How to generate a Child Public Key in HD wallets?

Question

is the first param of CKD must be parent private key ? if i want to extended public key.

can i do it like

ChildPublicKey = CKD(parent_public_key,parent_chain_num,child_index_num)[0:256]

or must use parent_private_key as param like

ChildPrivateKey = CKD(parent_private_key,parent_chain_num,child_index_num)[0:256]

ChildPublicKey = Elliptic_Curve_algorithm(ChildPrivateKey)