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Are You At Least 18 Years Old? Here’s How To Anonymously Prove It To Third-Parties Online

Most of us have encountered the infamous “Click here if you’re at least 18 years old” button online—no judgment on which site it was.

Despite its ubiquity, we all recognize that this mechanism offers little more than a speed bump for honest, rule-following seventeen-year-olds. It provides virtually no meaningful verification, yet it persists because age verification online is a technically challenging problem.
An image showing a warning and asking the user to verify that they are at least 18 years old.
We can all agree that turning eighteen years old doesn’t bestow us with some adults-only button-clicking ability, so this test doesn’t do much apart from deterring one or two very honest and patient seventeen-year-olds. (Note: Legally speaking, buttons of this variety may or may not provide some risk mitigation value to the site owner, but that topic is outside the scope of this blog post.)
 
With such an ineffective age verification mechanism in place so often, there is certainly room for an improved solution— you know, for all the ordinary seventeen-year-olds. This blog will focus on the technical challenges and propose two interesting cryptographic solutions that attempt to solve or mitigate many of the technical hurdles.

Rethinking the Problem: From Age Verification to Group Membership Proofs

While adult-content websites are the most obvious use case, age or eligibility verification is required across a much broader spectrum of online services:

  • Applying for government documents

  • Voter and residency eligibility checks

  • Purchasing age-restricted products (alcohol, tobacco, etc.)

  • Validating student, veteran, or regional affiliation

When generalized, the challenge becomes:

How can a user remotely—and privately—prove they belong to a specific group (e.g., “18+”, “resident of State X”, “graduate of University Y”)?

Unfortunately, most prior attempts either rely on shaky assumptions or require the user to expose far more identity data than necessary.

A Look at Previously Proposed Solutions

Three common concepts have been proposed:

1. Mobile phone ownership

Assumes users must be a certain age to purchase or own a phone.
This is false—and globally inequitable.

2. Bank account verification

Assumes users over 18 have credit/debit accounts, or banks can verify ages.
Again, untrue, and excludes many users.

3. Government-issued identity cards

These contain birthdate information and sometimes cryptographic signing capabilities.
However, providing this information directly sacrifices user privacy.

The fatal flaw:
All these methods reveal the user’s entire identity to the service—even when the service only needs to know a single attribute (e.g., “age ≥ 18”).

This leads us to the improved formulation:

How do we remotely and anonymously prove membership in a specific group?

Below are two cryptographic approaches that attempt to solve this.

Solution 1: Anonymous (Blinded) Certificates

In this model, a government-issued ID would contain multiple certificates, including:

  • A standard certificate (identity + attributes)

  • An anonymous certificate containing only the attributes needed for group validation (e.g., date of birth → age group)

The anonymous certificate would include no personally identifying information.
The user proves “I belong to the 18+ group” without revealing who they are.

Advantages

  • Straightforward to implement

  • Leverages existing certificate and smart card infrastructure

  • Provides a baseline level of anonymity

Limitations

  • Correlation risk: If an attacker or service provider can compare anonymous + non-anonymous certificates, anonymity collapses.

  • Multiple certificates increase user handling risk: A user may accidentally present an identifying certificate.

  • Attribute uniqueness: A birthdate or region can still uniquely identify users in small populations.

Bottom line: Anonymous certificates improve privacy but cannot guarantee it.

Additional drawbacks include the fact that each user needs multiple TLS keys and certificates, potentially leading to a user error where a user reveals their identity without intending to do so, and the reality that service providers may still be able to identify a user by their birth date and city of residence, if that information is provided in the ID card. Discovering users’ identities would be even easier with a small pool of users.
A table showing the pros and cons of the first proposed solution on how to anonymously prove one's age online

Solution 2: Ring Signatures

A ring signature proves that someone in a predefined group signed a message, but it is cryptographically impossible to determine which member.

How this applies to age verification

  1. Users form a cryptographic “ring” consisting of every user who is 18+, each with their own key pair.

  2. The user signs a challenge using a ring signature.

  3. The service verifies that:

    • The signature is valid

    • The signer is a member of the 18+ group

    • The service cannot determine which member produced the signature

This satisfies the requirement:
The verifier learns the attribute (18+) but not the identity.

Advantages

  • Strong anonymity guarantees

  • No identifying information ever exchanged

  • Resistant to correlation or database leakage

Limitations

  • Requires a publicly accessible database of eligible public keys

  • More complex for service providers to verify and manage

  • Government or trusted authority must maintain the eligibility list

Bottom line:
Ring signatures offer significantly stronger privacy—but with added complexity.

A ring signature is a type of digital signature that involves a group of users, where each individual has their own public-private key pair. A ring signature proves that one member of the group signed the data, but it is impossible to determine which individual created the signature. In other words, anyone can verify that the signature is valid and was signed by a member of the group, but they cannot discover the identity of the signer.
 
Knowing this, a remote user can form a ring of users, where every user is at least eighteen years of age. Then, the user can use a ring signature to complete a challenge-response handshake (as opposed to the standard digital signature currently used for challenge-response handshakes) with a remote service that needs to verify the user’s age.
 
At this point, the remote service can verify that someone in that specific group signed the challenge. Since every member of the group is at least eighteen, the remote service can be satisfied that the user accessing the system is at least eighteen, without knowing the identity of that user.
 
The ring signatures solution comes with the benefit of stronger security. At the same time, it’s more challenging to setup this type of solution, especially for the service provider that needs to verify the end users. Also, this solution would require a public database of government-issued certificates (potentially anonymous certificates).
A image of a table showing the pros and cons of the second proposed solution for anonymous online age verification

Future Enhancements

Both approaches can be strengthened using:

  • IP anonymization (e.g., Tor or VPN-based methods)

  • Mixing networks

  • Anonymous credentials (Idemix, U-Prove)

  • Zero-knowledge proofs (ZKPs)

These techniques can build toward fully decentralized, privacy-preserving identity systems

 

Where Garantir Fits In

While Garantir does not develop anonymizing networks or digital ID systems, we specialize in secure, high-performance cryptographic operations.

GaraTrust, our flagship platform, enables enterprises to:

  • Keep private keys secured inside HSMs or key managers

  • Enforce MFA, device authentication, approval workflows, and policy controls

  • Perform high-speed signing via client-side hashing

  • Support any public-private key use case, including:
    ✓ Code signing
    ✓ SSH
    ✓ Security signing
    ✓ S/MIME email
    ✓ Document signing
    ✓ TLS and more

GaraTrust offers the cryptographic foundation necessary for implementing secure, privacy-preserving digital signature workflows at scale.

Get in touch with the Garantir team to set up a call.

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