private synchronized MAC getHMACSHA1(){
    if(hmacsha1==null){
      try{
        Class c=Class.forName(jsch.getConfig("hmac-sha1"));
        hmacsha1=(MAC)(c.newInstance());
      }
      catch(Exception e){ 
        System.err.println("hmacsha1: "+e); 
      }
    }
    return hmacsha1;
  }