How to Register Signal Without Phone Number

Key Takeaways

• Signal is developing optional “numberless accounts” that will require a one-time payment to reduce spam, based on source-code references and the company’s statements.
• The feature, named “Signal Login” in unreleased code, allows users to register with a one-time in-app payment or an “account key” they already possess.
• Signal already uses zero-knowledge proofs and the Ristretto 25519 hashing scheme to protect usernames so the service cannot easily link a username back to an account.
• Automatic Key Verification, launched August 2026, introduces key transparency with three independent auditors: Signal, Cloudflare, and Trail of Bits.
• Accounts with and without phone numbers are treated as separate types; existing accounts cannot remove their number, and numberless accounts cannot add one later.

The Phone Number Has Been the Root of the Problem

For years, Signal’s phone-number requirement has been the main concern among privacy advocates. A phone number links back to a person through the SIM it is tied to, the carrier that issued it, and whoever paid for the plan. Requiring one to join a messenger promoted for private communication has always been a difficult fit.

Signal’s stance has been consistent: the phone number is not used to identify you; it is used to prevent abuse. As Signal CTO Ehren Kret explained at the FUTO “Don’t Be Evil” conference in a talk recorded in March 2026, the main reason the app still requires a number is that removing accounts that spam or abuse the service is the company’s primary method of protecting users from spam and maintaining operational stability. Phone numbers have monetary cost and limited availability, making it costly for a bad actor to create many accounts.

In February 2024, Signal made phone numbers hidden by default, introduced usernames so you can connect without sharing your number, and added a setting to block anyone from finding you by number. But the signup requirement remained: you still needed a number to register at all.

The foundation for the change has appeared in the public Signal-Server repository. Commits spotted by independent tracking site AboutSignal update registration security, account locking, and country statistics so all of it functions when an account has no number on file. One commit refuses to set a registration lock on an account without a phone number. Another records “n/a” for country and region statistics on accounts that may lack numbers. A third adds pessimistic locking of phone-number-less accounts, and a fourth blocks those accounts from performing PNI (phone number identity) key operations.

How Signal Login Works and What It Costs

The feature is called “Signal Login” in unreleased code. References in the Signal Android repository describe a purchase screen where users either make a one-time in-app payment or register with an “account key” they already have. The payment requirement directly addresses Kret’s stated challenge of finding “some way to induce cost for signing up without phone number.”

Signal is a nonprofit, and the code currently contains only example amounts, so the exact price is not yet known. The one-time fee is intended as a barrier against spam and abuse rather than a revenue source. The fees are also expected to help support Signal’s mission through the Signal Foundation.

A structural detail emerged from the server commits: “accounts without phone numbers can never get phone numbers, and accounts with them can never lose them.” Numberless accounts and phone-number accounts are two separate account types, at least in the current implementation. You cannot sign up with a number and later remove it, and you cannot start numberless and add one later. For now, the option appears limited to new sign-ups.

The “account key” path suggests a design where a user proves something about themselves without revealing their identity, which fits the use of zero-knowledge proofs.

Zero-Knowledge Proofs and What They Mean Here

Zero-knowledge proofs are a cryptographic method where one party (the prover) can show to another (the verifier) that a statement is true without revealing anything beyond the truth of that statement. In an identity context, that could mean proving “I am a legitimate human who has not mass-created accounts” without disclosing your name, location, or other identifying details.

Signal has used zero-knowledge proofs in production for years, specifically to protect usernames. In the official username announcement, Signal confirmed that usernames are protected using a “custom Ristretto 25519 hashing algorithm and zero-knowledge proofs,” so that Signal cannot easily see or generate the username of a given account. On Signal, there is no searchable directory of usernames, and the service cannot easily reverse a phone number into a username.

The same approach guides the numberless registration design. The anti-abuse problem is fundamentally a Sybil-resistance problem: how do you stop one actor from creating thousands of identities? A phone number addressed this through scarcity and cost. A zero-knowledge credential can address it through proof: a user shows they possess some scarce or costly attribute (a valid payment, a held account key) without the service learning their identity. The one-time payment is the cost signal; the cryptographic proof prevents registration from revealing identity.

Vitalik Buterin, the Ethereum co-founder, publicly supported the direction in August 2026, calling numberless registration “very welcome news.” His reasoning follows the privacy argument: reducing reliance on phone numbers lowers vulnerability to SIM-swap attacks and makes it harder for governments to pressure a platform into blocking citizens by location. But Buterin also offered a sharp caveat. In practice, he argued, pseudonymity is dead in 2026: message timing, patterns of who you message, frequency, and size all reveal identity regardless of whether a phone number is attached. The only effective form of privacy, in his view, is message-by-message unlinkability, which mixnet messengers like Session and SimpleX Chat are exploring.

