deriveChildNumber method
Derives a child private key specified by the index
Implementation
HDPrivateKey deriveChildNumber(int index) {
var elem = ChildNumber(index, false);
var fingerprint = _calculateFingerprint();
return _deriveChildPrivateKey(this.nodeDepth! + 1, Uint8List.fromList(this.keyBuffer), elem, fingerprint, this.chainCode, this.publicKey.getEncoded(true));
}