code: public Extension get(String alias) {
          X509AttributeName attr = new X509AttributeName(alias);
          String name;
          String id = attr.getPrefix();
          if (id.equalsIgnoreCase(X509CertImpl.NAME)) { // fully qualified
              int index = alias.lastIndexOf(".");
              name = alias.substring(index + 1);
          } else
              name = alias;
          return map.get(name);
      }
docs: Get the extension with this alias.
  
  @param alias the identifier string for the extension to retrieve.