The Rarimo team engaged Halborn to conduct a security assessment on their zero knowledge circuits and the verifier, beginning on 23/02/2024 and ending on 08/03/2024. The security assessment was scoped to the circuits provided to the Halborn team.

2. Assessment Summary

The team at Halborn was provided two weeks for the engagement and assigned a full-time security engineer to verify the security of the zero knowledge circuits and the verifier. The security engineer is a blockchain, smart-contract, and ZK security expert with advanced penetration testing, smart-contract hacking, and deep knowledge of multiple blockchain protocols.

The purpose of this assessment is to achieve the following :

- Ensure that the circuits operate as intended.

- Identify potential security issues within the circuits.

- Ensure that the verifier operate as intended.

- Identify potential security issues within the verifier contracts.

In summary, Halborn identified some security risks that were successfully addressed by the Rarimo team.

3. Test Approach and Methodology

Halborn performed a combination of manual and automated security testing to balance efficiency, timeliness, practicality, and accuracy in regard to the scope of this assessment. While manual testing is recommended to uncover flaws in logic, process, and implementation; automated testing techniques help enhance coverage of the code and can quickly identify items that do not follow the security best practices. The following phases and associated tools were used during the assessment:

Research into architecture and purpose.
Manual code review and walkthrough.
Graphing out functionality and contract logic/connectivity/functions.
Manual testing by custom scripts.
Used circomspect, a static analyzer and linter.
Used Picus, which checks for under-constrained bugs.

3.1 Out-of-scope

External libraries and financial-related attacks.
New features/implementations after/with the remediation commit IDs.

4. RISK METHODOLOGY

Every vulnerability and issue observed by Halborn is ranked based on two sets of Metrics and a Severity Coefficient. This system is inspired by the industry standard Common Vulnerability Scoring System.

The two Metric sets are: Exploitability and Impact. Exploitability captures the ease and technical means by which vulnerabilities can be exploited and Impact describes the consequences of a successful exploit.

The Severity Coefficients is designed to further refine the accuracy of the ranking with two factors: Reversibility and Scope. These capture the impact of the vulnerability on the environment as well as the number of users and smart contracts affected.

The final score is a value between 0-10 rounded up to 1 decimal place and 10 corresponding to the highest security risk. This provides an objective and accurate rating of the severity of security vulnerabilities in smart contracts.

The system is designed to assist in identifying and prioritizing vulnerabilities based on their level of risk to address the most critical issues in a timely manner.

4.1 EXPLOITABILITY

Attack Origin (AO):

Captures whether the attack requires compromising a specific account.

Attack Cost (AC):

Captures the cost of exploiting the vulnerability incurred by the attacker relative to sending a single transaction on the relevant blockchain. Includes but is not limited to financial and computational cost.

Attack Complexity (AX):

Describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Includes but is not limited to macro situation, available third-party liquidity and regulatory challenges.

Metrics:

EXPLOITABILIY METRIC ( $m_e$ )	METRIC VALUE	NUMERICAL VALUE
Attack Origin (AO)	Arbitrary (AO:A) Specific (AO:S)	1 0.2
Attack Cost (AC)	Low (AC:L) Medium (AC:M) High (AC:H)	1 0.67 0.33
Attack Complexity (AX)	Low (AX:L) Medium (AX:M) High (AX:H)	1 0.67 0.33

Exploitability

E

is calculated using the following formula:

$E = \prod m_e$

4.2 IMPACT

Confidentiality (C):

Measures the impact to the confidentiality of the information resources managed by the contract due to a successfully exploited vulnerability. Confidentiality refers to limiting access to authorized users only.

Integrity (I):

Measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of data stored and/or processed on-chain. Integrity impact directly affecting Deposit or Yield records is excluded.

Availability (A):

Measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. This metric refers to smart contract features and functionality, not state. Availability impact directly affecting Deposit or Yield is excluded.

Deposit (D):

Measures the impact to the deposits made to the contract by either users or owners.

Yield (Y):

Measures the impact to the yield generated by the contract for either users or owners.

Metrics:

IMPACT METRIC ( $m_I$ )	METRIC VALUE	NUMERICAL VALUE
Confidentiality (C)	None (I:N) Low (I:L) Medium (I:M) High (I:H) Critical (I:C)	0 0.25 0.5 0.75 1
Integrity (I)	None (I:N) Low (I:L) Medium (I:M) High (I:H) Critical (I:C)	0 0.25 0.5 0.75 1
Availability (A)	None (A:N) Low (A:L) Medium (A:M) High (A:H) Critical (A:C)	0 0.25 0.5 0.75 1
Deposit (D)	None (D:N) Low (D:L) Medium (D:M) High (D:H) Critical (D:C)	0 0.25 0.5 0.75 1
Yield (Y)	None (Y:N) Low (Y:L) Medium (Y:M) High (Y:H) Critical (Y:C)	0 0.25 0.5 0.75 1

Impact

I

is calculated using the following formula:

$I = max(m_I) + \frac{\sum{m_I} - max(m_I)}{4}$

4.3 SEVERITY COEFFICIENT

Reversibility (R):

Describes the share of the exploited vulnerability effects that can be reversed. For upgradeable contracts, assume the contract private key is available.

Scope (S):

Captures whether a vulnerability in one vulnerable contract impacts resources in other contracts.

Metrics:

SEVERITY COEFFICIENT ( $C$ )	COEFFICIENT VALUE	NUMERICAL VALUE
Reversibility ( $r$ )	None (R:N) Partial (R:P) Full (R:F)	1 0.5 0.25
Scope ( $s$ )	Changed (S:C) Unchanged (S:U)	1.25 1

Severity Coefficient

C

is obtained by the following product:

$C = rs$

The Vulnerability Severity Score

S

is obtained by:

$S = min(10, EIC * 10)$

The score is rounded up to 1 decimal places.

