The cipher used by the Zodiac killer in the 1970s. Zodiac Magic Leap, a billion-dollar startup building a pair of AR goggles that combine the real world with computer images, may be the most secretive tech company besides Apple.

Nobody has ever taken a demo of its technology without signing an NDA first.

In fact, its employees are so paranoid they keep their own notes in code, so that people can't look over their shoulder while they're writing.

Adrian Kaehler, a former vice president at Magic Leap, is currently suing Magic Leap for what he claims was wrongful termination, and he is being sued by Magic Leap for allegedly stealing trade secrets.

In a legal document, he says that he had a special notebook for the most sensitive Magic Leap ideas.

"Until July 2015, all of this work was done in detailed physical notebooks, whose dated entries not only catalog work, but also show the breakdown of that work at relatively high temporal resolution," Kaehler writes, saying that he's turned over all of his notebooks to his lawyers, because of the lawsuit.

"I kept the notebook used for Magic Leap's most confidential information in a cipher for Magic Leap's protection to preclude 'casual' perusal by strangers of the document either while I was writing in it, or if someone should have momentary access to it for some reason," he continued.

He does not explain which manual encryption technique he used or if coding his notes into a cipher slowed him down.

Magic Leap declined to comment, citing pending litigation.

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