code: public void encode(OutputStream out) throws IOException {
          DerOutputStream tmp = new DerOutputStream();
          if (extensionValue == null) {
              extensionId = PKIXExtensions.SubjectKey_Id;
              critical = false;
              encodeThis();
          }
          super.encode(tmp);
          out.write(tmp.toByteArray());
      }
docs: Write the extension to the OutputStream.
  
  @param out the OutputStream to write the extension to.
  @exception IOException on encoding errors.