title: What's the difference between NOT second preimage resistant and NOT collision resistant
body: <p>By definition, Not 2nd-preimage resistant means: there exists at least one x (which is known) such that it is easy to find another x', such that h(x) = h(x').</p>
  
  <p>While, Not collision resistant indicates: it is easy to find at least one such pair (x, x') that h(x) = h(x')</p>
  
  <p>I don't see any difference here, anyone can tell? Or do I give the wrong definitions?</p>
  
  <p>And, it is said that "Not collision resistant not necessarily means Not 2nd-preimage resistant", why is that?</p>