I've been reading about HD wallet derivation key mechanism. I understood that if an attacker gets somehow a child's private key plus the chaincode used to derive that child' key, then he could calculate the parent's private key. How could that be possible? Through a sort of brute force attack?
Furthermore, how can the hardened derivation scheme neutralize this problem by using the parent's private key instead of his public key as input for the HMAC-SHA512 functions?