Understanding Pedersen Hash Collisions in Cairo: A Security Guide

3 min readOct 23, 2024

Starknet’s rapid growth has brought exciting possibilities for scalable blockchain applications. However, with this growth comes the responsibility to understand and address potential security vulnerabilities. Today, we’ll examine one of the more nuanced security considerations in Cairo: Pedersen hash collisions.

The Foundation: Pedersen Hash in Cairo

Pedersen hash is a fundamental component of Cairo’s architecture, used extensively throughout the ecosystem for:

Storage slot computation,
Event emission,
Message hashing,
Signature verification,
State transitions.

Its popularity stems from its efficiency in zero-knowledge proof generation. However, this same property introduces subtle security implications that developers need to understand.

The Technical Challenge

Let’s look at a typical Cairo contract pattern:

While this looks secure at first glance, it relies on Pedersen hashing under the hood for storage mapping. The actual process looks more like:

The Vulnerability Explained

Pedersen hash, while perfect for zero-knowledge applications, isn’t classically collision-resistant. This means that theoretically:

For inputs a and b where a ≠ b:
pedersen(a) = pedersen(b)

This creates several potential attack vectors:

1. Storage Manipulation

An attacker finding a collision could access or modify storage slots they shouldn’t have access to.
Multiple data entries could unknowingly share the same storage slot.

2. Signature Forgery

Message verification systems relying solely on Pedersen hashing could be compromised.
Cross-chain message verification is particularly vulnerable.

3. State Confusion

State transitions could be manipulated if hash collisions are found.
Merkle tree implementations could be compromised.

Real-World Implications

Consider this vulnerable implementation:

An attacker who finds a collision could potentially access secrets stored under different keys.

Mitigation Strategies

1. Composite Key Hashing

Instead of using single values as keys, combine multiple elements:

2. Input Validation

Implement strict input validation:

3. Multi-Layer Hashing

Combine different hash functions for critical operations:

4. Bounded Storage Pattern

Implement storage patterns that limit the impact of potential collisions:

Best Practices

1. Input Boundaries

Always validate and bound input values.
Consider the practical limitations of your storage needs.

2. Defense in Depth

Don’t rely solely on Pedersen hash for critical security operations.
Implement multiple layers of validation.

3. Monitoring and Alerts

Implement event emission for sensitive operations.
Monitor for unusual patterns in storage access.

4. Testing

Create specific test cases for hash collision scenarios.
Implement fuzzing tests to detect potential collisions.

Conclusion

While Pedersen hash collisions in Cairo represent a theoretical vulnerability, proper implementation of security measures can effectively mitigate the risks. As the ecosystem grows, it’s crucial for developers to understand these nuances and implement appropriate safeguards.

Remember: Security isn’t about eliminating all risks, but about understanding and managing them effectively. Stay vigilant, implement proper controls, and keep learning as the ecosystem evolves.

This article is part of our ongoing commitment to securing the Starknet ecosystem. Follow us for more security insights and best practices in Cairo development.

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