Signal Introduces Phone-Numberless Registration Using Zero-Knowledge Proofs

Signal is expanding its privacy-centric registration model by allowing users to create accounts without providing a phone number, a feature currently appearing in Android and iOS beta releases. This development aims to provide an alternative for users who wish to avoid linking a mobile number to their messaging identity, while utilizing advanced cryptography to maintain the service's security standards.

Privacy via Zero-Knowledge Proofs

To facilitate registration without a phone number, Signal is employing zero-knowledge proofs (ZKP). This cryptographic method allows the system to verify that a user's input—such as a username—meets specific requirements, like character set limits or length, without the server ever seeing the actual content of the username.

Chart drawn by HN Digest from figures reported by community.signalusers.org; values are shown as written in the source.

ZKP technology is already integrated into other parts of the Signal ecosystem, including donation badges and group management. By applying it to registration, Signal can validate account credentials while ensuring that the underlying data remains private from the server.

“zero-knowledge proofs are not only the technology behind donation badges and backup payments, but also behind groups.”

— Registration without a phone number

Spam Mitigation and Payment Integration

Because phone numbers traditionally serve as a barrier to entry for automated bots, Signal is introducing a one-time fee for numberless accounts. This payment is intended to act as a friction point against mass-scale spam while allowing legitimate users to remain anonymous.

During the current beta period, this fee is processed via Google Play Billing. While this provides a functional way to verify a human user, it introduces a dependency on a third-party payment processor for a service that otherwise prioritizes independence from major tech platforms.

User Experience and Metadata Trade-offs

The numberless registration flow includes several specific configuration changes, such as automatically setting phone number discoverability to false and hiding certain settings from the user interface. These changes ensure that the account's lack of a phone number is reflected in its privacy profile.

Despite these protections, questions remain regarding the metadata that persists on Signal's servers for numberless accounts. While the registration itself is shielded by ZKP, the extent to which account identifiers remain linked or traceable in the backend is not fully detailed in the current release notes.