Signal has released version 8.30 of its apps, bringing encrypted backups to iPhone and desktop and completing a rollout that began on Android almost a year ago, BleepingComputer reported Tuesday. Backups made in the shared format can be restored on Android, iOS, Linux, macOS and Windows, so users can keep their message history when they switch platforms.

What Happened

Signal now offers two kinds of backup on every platform. Signal-hosted backups are stored on the company's servers, while local backups stay on the user's own device and have no size limit. Both are end-to-end encrypted and can only be decrypted with a recovery key that the user holds. According to BleepingComputer, a single recovery key can decrypt every past backup it encrypted, wherever those files are stored.

Hosted backups have a free tier that covers text messages and the last 45 days of media, with individual messages capped at 128 KB. A paid plan raises the media allowance to 100 GB for a monthly fee. Help Net Security reported that paying users can also swap older media for thumbnails and pull the full files back from the hosted backup when needed; Signal's own example shrank 5 GB of unused media to about 600 MB.

RelatedSuspected Chinese Spies Exploited BeyondTrust Remote Support Flaw Since August, Google Says

The update also changes how the backups work. The Android format has been revised to match the new cross-platform one. Media is now kept separately from the backup archive and stored only once even when it appears in several chats. A device newly linked to a primary phone can fetch older attachments that are missing from its chats. On iOS, users can move directly from one iPhone to another over a local, end-to-end encrypted Wi-Fi Aware link.

Signal says that with the rollout finished, its engineers will turn to other parts of the app.

Why It Matters

Losing message history when a phone breaks or when switching between Android and iPhone has long been a usability gap for Signal compared with rivals that back up chats to the cloud by default. Signal's approach tries to close that gap without giving itself access to the content.

For hosted backups, the recovery key works together with a second, supplemental key that rotates daily and is held inside a trusted execution environment. That key cannot decrypt backups on its own. Signal says the arrangement provides forward secrecy and calls trusted execution environments the best current way to protect the supplemental key.

Disappearing messages set to vanish within 24 hours are excluded from both hosted and local backups, which keeps the backup feature from undermining users who rely on short-lived messages.

The Recovery Key Is the Target

The design concentrates risk in one secret. BleepingComputer noted that attackers began including Signal's backup recovery key in their targeting shortly after the feature appeared, and the outlet previously reported an FBI warning that Russian hackers were going after these keys. Because the key is what unlocks a user's backups, Signal tells users never to share it. With backups now available on iPhones and desktops, far more users will be holding one.

What to Watch

Uptake of the paid storage tier will show whether Signal, which is run by a nonprofit foundation, can turn backups into a source of revenue. Security teams at organizations that use Signal may also want to update guidance on storing recovery keys, given the FBI's earlier warning that Russian hackers are trying to obtain it.