Fortress Mode
Encrypt the local mail store of an account with a passphrase that FOLD never stores.
Fortress Mode encrypts what FOLD keeps on your device for one account, on top of FileVault or the data protection of iPhone and iPad. It is off by default and you turn it on per account.
Turning it on
On the Mac, open Settings (⌘,) > Security > Fortress encryption and switch on the account. On iPhone and iPad, tap your account picture, then Security & Privacy and use the Fortress section.
For the first account you choose a passphrase (Set Fortress passphrase). Every further account uses the same passphrase. FOLD then encrypts the existing data of the account.
The passphrase is stored nowhere. If you lose it, the local data of your Fortress accounts is gone; the mail on your server is not affected and can be loaded again after Reset Fortress ….
How it works
- Argon2id with 64 MiB of memory, 3 passes and 4 lanes turns the passphrase into a 256-bit master key. The master key exists only in memory.
- Each account gets its own key, derived with HKDF-SHA256.
- Every stored record is encrypted with AES-256-GCM and bound to its place, so records cannot be swapped between messages.
Encrypted: message bodies and attachments, subject, addresses, preview, raw headers and authentication results. Not encrypted: message IDs, dates, flags and folders, which FOLD needs to build the message list.
Limits you notice
- Search: FOLD does not put the content of Fortress accounts into its full-text index, so searching for words finds nothing in those accounts.
- Spotlight: Fortress accounts are kept out of system search.
- While locked: sync pauses for Fortress accounts and their messages cannot be opened.
Locking and unlocking
- On the Mac, Fortress locks after 5 minutes without activity and with ⌘L.
- On iPhone and iPad there is no separate inactivity timer yet; Fortress locks together with the app lock.
- On both platforms, leaving the app locks Fortress only if the app lock is on, and only when the app lock itself locks. FOLD does not react to the system screen lock.
To unlock, enter the passphrase. You can also turn on unlocking with Touch ID or Face ID: FOLD then keeps the master key in the keychain of this device, protected by biometrics and never synced. With the app lock on, one Touch ID or Face ID prompt unlocks both.
Turning it off
Switching an account off asks Disable Fortress?. Disable & delete locally removes the encrypted local data, and FOLD loads the mail from the server again.