Question

For hash function, what’s the difference for collision protection and preimage protection?

Answer 1

from wikipedia: http://en.wikipedia.org/wiki/Cryptographic_hash_function

Properties

Most cryptographic hash functions are designed to take a string of any
length as input and produce a fixed-length hash value. A cryptographic
hash function must be able to withstand all known types of
cryptanalytic attack. As a minimum, it must have the following
properties:

Preimage resistance Given a hash h it should be difficult to find any message m such that h = hash(m). This concept is related to that of one-way
function. Functions that lack this property are vulnerable to preimage
attacks.

Second-preimage resistance Given an input m₁ it should be difficult to find another input m₂ — where m₁ != m₂ — such that hash( m₁ ) = hash( m₂ ). This property is
sometimes referred to as weak collision resistance, and functions that
lack this property are vulnerable to second-preimage attacks.

Collision resistance It should be difficult to find two different messages m₁ and m₂ such that
hash( m₁ ) = hash( m₂ ). Such a pair is called a cryptographic hash
collision. This property is sometimes referred to as strong collision
resistance. It requires a hash value at least twice as long as that
required for preimage-resistance, otherwise collisions may be found by
a birthday attack.