A cryptex is a type of cylindrical lock with a password, that holds secret messages on a papyrus rolled around a glass vile of vinegar. Author Dan Brown said in "The Da Vinci Code", a book from some years ago, that Leonardo da Vinci invented it to allow messengers to carry secret papyrus rolls that they themselves could not read; only the sender and the recipient knew the password. If the messenger or his captor smashed the cryptex to read the message, a vial containing vinegar which was rolled inside of the papyrus roll would break instantly spreading the vinegar over the message, dissolving the ink, and rendering the tactical information unreadable.
The term “cryptex” is actually made up by Dan Brown. We don’t really know for sure that Leonardo invented it, he never actually made one. If he had, he would have made a drawing in his treatise, but one was never found so the artist’s work is now an after thought. Most scientists now feel that the vinegars result may have been less than perfect. It doesn't dissolve the ink so much that it is unreadable, but only renders it a little more vague. It was tested with Black Indian ink, several kinds of pen inks, marker pens, ecoline pencils, and it failed to destroy the information completely just as Wikipedia reports.

In The Da Vinci Code, the cryptex was used differently than was meant by Leonardo. In the story they use it as part of a big puzzle, the answer to a riddle is the password of a cryptex, which contains another riddle that leads to another cryptex, etc. Widely held opinion is that the use of cryptexes in such a way is a bit ridiculous, the characters in the book could've put the cryptex in the freezer so that the vinegar froze, then they could saw it open without risk of damage to the information contained within; but they didn't think of that easy solution in the book.