About convert from BASE64URL to BASE64.

func base64(from base64url: String) -> String {
  var base64 = base64url.replacingOccurrences(of: "-", with: "+").replacingOccurrences(of: "_", with: "/")
  if base64.count % 4 != 0 {
    base64.append(String(repeating: "=", count: 4 - base64.count % 4))
  }
  return base64
}