Does it possible to decrypt the hash from this function?
Well, you map a string of arbitrary length to a finite field, so you do have some collisions. It therefore is not always possible to 'decrypt' the hash (in the sense of getting s from h(s), h being the hash function), but you can always find a string s' that hashes to the same value, hence h(s) = h(s'). The next question would then propably be how long it takes to find such string.
greetz
Mike