I'm not planning to use them.
Reply / PM if you're interested!

Seriously, if someone has a technical issue with bitcoin they shouldn't be forced to post that issue on newbie forum because no one will reply here.

Will definitely give this a try.

We're testing how things work here e.g. how forum posts work

ok so i found the way to do it:
when deriving key K_i from (K_par,c_par) you actually calculate I_L and since k_i=I_L+k_par then you can easily get k_par=k_i-I_L

I hate being stuck on newbie board

In BIP 0032 two key derivation functions are introduced. These functions can be used to derive deterministically private keys k_i knowing an extended private key denoted (k_par, c_par) and correspondingly public keys K_i from extended public key (K_par, c_par).

Knowing extended public key it is not possible to generate private keys however this BIP briefly mentions that knowing extended public key (K_par, c_par) and a derived private key k_i (could be at any depth since public keys can be derived to any depth but let's say it's at depth 1) allows recovering private key k_par which is terrible (security-wise).

I tried analyzing how is that possible and i can't see how (isn't the HMAC-SHA512 function supposed to make it impossible to go 'back the chain')?

On a side note how can we teach a random newbie (apart from posting on newbie board) about that security risk before they start using deterministic wallets? (or should we even care?)