Introduction to Hash Functions
From The ECRYPT Hash Function Website
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Security Requirements
The security properties that hash functions are expected to provide, are summarized in the following three basic requirements:
- Collision resistance: it is infeasible in practice to find two messages m and m^* != m such that h(m) = h(m^*).
- Second preimage resistance: for a given message m, it is infeasible in practice to find a second message m^* != m such that h(m) = h(m^*).
- Preimage resistance: it is infeasible in practice to find, for a given hash value y, a message m such that h(m) = y.
In practice there are several other requirements, but for sake of simplicity we stick to them.