World
ID Nullifier Contracts

Covered end-to-end changes to World's nullifier contracts.

report id

0411-0693

1 Executive Summary

This document presents the results of the security review conducted by Nethermind Security for World's Nullifier contracts.

This audit focused on the end-to-end review of the above described changes. The audit comprises 774 lines of Solidity code. The audit was performed using (a) manual analysis of the codebase, and (b) automated analysis tools.

Audit assessment
Audit Type	Security Review
Initial Report	October 31, 2025
Final Report
Initial Commit Hash	`d3350e4c4a`
Final Commit Hash	`06d1ecb845`
Documentation Assessment	High
Test Suite Assessment	High

Total findings: 2
Fixed: 2
Acknowledged: 0
Mitigated: 0
Unresolved: 0

Findings by severity

Critical0
High0
Medium1
Low1
Informational0
Best Practices0

2 Audited Files

#	Contract	LoC
1	`contracts/src/BabyJubJub.sol`	45
2	`contracts/src/Groth16VerifierKeyGen13.sol`	191
3	`contracts/src/Groth16VerifierNullifier.sol`	145
4	`contracts/src/RpRegistry.sol`	340
5	`contracts/src/Types.sol`	53
Total		774

3 Summary of Issues

#	Finding	Severity	Update
1	Check BabyJubJub element is on curve	Medium	Fixed
2	`numPeers` should be exact	Low	Fixed

4 System Overview

The RpRegistry contract coordinates a three-round key generation process among registered OPRF peers to securely produce and register a nullifying key. The contract is initialized by an admin, who defines the set of OPRF peers and key generation parameters. Once the system is marked ready, the admin can start a new key generation session for a specific RP ID, creating a state that tracks contributions across rounds.

Following the specification in the paper "A Nullifier Protocol based on a Verifiable, Threshold OPRF", the three round contribution acts as follows:

Round 1: each peer submits a Round1Contribution containing commitment data; the contract aggregates these BabyJubJub points to build a collective key. When all peers contribute, a Round 2 event is emitted.
Round 2: peers submit ciphertexts and accompanying zero-knowledge proofs (verified via the Groth16 verifier). These contributions are validated and stored for all peers. Once every participant completes Round 2, the contract emits a Round 3 event.
Round 3: peers simply acknowledge completion; when all peers have done so, the contract finalizes the process by registering the RP’s material — an ECDSA public key and a derived BabyJubJub nullifier key — and cleaning up intermediate data.

Access control in the RpRegistry contract separates administrative and participant privileges to maintain security and order. The owner exclusively registers OPRF peers, while the KeyGenAdmin alone can start new key generation sessions. Only registered OPRF ppers may submit round contributions, verified through strict address-to-peer checks. Modifiers like onlyOwner, onlyAdmin, and isReady enforce correct authorization and initialization, and contract upgrades are restricted to the owner, ensuring controlled governance and secure operation.

The BabyJubJub contract provides arithmetic operations for the BabyJubJub elliptic curve over the BN254 scalar field. It defines core curve parameters and implements point addition and curve validation with modular arithmetic under the field modulus Q. The contract performs safe and deterministic elliptic curve computations, including modular inversion and exponentiation, to support cryptographic protocols that rely on BabyJubJub point operations.

5 Risk Rating Methodology

The risk rating methodology used by Nethermind Security follows the principles established by the OWASP Foundation. The severity of each finding is determined by two factors: Likelihood and Impact.

Likelihood measures how likely the finding is to be uncovered and exploited by an attacker. This factor will be one of the following values:

High: the issue is trivial to exploit and has no specific conditions that need to be met;
Medium: the issue is moderately complex and may have some conditions that need to be met;
Low: the issue is very complex and requires very specific conditions to be met.

When defining the likelihood of a finding, other factors are also considered. These can include but are not limited to motive, opportunity, exploit accessibility, ease of discovery, and ease of exploit.

Impact is a measure of the damage that may be caused if an attacker exploits the finding. This factor will be one of the following values:

High: the issue can cause significant damage, such as loss of funds or the protocol entering an unrecoverable state;
Medium: the issue can cause moderate damage, such as impacts that only affect a small group of users or only a particular part of the protocol;
Low: the issue can cause little to no damage, such as bugs that are easily recoverable or cause unexpected interactions that cause minor inconveniences.

When defining the impact of a finding, other factors are also considered. These can include but are not limited to data and state integrity, loss of availability, financial loss, and reputation damage. After defining the likelihood and impact of an issue, the severity is determined according to the table below.

Impact / Likelihood	Low	Medium	High
High	Medium	High	Critical
Medium	Low	Medium	High
Low	Info / Best Practices	Low	Medium
Undetermined	Undetermined	Undetermined	Undetermined

To address issues that do not fit a High, Medium or Low severity, Nethermind Security also uses three more finding severities: Informational, Best Practices, and Undetermined.

Informational findings do not pose any risk to the application, but they carry some information that the audit team intends to pass to the client formally;
Best Practice findings are used when some piece of code does not conform with smart contract development best practices;
Undetermined findings are used when we cannot predict the impact or likelihood of the issue.

6 Issues

6.1 Check BabyJubJub element is on curve

File	Fix commit	Type	Status
`RpRegistry.sol` `BabyJubJub.sol`	PR #175	Medium	Fixed

There are multiple instances where an elliptic curve point is used as input, but there is no checking if the point is actually on the curve.

