Proof of personhood

Proof of personhood is a way to confirm to a website that a living human is behind an account and that this human has only one account, ideally without saying who they are. It protects against a “Sybil attack”, in which one participant poses as many: inflating votes, collecting giveaways several times over, running armies of bots.

The term appeared in 2017 in a paper by Bryan Ford’s group at EPFL.¹ The largest project is World (Worldcoin until 10.2024), founded by Sam Altman: a special device, the Orb, scans the iris of the eye; according to the company, more than 18 million people have been verified this way.² With the arrival of generative AI and AI agents, the problem has moved beyond the crypto community: CAPTCHA no longer tells humans from machines, and websites need a different signal.

History

• 01.10.1950Alan Turing replaces the question “Can machines think?” with the “imitation game”: a machine has to pass itself off as a human in written conversation.³ ¶ ccvqhw9q7
• 2000Luis von Ahn and colleagues at Carnegie Mellon University invent CAPTCHA, the “Completely Automated Public Turing test to tell Computers and Humans Apart”.⁴⁵ ¶ cu5ug5j2c
• 2002John Douceur of Microsoft Research describes the “Sybil attack” and proves that without a central authority vouching for identities, there is no defence against it.⁶ ¶ cn8kxp88j
• 01.04.2008Bryan Ford and Jacob Strauss (MIT) propose “pseudonym parties”: people gather in one place at the same time and each receives one anonymous credential, so that “one real person gets one virtual persona”.⁷ ¶ c75h4qypc
• 29.04.2017Ford’s group coins the term proof-of-personhood: a mechanism that “binds physical entities to virtual identities” while preserving anonymity.¹ ¶ c3a2ijgwe
• 21.10.2021Worldcoin, from Sam Altman and Alex Blania, comes out of stealth with $25 million in funding and a promise to hand out tokens for iris scans.⁸ ¶ cn5cymgdm
• 24.07.2023Worldcoin launches globally: about 2 million people got a World ID during the beta.⁹ ¶ c3umipnf5
• 15.08.202432 researchers from OpenAI, Microsoft, MIT, Harvard and Oxford propose “personhood credentials”: CAPTCHA, they write, is powerless against sophisticated AI, while checking a passport reveals too much about a person.¹⁰ ¶ chypg5htt
• 17.10.2024Worldcoin becomes World and learns to verify a human not only with the Orb but also with a passport chip.¹¹¹² ¶ c7pn58tc2
• 30.04.2025World launches in the US; partners include Visa, Tinder and Stripe.¹³¹⁴ ¶ cc58j93ew
• 17.03.2026World and Coinbase release AgentKit: a person delegates their World ID to an AI agent, and a website sees that one unique human stands behind the agent.¹⁵ ¶ cgjn2vkjr
• 25.03.2026Reddit starts verifying accounts that look like bots, via passkey, Face ID or World ID. In the words of CEO Steve Huffman: “Our aim is to confirm there is a person behind the account, not who that person is.”¹⁶ ¶ cgwzu97ih

How it works

The system has to be sure of two things: it is facing a human, and that human is not registered yet. Approaches differ in what they trust.

Phone number. The most widespread check is a code by SMS. In 157 countries a SIM card is sold only with the owner’s registration,¹⁷ so a number seems tied to a person. But an SMS proves only access to the number: services sell verification codes for mass account sign-ups, and one of them, according to Trend Micro, ran on a botnet of thousands of infected Android phones.¹⁸ On top of that, a number makes it easy to link all of a person’s accounts to each other and to their passport. ¶ c8kr328th

In-person meeting and simultaneous test. A person cannot be in two places at once; that is the idea behind “pseudonym parties”.⁷ Idena takes it online: all participants solve picture puzzles in the very same minute.¹⁹ ¶ cba6y7mp3

Social graph. In BrightID, people vouch for each other, and the system analyses the resulting graph of acquaintances without storing personal data.²⁰ Proof of Humanity adds a video, a deposit and a voucher; anyone can challenge a profile, and disputes are settled by jurors.²¹ ¶ cyy6w7j7q

Biometrics. The Orb turns the iris into an “iris code” and checks whether the database already has one like it. The code is split into parts held by several independent parties, mostly universities, so nobody sees it whole.²² To a website, the user presents a zero-knowledge proof: the site learns “this is a unique human” but not which one, and cannot link them to other sites.²³ ¶ cgqepmunq