Severity	Score Value Range
Critical	9 - 10
High	7 - 8.9
Medium	4.5 - 6.9
Low	2 - 4.4
Informational	0 - 1.9

5. SCOPE

Files and Repository

(a) Repository: passport-zk-circuits

(b) Assessed Commit ID: https://github.com/rarimo/passport-zk-circuits/tree/48793f924383cd11d4c3426b4697c2f9b4efa60f

- dualMux.circom
- hashLeftRight.circom
- merkleTree.circom

↓ Expand ↓

Out-of-Scope:

Remediation Commit ID:

Out-of-Scope: New features/implementations after the remediation commit IDs.

6. Assessment Summary & Findings Overview

Critical

High

Medium

Low

Informational

Security analysis	Risk level	Remediation Date
Missing constraint in the output of `passportVerificationSHA1` circuit	Low	Solved - 03/15/2024
Different Circom versions used along the project	Informational	Solved - 03/14/2024

7. Findings & Tech Details

7.1 Missing constraint in the output of `passportVerificationSHA1` circuit

// Low

Description

The output signal of the passportVerificationSHA1 circuit is formed by the following elements:

out[0] -> SHA1 hash of the in input signal.
out[1] -> not used.
out[2] -> number representation of the passport issuer code.

Both out[0] and out[2] are properly constrained, but out[1] is not constrained in any way. That makes it possible for the prover to put whatever value he wants in there. This may not pose a serious risk, but It is recommended constraining out[1] to be zero:

passportVerificationSHA1, line 50template PassportVerificationSHA1(N) {
    
    ...

    out[0] <== bits2NumHash.out;
+   out[1] <== 0;
}

BVSS

AO:A/AC:L/AX:L/C:L/I:N/A:N/D:N/Y:N/R:N/S:C (3.1)

Recommendation

It is recommended to constrain out[1] to be zero: [**passportVerificationSHA1, line 50**](https://github.com/rarimo/passport-zk-circuits/blob/48793f924383cd11d4c3426b4697c2f9b4efa60f/passportVerification/passportVerificationSHA1.circom#L50)

template PassportVerificationSHA1(N) {
    
    ...

    out[0] <== bits2NumHash.out;
+   out[1] <== 0;
}

Remediation Plan

SOLVED : The Rarimo team solved the issue by implementing the provided suggestion.

Remediation Hash

https://github.com/rarimo/passport-zk-circuits/commit/f0e8d5096c26d425571a25c1ff83f1eff5773dac#diff-26241884654ea3527f948b3332d3f586fb63fd7815a1ea6c4c7ec1267ba59242

7.2 Different Circom versions used along the project

// Informational

Description

During the code review of the one circuit, namely the SMTVerifier circuit, uses a different version of the Circom compiler (2.0.0.) instead of the one the whole project uses (2.1.6). Using the same version for all circuits is recommended, as older compiler versions may have bugs that recent releases may have fixed.

Score

AO:A/AC:L/AX:L/C:N/I:N/A:N/D:N/Y:N/R:N/S:C (0.0)

Recommendation

Using the same version for all circuits is recommended, as older compiler versions may have bugs that recent releases may have fixed.

Remediation Plan

SOLVED : The Rarimo team solved the issue by implementing the provided suggestion.

Remediation Hash

https://github.com/rarimo/passport-zk-circuits/commit/eb5bb5dd5522533c443e7c4726ea725eda81ec3e#diff-5bd3302d8df02c76860fef4213bf2d8422b29e0a23e685212a410978aa06ad5e

8. Automated Testing

Halborn used automated testing techniques to enhance coverage of certain areas of the scoped component. Among the tools used were circumspect and picus. After Halborn verified all the code and scoped structures in the repository and could compile them correctly, these tools were leveraged on scoped structures. With these tools, Halborn can statically verify security-related issues across the entire codebase.

Halborn strongly recommends conducting a follow-up assessment of the project either within six months or immediately following any material changes to the codebase, whichever comes first. This approach is crucial for maintaining the project’s integrity and addressing potential vulnerabilities introduced by code modifications.

1. Introduction
2. Assessment summary
3. Test approach and methodology

3.1 Out-of-scope

4. Risk methodology
5. Scope
6. Assessment summary & findings overview
7. Findings & Tech Details

7.1 Missing constraint in the output of `passportverificationsha1` circuit
7.2 Different circom versions used along the project

8. Automated Testing

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Passport ZK Circuits Security Assessment - Freedom Tool

Rarimo

Table of Contents

1. Introduction

2. Assessment Summary

3. Test Approach and Methodology

3.1 Out-of-scope

4. RISK METHODOLOGY

4.1 EXPLOITABILITY

Attack Origin (AO):

Attack Cost (AC):

Attack Complexity (AX):

Metrics:

4.2 IMPACT

Confidentiality (C):

Integrity (I):

Availability (A):

Deposit (D):

Yield (Y):

Metrics:

4.3 SEVERITY COEFFICIENT

Reversibility (R):

Scope (S):

Metrics:

5. SCOPE

6. Assessment Summary & Findings Overview

7. Findings & Tech Details

7.1 Missing constraint in the output of `passportVerificationSHA1` circuit

Description

BVSS

Recommendation

Remediation Plan

Remediation Hash

7.2 Different Circom versions used along the project

Description

Score

Recommendation

Remediation Plan

Remediation Hash

8. Automated Testing

Table of Contents