Readers of Dan Brown's blockbusting thriller, "The Da Vinci Code," will recall that the atbash cipher plays a crucial role in the verse clue that Jacques Saunière constructs as the key to the outer cryptex. (If you don't know what I'm talking about, don't worry and read on.)

Dan Brown's knowledge of Judaism is highly unreliable, but this happens to be one of the few things he got right.

The atbash cipher is a Hebrew code, which works by substituting the first letter of the aleph bet for the last, the second for the second last, and so on. So aleph becomes tav, bet becomes shin, hence the name, atbash.

The Talmud discusses the method and gives a mnemonic for recalling the substitutions (Shabbat 104a).

According to the Talmud, there are atbash-coded words in the Bible itself. Jeremiah (25:26) says that, at last, the king of Sheshach will be forced to drink from the cup of divine wrath.

Sheshach is not a town of which the Talmud (Megillah 6a) has heard, and it states that Sheshach is, in fact, Bavel, or Babylon. