Signal will use zero-knowledge proofs for phone-number-free registration

Signal's plan to let people register without a phone number will rely on zero-knowledge proofs, according to a long-running thread on the Signal community forum that surfaced on the front page of Hacker News. The discussion covers both how the cryptography fits into Signal's existing architecture and the questions the feature still has to answer before it ships.

How zero-knowledge proofs fit in

A zero-knowledge proof lets one party demonstrate that something is true — a value is well-formed, unique, or inside allowed bounds — without revealing the value itself. Writing in the thread under the alex-signal account, Alex Hart pointed to username handling as a concrete example: the same family of techniques can check that a username's character set and length are valid while keeping its contents hidden from the server. Hart, who admitted the underlying mathematics is well beyond his own expertise, was responding to a user who questioned whether the approach could be trusted.

Not Signal's first zero-knowledge deployment

Another participant, Avitus, argued that the technique is already woven through the app. According to Avitus, zero-knowledge proofs sit behind Signal's donation badges, its paid backups, and its group system, and the server can no more link a person to a specific donation than it can to a specific group. Avitus also made a broader architectural point: the Signal client is deliberately written so that it does not trust the server, which means the security argument can be made from the client side alone. For users worried that removing phone numbers weakens verification, the thread's defenders frame it as the opposite — a way to prove eligibility without handing over the identifier at all.

Open questions around existing accounts

Much of the recent discussion concerns people who already registered with a number. One user, signalOne, asked whether an existing account can simply unlink its phone number without re-registering; the visible portion of the thread contains no definitive answer.

Another participant, zetabeta, pressed for technical specifics: whether the phone number would be fully separated from the username, and what traces would survive on the device and on Signal's servers after an unlink. Zetabeta's suspicion is that the separation would not be total, and that creating a fresh account without a number might offer cleaner privacy guarantees than unlinking an old one.

Abuse prevention emerged as the sharpest sticking point. Desert_nip noted that if unlinking were free and unrestricted, a spammer could register with a single number, unlink, and repeat indefinitely to mint unlimited accounts, so some friction such as a cooldown would be necessary. Zetabeta countered that even a month-long cooldown might not deter determined abusers, while also observing that the waiting period cannot be stretched indefinitely without hurting legitimate users. Signal has not publicly detailed how it intends to resolve this tension.

Why it matters

Phone numbers have been Signal's identity anchor since the service launched, and they are exactly the kind of metadata that enables account enumeration, correlation, and social graph reconstruction. Moving registration to zero-knowledge proofs would let Signal enforce its rules — one account per person, well-formed identifiers, abuse limits — while learning nothing it does not strictly need. The organization also has working precedent, since the same cryptographic primitive already runs groups, badges, and backup payments at scale. The unresolved challenge, as the forum exchange makes clear, is balancing anonymity against spam resistance, and that trade-off will likely determine how useful number-free registration turns out to be in practice.