching-a-zero-trust-storage-layer-for-deltachat-core-rust-thoughts/4554/7

rPGP crate that we use supports streaming encryption and decryption with AEAD modes of the latest standard of OpenPGP. Mandatory to implement AEAD mode is OCB, so encrypted backup format should likely be an OpenPGP binary message with an SKESK packet (with Argon2 which is also supported) followed by SEIPD v2 in OCB mode. We already have a function to export backups into streams, this is used both when exporting into files and when exporting over the network when "Add second device" procedure is used:

core/src/imex.rs

Line 452 in 82a0d6b

async fn export_backup(context: &Context, dir: &Path, passphrase: String) -> Result<()> {

We definitely don't want users to provide their own passwords in the UIs, and don't want each UIs to have its own code to generate passwords. Passwords should be generated by the application as a string of digits separated by -, similar to Autocrypt Setup Message or Signal passwords for backups.

I have not figured out API details, but one way is to have a separate API to generate the password and then UI can pass it back to backup API after displaying the password to the user.

[link2xt]

Current API for exporting backups already has a "passphrase" argument, this can be reused. No UIs currently use it, it exports unencrypted backup with SQLCipher-encrypted database and we don't want to maintain SQLCipher indefinitely. So there will be no change in import/export API, the change is that the whole backup will be encrypted with OpenPGP.

Generating backup passphrase is a separate topic. In Autocrypt it was 36 digits (9 x 4), Signal uses 30 digits for on-device backups in 6 groups of 5 digits. For "Signal Secure Backups" Signal now uses "64-character recovery key", this is not what we want, don't know why it has to be that large: https://support.signal.org/hc/en-us/articles/9708267671322-Signal-Secure-Backups

[r10s]

ftr, older feature proposal in the forum: https://support.delta.chat/t/encrypted-backup-of-chats/3119