Automatic Key Verification: The Other Half of the Story

The numberless registration work is arriving alongside another major cryptographic upgrade. On August 11, 2026, Signal launched Automatic Key Verification, a feature built on “key transparency” that closes a long-standing gap in end-to-end encryption.

The gap: when you send a Signal message, your app asks the Signal server for the public key associated with your contact’s phone number or username. But how do you know the server gave you the correct key? A compromised or malicious server could return a false key, causing you to encrypt your message to an attacker instead of your intended recipient. This is the “Mallory in the middle” attack that Signal’s own blog post explains in detail.

Until now, the only defense was manually comparing safety numbers in person or over a trusted secondary channel, something most people never do. Automatic Key Verification makes the key-to-identity mapping globally consistent and transparent. The system works through a chain of verifications performed by you, your Signal contacts, and third-party auditors, which together provide the same assurance as manually verifying safety numbers, without requiring an in-person meeting.

The architecture relies on three independent auditors: one operated by Signal, one by Cloudflare, and one by Trail of Bits. Each auditor keeps a copy of the public-key map as a Merkle tree and periodically signs the head of the tree with a signing key only it knows. The Signal client requires signatures from all three auditors. If a malicious server tries to show different views of the key map to different users, it can only produce a valid signature for one of them, and the split view collapses within about a week, at which point client apps begin displaying warnings.

Trail of Bits, which built its auditor from scratch based on Signal’s specification rather than reusing Signal’s reference implementation, explains the value precisely: “Encryption proves nobody read your message in transit. It never proved you were sending it to the right person.” Key transparency is the missing proof. Users can enable it under Settings, Privacy, Advanced, and verify a contact by tapping “Verify Automatically” on the safety number screen.

Trade-offs and Open Questions

The payment is the clearest trade-off. Requiring a one-time fee to exercise a privacy right has drawn criticism, with some users noting that “pay to exercise your right to privacy” is a hard sell, even if the fee is small. The counterpoint is structural: without a cost signal, numberless accounts would be easily mass-created and the entire anti-spam defense would fail. Signal is a nonprofit, so the fee is not a profit source, but the tension between accessibility and abuse-resistance is real and unresolved.

The two-account-type limitation is another restriction. Because existing accounts cannot remove their number and numberless accounts cannot add one, users who want the new option must start fresh, losing their message history and contacts. This migration cost will limit adoption among existing users.

The zero-knowledge aspect also has a nuance: Signal’s use of zero-knowledge proofs for usernames is documented and deployed, but the specific method for numberless registration, whether it uses full zero-knowledge proofs or a simpler account-key credential, has not been publicly detailed by Signal. The community forum thread that gave the feature its “zero-knowledge proofs” label points toward the direction, but the exact cryptographic design remains to be confirmed when the feature ships.

Finally, key transparency itself involves a trust boundary. The system still depends on three auditors, and a user who wants to avoid trusting Signal or any auditor can disable Automatic Key Verification and rely on manual safety-number comparison. The feature complements, rather than replaces, the existing safety-number system.

What to Watch

The numberless registration feature has merged into the Android beta 8.26 codebase but is not yet enabled, so users cannot try it. Kret said in March 2026 that Signal aims to ship phone-number-free signup “later this year.” The exact price of the one-time payment, the precise cryptographic design for the account-key path, and whether the two-account-type limitation is relaxed over time are three open questions that will shape how widely the feature is adopted.

The broader significance: Signal is challenging a core assumption of most identity systems, that a persistent, linkable identifier is necessary to control abuse. If a nonprofit messenger can combine a small payment with zero-knowledge-style proofs to offer truly numberless accounts at scale, it will set a precedent that privacy-focused services from mixnet messengers to decentralized identity projects will watch closely.

For practitioners building secure systems, the lesson is clear: anti-abuse defenses do not have to collect identity. A cost signal (payment, proof-of-work, or scarce credential) combined with a zero-knowledge proof can provide Sybil resistance while minimizing the data a service holds. Signal’s usernames already follow this pattern in production, and numberless registration is the next, more difficult test of the same idea. The broader move toward verifiable trust without identity collection is also appearing in how AI agents handle honesty and cooperation, which is worth examining if you build systems where trust must be established without a central authority.

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Sources and References

Sources cited while researching and writing this article:

• Signal Login: optional registration without a phone number will require …
• Signal >> Blog >> Introducing Automatic Key Verification