We are talking about obtaining a 128-bit or 256-bit seed which is then passed through an HMAC512 algorithm, the resulting hash is used to derive the private and public keys, it would not be easy or computationally feasible to perform this attack. Suffice it to say that bitcoin uses the sha256 algorithm, that is, it works with 256bit. Have you heard of bitcoin being hacked and its security broken? Surely not.