There are many competing explanations for myrtus, which could simply signify myrtle, a plant important to many cultures in the region. But some security experts see the reference as a signature allusion to Esther, a clear warning in a mounting technological and psychological battle as Israel and its allies try to breach Tehran’s most heavily guarded project. Others doubt the Israelis were involved and say the word could have been inserted as deliberate misinformation, to implicate Israel.

“The Iranians are already paranoid about the fact that some of their scientists have defected and several of their secret nuclear sites have been revealed,” one former intelligence official who still works on Iran issues said recently. “Whatever the origin and purpose of Stuxnet, it ramps up the psychological pressure.”

So a calling card in the code could be part of a mind game, or sloppiness or whimsy from the coders.