code: public final void update(byte b) throws SignatureException {
          if (state == VERIFY || state == SIGN) {
              engineUpdate(b);
          } else {
              throw new SignatureException("object not initialized for "
                                           + "signature or verification");
          }
      }
docs: Updates the data to be signed or verified by a byte.
  
  @param b the byte to use for the update.
  
  @exception SignatureException if this signature object is not
  initialized properly.