NullKey 
Offline does not mean backed up
A vault stored only on a phone can disappear with the phone. Encryption protects confidentiality; it does not protect against loss, uninstalling an app or failing storage. Plan for losing the entire device.
Save and check a fresh export
On your unlocked phone, open Settings and choose “Create encrypted backup”. In Android’s document picker choose the destination. NullKey then rereads the saved archive and checks the index, encrypted items and thumbnails against the current vault. If the app reports that the check failed, make another backup and do not rely on that file.
Export ciphertext, not a folder of originals
An encrypted vault backup preserves protected files in an export you can store elsewhere. A decrypted export creates another readable copy. Know which operation you are using. In NullKey, the whole-vault backup contains ciphertext and requires the passphrase or recovery key from that vault.
Check the backup, then practise restoring
NullKey rereads the exported archive, authenticates its index, every encrypted item segment and every thumbnail, and matches the current vault snapshot. A restore exercise with sample data also checks that you have the right secret and understand the process. Restore requires an empty vault; never erase a working vault just to try it. To understand which secret opens an older export, read recovery key or backup. Before removing source files, follow the copy-check steps. The encrypted backup feature describes the read-back check.
Make the routine repeatable
Keep an encrypted backup away from the phone and the unlocking secret separately. Refresh the backup after important changes. Label which passphrase or recovery key belongs to older exports: changing the current vault’s credentials does not rewrite backup files you already made.
