Sprint 1 - Rate Limiting Nullifier (RLN) Protocol - MarkuSchick, Elpacos

[markus0x1]

yAcademy Rate-Limit Nullifier Review

Review Resources:

Repo

Docs

Specs

Auditors:

• @MarkuSchick
• @Elpacos

Table of Contents

1. Review Summary
2. Scope
3. Assumptions
4. Issues not addressed
5. Tools used
6. Code Evaluation Matrix
7. Findings Explanation
   • Low 1
   • Low 2
8. Final remarks
9. CircomSpect Output

Review Summary

Rate-Limit Nullifier

The main goal of RLN v2 circuits is to make it possible to have a custom amount of messages (signals) per epoch without using a separate circuit or high-degree polynomials for Shamir's Secret Sharing.

The circuits of the Rate-Limit Nullifier Github were reviewed over 15 days. The code review was performed by 1 auditor between 31st May, 2023 and 14th June, 2023. The repository was static during the review.

Scope

The scope of the review consisted of the following circuits within the repo:

• Circuits
• rln.circom
• utils.circom
• withdraw.circom

The scope of the review consisted of the following contracts at the specific commit:

37073131b9c5910228ad6bdf0fc50080e507166a

After the findings were presented to the Rate-Limit Nullifier team, fixes were made and included in several PRs.

This review is a code review to identify potential vulnerabilities in the code. The reviewers did not investigate security practices or operational security and assumed that privileged accounts could be trusted. The reviewers did not evaluate the security of the code relative to a standard or specification. The review may not have identified all potential attack vectors or areas of vulnerability.

yAcademy and the auditors make no warranties regarding the security of the code and do not warrant that the code is free from defects. yAcademy and the auditors do not represent nor imply to third parties that the code has been audited nor that the code is free from defects. By deploying or using the code, Rate-Limit Nullifier and users of the contracts agree to use the code at their own risk.

Code Evaluation Matrix

Category	Mark	Description
Mathematics	Good	Math is relative simple and well described
Complexity	Very Good	Clean and simple implementation
Libraries	Good	Uses well-tested standard library
Decentralization	Good	No privileged actors
Code stability	Good	Minimal changes
Documentation	Very Good	Full documentation and specification
Monitoring	-	-
Testing and verification	Good	High coverage, but no automated testing

Findings Explanation

Findings are broken down into sections by their respective impact:

• Critical, High, Medium, Low impact
  • These are findings that range from attacks that may cause loss of funds, impact control/ownership of the contracts, or cause any unintended consequences/actions that are outside the scope of the requirements
• Gas savings
  • Findings that can improve the gas efficiency of the contracts
• Informational
  • Findings including recommendations and best practices

---

Low Findings

REPORTED BY markus, elpacos:

1. Low - Unused public inputs ban be optimized out

As described in the 0xParc ZK Bug Tracker the circom optimizer can remove public inputs that are unused.

Technical Details

The Withdraw circuit has a public input address that is not used in any constraints. Hence, the circom optimizer might remove this variable. But the address has to be part of the proof to prevent users from front-running a withdraw transaction.

Impact

Low. Most libraries (snarkjs, arkworks) create constraints for all public inputs. We were unable to replicate this bug with snarkjs and arkworks.

Recommendation

Add a dummy constraint that uses the public input

template Withdraw() {
    signal input identitySecret;
    signal input address; 

+   signal addressSquare;
+   addressSquare <== address * address;
    signal output identityCommitment <== Poseidon(1)([identitySecret]);
}

component main { public [address] } = Withdraw();

Developer Response

2. Low - Specification uses incorrect definition of identity commitment

REPORTED BY markus:

The V2 Specification uses the identity_secret to compute the identity_commitment instead of the identity_secret_hash. The identity_secret is already used by the Semaphore circuits and should not get revealed in a Slashing event.

Technical Details

RLN stays compatible with Semaphore circuits by deriving the secret ("identity_secret_hash") as the hash of the semaphore secrets identity_nullifier and identity_trapdoor.

RLN V2 improves upon the V1 Protocol by allowing to set different rate-limits for users. Hence, the definition of the user identity changes from the V1 definition:

identity_secret: [identity_nullifier, identity_trapdoor],
identity_secret_hash: poseidonHash(identity_secret),
identity_commitment: poseidonHash([identity_secret_hash])
+rate_commitment: poseidonHash([identity_commitment, userMessageLimit])

The RLN-Diff flow wrongfully derives the identity_commitment from the identity_secret directly instead of the identity_secret_hash.

Impact

Medium. Using the identity_secret as secret value is problematic since a slasher can now compromise the semaphore identity. The official sdk implements the correct definition of the identity commitment. But an incorrect specification can lead to future implementation bugs.

Recommendation

Short term:

Modify the following part of the V2 Specification:

Registration

-id_commitment in 32/RLN-V1 is equal to poseidonHash=(identity_secret). 
+id_commitment in 32/RLN-V1 is equal to poseidonHash=(identity_secret_hash). 
The goal of RLN-Diff is to set different rate-limits for different users. It follows that id_commitment must somehow depend on the user_message_limit parameter, where 0 <= user_message_limit <= message_limit. There are few ways to do that:
1. Sending identity_secret_hash = poseidonHash(identity_secret, userMessageLimit) 
and zk proof that user_message_limit is valid (is in the right range). This approach requires zkSNARK verification, which is an expensive operation on the blockchain.
-2. Sending the same identity_secret_hash as in 32/RLN-V1 (poseidonHash(identity_secret)) 
+2. Sending the same identity_commitment as in 32/RLN-V1 (poseidonHash(identity_secret_hash)) 
and a user_message_limit publicly to a server or smart-contract where 
-rate_commitment = poseidonHash(identity_secret_hash, userMessageLimit) is calculated. 
+rate_commitment = poseidonHash(identity_commitment, userMessageLimit) is calculated. 
The leaves in the membership Merkle tree would be the rate_commitments of the users. This approach requires additional hashing in the Circuit, but it eliminates the need for zk proof verification for the registration.

Long-term:

Rename the variable identity_secret in the circuit to avoid further confusion with a variable of the same name derived from Semaphore.

Developer Response

[curryrasul]

Hi, thanks for your report!

Unused public inputs ban be optimized out

Good find! We'll add this "dummy constraint" to the withdraw circuit.

Specification uses incorrect definition of identity commitment

Spec/docs is out of date, we'll update it soon. Thanks for your review.