To be pedantic: given x and H(x), it is impossible to find x’ such that H(x’) == H(x) in a practical amount of time. (You need x, and an attack doesn't need to

To be pedantic: given x and H(x), it is impossible to find x’ such that H(x’) == H(x) in a practical amount of time. (You need x, and an attack doesn't need to be polynomial to break the hash function; it just needs to be fast enough, considering constant factors, to fit within some plausible attacker's computational resources.)