In Wikipedia's bibliography of Martin Kruskal it is said:
Martin, who had a great love of games, puzzles, and word play of all kinds, also invented several quite unusual origami models including an envelope for sending secret messages (anyone who unfolded the envelope to read the message would have great difficulty refolding it to conceal the deed).
Unfortunately, this is the only information in Wikipedia about Kruskal's secret message envelope.
Does anyone know the history of this envelope, or where I can find more information about it?
This envelope seems like a puzzle, since there may have been a clever trick to refold it.