Multicollision
A multicollision is when k distinct inputs to a hash function map to the same hash value; Joux introduced it to analyze iterated hash security.
Purpose
It analyzes the security degradation of iterated hash functions (Merkle-Damgård) under multicollisions, with consequences for cascaded hash designs.
Mechanism
Joux showed that for an n-bit iterated hash, building a 2^k-way collision takes only k×2^(n/2) operations, so cascading two n-bit hashes yields security of only 2^(n/2)+2^(n/2) rather than 2^n.
Use cases
- Iterated hash security analysis
- Cascaded hash security assessment
- Hash function design