The word cryptex is a neologism coined by the author Dan Brown for his 2003 novel The Da Vinci Code, denoting a portable vault used to hide secret messages. It is a combination of the words cryptology and codex; "an apt title for this device" since it uses "the science of cryptology to protect information written on the contained scroll or codex" (p. 199 of the novel). The first physical cryptex was created by Justin Kirk Nevins in 2004.
The (first) cryptex featured in the novel is described as a stone cylinder comprising "six doughnut-sized disks of marble [that] had been stacked and affixed to one another within a delicate brass framework"; end caps make it impossible to see inside the hollow cylinder. Each of the disks is carved with the entire alphabet and, since they can be rotated individually, the disks can be aligned to spell different five-letter words.
The cryptex works "much like a bicycle's combination lock", and if one arranges the disks to spell out the correct password, "the tumblers inside align, and the entire cylinder slides apart" (p. 200). In the inner compartment of the cryptex, secret information can be hidden, written on a scroll of thin papyrus wrapped around a fragile vial of vinegar as a security measure: if one does not know the password but tries to force the cryptex open, the vial will break and the vinegar will dissolve the papyrus before it can be read.