code: func (k *ecdsaPrivateKey) SKI() []byte {
  	if k.privKey == nil {
  		return nil
  	}
  
  	// Marshall the public key
  	raw := elliptic.Marshal(k.privKey.Curve, k.privKey.PublicKey.X, k.privKey.PublicKey.Y)
  
  	// Hash it
  	hash := sha256.New()
  	hash.Write(raw)
  	return hash.Sum(nil)
  }
docs: // SKI returns the subject key identifier of this key.