Signal's Phone-Free Registration Push Relies on Zero-Knowledge Proofs

Signal's long-running effort to let people register without a phone number is showing up in code. The public commit log of the Android repository carries a series of changes labeled as numberless-account work, and the community thread that has tracked the request since 2018 has spent recent weeks discussing what the feature will lean on: zero-knowledge proofs.

What the commits show

The scaffolding appears in stages. On August 25, greyson-signal committed “Add signal login screen scaffolding” and “Allow setting username in registration module”; the next day mtang-signal added a commit titled “Add new signal login strings”. A larger batch followed on September 2: “Add basic ability to register numberless account”, “Add basic ability to login to an existing numberless account”, “Use new zkgroup credential for numberless accounts”, “Add password manager support to numberless login”, plus several changes that hide settings which only make sense with a phone number, among them the PIN reminder and the phone-number discoverability option.

On September 9 the same repository received two smaller fixes: “Fix setting name rendering when there's no phone number” and “Allow full CDS refreshes for numberless accounts”. The diffs are the only public trace of the work so far — the thread contains no release date and no official announcement, and the changes concern the Android client.

Zero-knowledge proofs are already inside Signal

In the thread, Signal staff member alex-signal (Alex Hart) noted that zero-knowledge proofs are already used to verify a username's character set and length “without actually revealing contents”. A long-time contributor posting as rassilon1963 explained that the same technology underpins groups, donation badges and backup payments: Signal holds a list of payers but cannot link an account to any particular one, and the client proves membership of that list without revealing which entry belongs to you.

Questions about anonymity remain open

Not everyone in the thread expects the change to deliver anonymity. One participant, kevino, argued that paid features attract tax, record-keeping and logging obligations that donations do not, and that Signal could be subpoenaed for metadata about specific accounts. rassilon1963 replied that Signal's client is designed not to trust the server and pointed to a public review of the protocol's cryptography. Either way, the discussion documents code in progress rather than a shipped feature: the request thread, opened in March 2018, now runs to 217 posts and about 37,000 views.