code: func (p *PrivateKey) Serialize() []byte {
  	b := make([]byte, 0, PrivKeyBytesLen)
  	return paddedAppend(PrivKeyBytesLen, b, p.ToECDSA().D.Bytes())
  }
docs: // Serialize returns the private key number d as a big-endian binary-encoded
  // number, padded to a length of 32 bytes.