What is collision resistance?

Collision resistance refers to a desired property of cryptographic hash functions where it is difficult to find two different input values that result in the same hash value output. This makes it infeasible for attackers to find inputs that collide or match the same hash.

Hash functions with strong collision resistance are crucial for blockchain security and data integrity verification in many cybersecurity applications.

Collision resistance allows hash functions to be used as the basis for efficient probabilistic data structures like Count Min Sketches for analytics use cases such as data pruning.

What does it do/how does it work?

Collision resistant hash functions rely on the avalanche effect where even a small change in the input results in significant unpredictable changes to the output hash. This makes intentionally generating colliding hashes practically impossible, especially as the output space grows larger.

Cryptographic hash algorithms like SHA-256 are designed to provide near-optimal collision resistance based on mathematical properties for the given output size. Security rests on computational hardness assumptions.

Why is it important? Where is it used?

Collision resistance is vital for security properties like data integrity, digital fingerprints, message authentication codes (MAC), cryptocurrencies, and digital signatures which rely on the uniqueness of hash values.

It prevents tampering with data by ensuring any changes result in a different hash. Colliding hashes would undermine certificate authorities, cryptocurrency ledgers, file verification, and other applications relying on hash uniqueness.

FAQ

How is collision resistance measured?

Collision resistance is measured by the difficulty of finding two inputs with the same cryptographic hash value. Key metrics include:

• Collision probability: Likelihood of randomly finding a collision
• Work factor: Computational effort required to find a collision by brute force
• Collision attack complexity: Fastest known computational complexity to find a collision

What affects the collision resistance of a hash function?

The collision resistance depends on mathematical properties and implementation choices:

• Digest size: Larger output size reduces collision probability
• Internal design: Structures like Merkle–Damgård construction improve resistance
• Randomization: Random salts/initialization vectors make collisions harder
• Security margin: Output much larger than the input number of bits

What breaks collision resistance?

Despite mathematical assumptions, weaknesses can still emerge to break collision resistance:

• Mathematical flaws in the design such as in MD5 and SHA-1 hashes
• Shorter digest size reduces collision resistance due to the birthday attack
• Rainbow tables and precomputed databases of hashes
• Quantum computing may undermine collision resistance of current hashes

How can collision resistance be improved?

Strategies to strengthen collision resistance include:

• Using hashes with larger output sizes like SHA-256+
• Adding random salts to inputs before hashing
• Switching to quantum-resistant hash algorithms like SHA-3
• Combining multiple hash functions in a hash tree

References:

• [Book] Real-World Cryptography by Manning Publications
• [Book] Encyclopedia of Cryptography and Security
• [Article] Developing a New Collision-Resistant Hashing Algorithm
• [Article] Cryptographic Hash-Function Basics: Definitions, Implications, and Separations for Preimage Resistance, Second-Preimage Resistance, and Collision Resistance