https://medium.com/canopas/golang-handling-appstore-server-to-server-v2-notifications-ef1e8eb05118- extract_header

app_store-extract-header.go

// prepare header structure contains algorithm and token type
type NotificationHeader struct {
	Alg string   `json:"alg"`
	X5c []string `json:"x5c"`
}

// extract header from given JWS formatted token
func extractHeaderByIndex(tokenStr string, index int) ([]byte, error) {

	tokenArr := strings.Split(tokenStr, ".")  // get header from token
  
	headerByte, err := base64.RawStdEncoding.DecodeString(tokenArr[0])  //convert header to byte
	if err != nil {
		return nil, err
	}

	var header NotificationHeader
	err = json.Unmarshal(headerByte, &header)  // bind byte to header structure
	if err != nil {
		return nil, err
	}

	certByte, err := base64.StdEncoding.DecodeString(header.X5c[index])  //decode x.509 cerificate headers to byte
	if err != nil {
		return nil, err
	}

	return certByte, nil
}