The word cryptex is a neologism coined by the author Dan Brown for his novel The Da Vinci Code, denoting a portable vault used to hide secret messages. It is a portmanteau of 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) - although actually a codex is a flat book.

The (first) cryptex featured in the novel is described as a stone cylinder made up of "five 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 pry the cryptex open by force, the vial will break and the vinegar will dissolve the papyrus before it can be read. (It is not clear how effective vinegar would really be; while liquids certainly damage ancient documents, they would not necessarily render them instantly illegible. See [1] for a Gothic document which was immersed in a flood.)

In the main part of Brown's novel, the characters (while pursued by various sinister agencies) are trying to access vital information by figuring out the passwords that will open two different cryptexes, one hidden within the other to provide extra security.

Some fans of the best-selling novel, wishing to construct a real cryptex, have tried to come up with the blueprints for one. It is claimed in the novel that the original design came from the secret diaries of Leonardo da Vinci; whether this claim is based on fact remains undetermined.

In the modern-day world, such a device would in any case provide poor security. Modern scanning methods (e.g. ultrasound or X-rays) could be used to display the inner mechanisms of the cryptex, revealing how it must be aligned to open it. Another possibility, which the characters of Brown's novel never think of, would be to simply place the cryptex in a freezer so that the vinegar would freeze to ice. (The freezing point of vinegar depends on the strength, but is at most 2°C below zero.) Thereafter one could smash open the cryptex without risking that the vinegar would dissolve the papyrus hidden within.