Irreversible Origami Envelope In Patrick Rothfuss' novel The Wise Man's Fear, the main character briefly mentions that he has a way of folding a letter to be its own envelope in such a way that it is not possible to reopen the envelope/letter without tearing it, giving a method of secure communication without sealing wax or similar. Does such a folding method actually exist?

I've done some experimenting myself, and some searching online, but the closest I've found are envelope designs that don't need tape/glue to stay closed. Nothing I've found has the key property of being impossible (or at least very difficult) to unfold again without tearing.
While not impossible to open without tearing, and requiring cuts as well as folds (which disqualifies the method as origami to an origami purist), there is a form of letterlocking that involves cutting a thin strip from the blank margin of the letter, folding the letter, and then punching holes through the letter with an awl, and threading the strip through the holes in an interlocking knot-like pattern, which would make it very difficult to open the letter without tearing the thin delicate strip of paper.

See: https://vimeo.com/189348055

http://letterlocking.org/categories states that this blank margin method of letterlocking was "used by Queen Elizabeth I of England, Mary Queen of Scots, Catherine DeMedici, and many others."