Recommendation

Require that inputs to add(...), submod(...), modInverse(...) and expmod(...) are within the field and inputs in the `RpRegistry` contract that are treated as elliptic curve elements be checked to be on the curve.

6.2 `numPeers` should be exact

File	Fix commit	Type	Status
`RpRegistry.sol`	PR #175	Low	Fixed

The circuit using the number of peers requires it to be a fixed number, but the initialiser allows for any value for the number of peers that is greater than 2.

Recommendation

Restrict peers and threshold to be the corresponding values required by the circuit.

7 Documentation Evaluation

Software documentation refers to the written or visual information that describes the functionality, architecture, design, and implementation of software. It provides a comprehensive overview of the software system and helps users, developers, and stakeholders understand how the software works, how to use it, and how to maintain it. Software documentation can take different forms, such as user manuals, system manuals, technical specifications, requirements documents, design documents, and code comments. Software documentation is critical in software development, enabling effective communication between developers, testers, users, and other stakeholders. It helps to ensure that everyone involved in the development process has a shared understanding of the software system and its functionality. Moreover, software documentation can improve software maintenance by providing a clear and complete understanding of the software system, making it easier for developers to maintain, modify, and update the software over time. Smart contracts can use various types of software documentation. Some of the most common types include:

Technical whitepaper: A technical whitepaper is a comprehensive document describing the smart contract’s design and technical details. It includes information about the purpose of the contract, its architecture, its components, and how they interact with each other;
User manual: A user manual is a document that provides information about how to use the smart contract. It includes step-by-step instructions on how to perform various tasks and explains the different features and functionalities of the contract;
Code documentation: Code documentation is a document that provides details about the code of the smart contract. It includes information about the functions, variables, and classes used in the code, as well as explanations of how they work;
API documentation: API documentation is a document that provides information about the API (Application Programming Interface) of the smart contract. It includes details about the methods, parameters, and responses that can be used to interact with the contract;
Testing documentation: Testing documentation is a document that provides information about how the smart contract was tested. It includes details about the test cases that were used, the results of the tests, and any issues that were identified during testing;
Audit documentation: Audit documentation includes reports, notes, and other materials related to the security audit of the smart contract. This type of documentation is critical in ensuring that the smart contract is secure and free from vulnerabilities.

These types of documentation are essential for smart contract development and maintenance. They help ensure that the contract is properly designed, implemented, and tested, and they provide a reference for developers who need to modify or maintain the contract in the future.

8 Remarks & Test Suite

The World team provided a clear and comprehensive overview of the World nullifier's contracts during the kick-off call. This was supported with written documentation detailing the different features present in the contracts.

Test suite

19Tests passed

0Failed

The complete test output is in the full report.

General advisory to clients

As auditors, we recommend that any changes or updates made to the audited codebase undergo a re-audit or security review to address potential vulnerabilities or risks introduced by the modifications. By conducting a re-audit or security review of the modified codebase, you can significantly enhance the overall security of your system and reduce the likelihood of exploitation. However, we do not possess the authority or right to impose obligations or restrictions on our clients regarding codebase updates, modifications, or subsequent audits. Accordingly, the decision to seek a re-audit or security review lies solely with you.

Disclaimer

This report is based on the scope of materials and documentation provided by you to Nethermind in order that Nethermind could conduct the security review outlined in 1. Executive Summary and 2. Audited Files. The results set out in this report may not be complete nor inclusive of all vulnerabilities. Nethermind has provided the review and this report on an as-is, where-is, and as-available basis. You agree that your access and/or use, including but not limited to any associated services, products, protocols, platforms, content, and materials, will be at your sole risk. Blockchain technology remains under development and is subject to unknown risks and flaws. The review does not extend to the compiler layer, or any other areas beyond the programming language, or other programming aspects that could present security risks. This report does not indicate the endorsement of any particular project or team, nor guarantee its security. No third party should rely on this report in any way, including for the purpose of making any decisions to buy or sell a product, service or any other asset. To the fullest extent permitted by law, Nethermind disclaims any liability in connection with this report, its content, and any related services and products and your use thereof, including, without limitation, the implied warranties of merchantability, fitness for a particular purpose, and non-infringement. Nethermind does not warrant, endorse, guarantee, or assume responsibility for any product or service advertised or offered by a third party through the product, any open source or third-party software, code, libraries, materials, or information linked to, called by, referenced by or accessible through the report, its content, and the related services and products, any hyperlinked websites, any websites or mobile applications appearing on any advertising, and Nethermind will not be a party to or in any way be responsible for monitoring any transaction between you and any third-party providers of products or services. As with the purchase or use of a product or service through any medium or in any environment, you should use your best judgment and exercise caution where appropriate. FOR AVOIDANCE OF DOUBT, THE REPORT, ITS CONTENT, ACCESS, AND/OR USAGE THEREOF, INCLUDING ANY ASSOCIATED SERVICES OR MATERIALS, SHALL NOT BE CONSIDERED OR RELIED UPON AS ANY FORM OF FINANCIAL, INVESTMENT, TAX, LEGAL, REGULATORY, OR OTHER ADVICE.

WorldID Nullifier Contracts

1

Executive Summary

2

Audited Files

3

Summary of Issues

4

System Overview

5

Risk Rating Methodology

6

Issues

6.1 Check BabyJubJub element is on curve

6.2 numPeers should be exact

7

Documentation Evaluation

8

Remarks & Test Suite

World
ID Nullifier Contracts

6.2 `numPeers` should be exact