Signal completes encrypted backups on iOS and desktop

Signal has shipped version 8.30 of its messaging app, bringing end-to-end encrypted backups to every platform it supports: Android, iOS, Linux, macOS and Windows.

Android users got the backup feature nearly a year ago. It lets people create encrypted copies of their chats and restore them on another device, for example after a phone is lost or stolen. With the new release, iOS and desktop users can now make on-device backups too.

Two ways to back up

Signal offers two backup options.

The first is a hosted backup stored by Signal. Free users are limited to 45 days of media and 128 KB per message. Paid plans raise the cap to 100 GB.

The second is a local, on-device backup, which has no size limits.

Both options are encrypted, and a recovery key is needed to decrypt them. Hosted backups add a second, supplemental key that rotates every day inside a Trusted Execution Environment. Signal says this provides extra protection and forward secrecy, so a single compromised key does not expose older data.

The local option works differently. According to Signal, the on-device recovery key can decrypt every past backup file it was used to encrypt, no matter where those files end up stored. That makes the key a valuable target, and threat actors have noticed. Shortly after Signal introduced the feature, attackers added the recovery key to the list of things they go after.

This matters most for people who are already of interest to well-resourced attackers, including those watched with commercial spyware tools. Anyone who gets hold of the key and the backup files could read an archive of past conversations.

One format across platforms

Besides bringing local backups to iOS and desktop, version 8.30 reworks Android backups so they use the same cross-platform format as the other clients.

Several other changes come with the update:

• When a new device is linked from the primary device, it now pulls in older attachments that are missing from the chat history.
• Media is stored apart from the backup archive, and duplicate files are kept only once. Signal says this removes unnecessary backing up and syncing.
• Paying users get an option to delete old media from local storage and keep only thumbnails. The full files are fetched automatically from the hosted backup when needed.

Disappearing messages stay out

Disappearing messages set to vanish within 24 hours are not included in either type of backup, so they never land in the archive.

Users who run both hosted and local backups will find that the hosted one does not overwrite the local copy. When restoring, however, any disappearing messages in the local backup that have already expired are skipped.

iPhone-to-iPhone transfer

The iOS version of 8.30 also adds direct transfers between iPhones. The transfer runs over a local Wi-Fi Aware connection that is end-to-end encrypted. Signal says the new method is faster and more reliable.

Signal describes this release as the end of the first phase of its backups project. Its engineers will now turn to improving parts of the app that are not related to backups.

Our Take

For Signal users, especially on iPhone and desktop, this closes a long-standing gap. Until now, losing a phone often meant losing chat history, and that pushed some people toward less private workarounds. An encrypted, cross-platform backup that Signal itself cannot read is a sensible answer.

The trade-off is that the recovery key now carries a lot of weight. Because a local backup key can unlock every archive it created, it belongs in the same category as a password manager's master password. Users should store it offline and away from the device it protects. The fact that attackers began targeting the key soon after the Android launch suggests this is not a theoretical concern.

It is worth watching whether Signal adds key rotation or similar forward secrecy to local backups, as it already does for hosted ones. Organisations that allow Signal on work devices may also want to check where desktop backup files are stored and who can reach them. Keeping clients patched remains basic hygiene, as fixes like Apple's recent zero-day patch show that the device underneath matters as much as the app.