let child_ix = self.session_child_index.fetch_add(1, Ordering::AcqRel);
let child_privkey = self.session_master_key.ckd_priv(&self.secp_ctx, ChildNumber::from_hardened_idx(child_ix as u32)).expect("Your RNG is busted");
sha.input(&child_privkey.secret_key[..]);
let child_ix = self.session_child_index.fetch_add(1, Ordering::AcqRel);
let child_privkey = self.session_master_key.ckd_priv(&self.secp_ctx, ChildNumber::from_hardened_idx(child_ix as u32)).expect("Your RNG is busted");
sha.input(&child_privkey.secret_key[..]);