Clear DefinitionsCollision Resistance
1 min read
Pronunciation
[kuh-lizh-uhn ri-zis-tuhns]
Analogy
Imagine a perfect fingerprinting system for documents. Collision resistance means it's practically impossible to find two different documents that somehow end up having the identical fingerprint. If you find two documents with the same fingerprint, you can be almost certain they are the same document.
Definition
Key Points Intro
Collision resistance is a critical security property for hash functions used in digital signatures and data integrity checks.
Key Points
It should be computationally hard to find any two distinct inputs m1 and m2 such that H(m1) = H(m2).
Stronger than second preimage resistance.
Essential for the security of digital signatures (prevents forging a signature for a different message with the same hash).
Vulnerabilities in collision resistance (like in MD5 and SHA-1) have led to their deprecation for many uses.
Example
If a hash function used for digital signatures lacked collision resistance, an attacker could create a malicious document that has the same hash as a legitimate document. They could then get the legitimate document signed, and substitute the malicious one, which would still validate with the same signature.
Technical Deep Dive
The difficulty of finding a collision is related to the hash function's output size. Due to the birthday problem (or birthday attack), finding a collision in a hash function with an n-bit output takes roughly 2^(n/2) operations on average. Therefore, a 160-bit hash function (like SHA-1, now broken) offers about 80 bits of collision resistance, while a 256-bit hash function (like SHA-256) offers about 128 bits of collision resistance, which is currently considered strong. Attacks that find collisions faster than this brute-force bound indicate a weakness in the hash function.
Security Warning
Using hash functions with known collision vulnerabilities (e.g., MD5, SHA-1) for security applications like digital signatures or certificate issuance is dangerous and can lead to forged documents or compromised trust.
Collision Resistance - Related Articles
No related articles for this term.