Passport and zero knowledge. The phone reads the passport chip and proves a single attribute, such as “over 18”, “citizen of country X” or “this passport has not been used here before”, without showing the rest. This is how Self, ZKPassport and the passport version of World ID work;²⁴²⁵¹² the European Commission is building its age verification app on the same principle.²⁶ ¶ cqqtb3765

Aggregators. Human Passport (Gitcoin Passport until 2025) collects “stamps” from different sources and computes a score from them; grant funds use it to check that a human is voting.²⁷²⁸ ¶ cwegwiw9p

The ideal all these approaches aim for is set out in the paper on personhood credentials: one credential per person, and on each site a separate pseudonym that cannot be linked either to other sites or to whoever issued the credential.¹⁰ ¶ chw2wvxwp

Data

Bans and court rulings

Most of the complaints against World concern collecting biometrics in exchange for tokens.

Incidents

Worldcoin: how the first users were recruited, April 2022 ¶ cbxfcdxx4

MIT Technology Review found out how Worldcoin recruited its first 450,000 people in 24 countries. People were lured to scans with cash, AirPods and the promise of tokens, without being told they were taking part in testing; in Indonesia, local officials were paid to help with recruitment. Alex Blania promised at the time: “We will let privacy experts take our systems apart, over and over, before we actually deploy them on a large scale”.⁴⁴

Stolen Orb operator passwords, May 2023 ¶ ce4e49fcn

Malware stole the passwords of Orb operators, the local contractors who scan people, and they were put up for sale on the dark web. The operator dashboard had no two-factor authentication. Worldcoin reset all passwords and said the attackers had not reached user data.⁴⁵

World ID black market, May 2023 ¶ cahf8xmmb

Buyers in China, where Worldcoin did not operate, paid $20–30 for the World IDs of people scanned in Cambodia and Africa. The company called it “a few hundred instances” and made transfers harder, but admitted that if seller and buyer have struck a deal, there is no way to stop them.⁴⁶

Criticism

Two days after Worldcoin was announced, Edward Snowden wrote that the project keeps iris hashes that will match future scans: “Don’t catalogue eyeballs.” He added: “The human body is not a ticket-punch.”⁴⁷ ¶ cw8wzdmhz

In 2023 Vitalik Buterin named four risks of biometrics: a scan can reveal whether a person has a World ID; you have to get to an Orb; there is no way to check that the device has no backdoor for creating fake identities; people can be coerced into being scanned. His conclusion: “There is no ideal form of proof of personhood”, but a world without it at all would be left to centralised identity systems and money.⁴⁸ ¶ c4figtgx5

Cryptographer Matthew Green found fewer technical problems in Worldcoin than he expected, but named the database itself as the main risk: its existence “could potentially create incentives that force the company to put this data to more dangerous use”.⁴⁹ ¶ c5nprfadb

The authors of the personhood credentials paper warn about it themselves: if a credential can be secretly lent out, “authentication farms” for bots will appear, and people without credentials will be cut off from services.¹⁰ ¶ cj8npeguq

In 2025 Buterin added that the rule “one credential per person” kills anonymity: in real life, pseudonymity usually requires several accounts. He proposes a system with no single issuer, where you can create N identities, but the cost grows as N².⁵⁰ ¶ cygu3esri

In culture

In Philip K. Dick’s novel Do Androids Dream of Electric Sheep? (1968), androids cannot be told apart from humans either by looks or by intelligence. Only their lack of empathy gives them away, and that is what the Voigt-Kampff test catches: the device tracks involuntary reactions to emotionally charged questions.⁵¹ ¶ c36x82k7c

In the film Blade Runner (1982), Deckard tests Rachael by watching her pupils. Twenty or thirty questions are usually enough; she needs about a hundred: Rachael is a replicant but believes she is human.⁵² ¶ c7iar88nu

Outlook

The main new customers are AI agents. A payment does not show how many people stand behind a swarm of agents: one person can launch thousands.¹⁵ So World promotes “human-backed agents” under the rule “one human, one agent, one allocation”; Okta and Vercel have signed on.⁵³⁵⁴ In October 2026 World puts it bluntly: “Agents that cannot answer [who is behind them] are likely to be blocked”.⁵⁵ ¶ cheb4ikj7

Ordinary services are starting to check personhood too: Zoom, Docusign and Tinder have integrated World ID,⁵⁶ and OpenAI, according to Forbes, is discussing a “humans only” social network (the company has not confirmed this).⁵⁷ There is also the opposite route, labelling bots instead of humans: from 02.08.2026 the EU AI Act requires that people know they are talking to an AI,⁵⁸ and Reddit puts a label on approved bots.¹⁶ ¶ c3zkmyesq