Signal cloud backups

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Signal is a privacy-focused, open source encrypted messaging service. In 2020, the company was accused of collecting and storing sensitive user data on their cloud database without user consent.

Background

Signal states on their website that they can't "read your messages or listen to your calls, and no one else can either."

"We’ve designed the Signal service to minimize the data we retain about Signal users, so the only information we can produce in response to a request like this is the date and time a user registered with Signal and the last date of a user’s connectivity to the Signal service. Notably, things we don’t have stored include anything about a user’s contacts (such as the contacts themselves, a hash of the contacts, any other derivative contact information), anything about a user’s groups (such as how many groups a user is in, which groups a user is in, the membership lists of a user’s groups), or any records of who a user has been communicating with."

Incident

In 2019, Signal previewed a feature called "secure value recovery" which would allow users installing the app on a new device to retrieve data from cloud servers. While the data is stored in Signal's cloud, it is stored in a securely encrypted manner. The data collected and stored includes the user's name, photo, phone number, and a list of each Signal user that had been contacted.^{[disputed contact discovery on Signal is private and does not share the phone number as explained later in the cited sources - discuss]} Messages are not saved.

Some users objected on philosophical grounds, requesting that Signal instead provide a means to export encrypted backups that could be imported locally which would eliminate dependence on cloud-based servers. Some users also raised technical concerns about the security of the system and doubt it could sufficiently protect their data. Some of these concerns were also shared by cybersecurity experts, and security researchers demonstrated that the system was vulnerable to attacks, which allowed them to access the user data being stored.^{[disputed "In recent weeks, Signal has introduced more features that make it more user friendly to people who may not have extremely paranoid threat models. For example, it’s now possible to migrate all Signal data, including message history, from one phone to another, using a feature that does not rely on cloud servers and is also encrypted, according to Signal. " - discuss]}

Signal's privacy policy remains unaltered from its latest version updated in 2018 and does not reflect the new cloud feature.

Signal's response

Signal began to roll out the cloud-based recovery feature in 2020 without clear communication with the public or app users about the new feature. Signal users queried the need for the PIN, and communication from the team suggested it was to ensure messages were not missed or lost by keeping messages logged in a cloud format. A more detailed response from Signal following user backlash explains that cloud backups were encrypted in the same way as messages, and were safe. Several users requested that Signal update their policy following the change in data collection.