How get fingerprint after derive parent key?

Question

After derive parent key i have IL, IR and pIL

Then i get ki:

curve_n = 'fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141';

ki = remainder((pIL + priv), curve_n);

I know that parentFingerpring is:

parentFingerprint = slice(rdm160(sha256(UNKNOW)), 4);

what varable or operation must be instead of UNKNOW?

Answer 1

UNKNOWN is the serialized compressed public key for ki. ki is the private keys, so compute the public key for it and serialize it in compressed form.