code: public KeyIdentifier getSubjectKeyId() {
          SubjectKeyIdentifierExtension ski = getSubjectKeyIdentifierExtension();
          if (ski != null) {
              try {
                  return (KeyIdentifier)ski.get(
                      SubjectKeyIdentifierExtension.KEY_ID);
              } catch (IOException ioe) {} // not possible
          }
          return null;
      }
docs: Returns the subject's key identifier, or null