"to succed in the attack, an event whose probability is considered to be neglible" - sry like said im not a math guru, perhaps im wrong, but how could that be something valid proven
The context is that it would be equivalent to "breaking" a hash function. That is considered impossible so showing the equivalence is a valid security proof.
find a pre-image of Hs
http://en.wikipedia.org/wiki/Preimage_attack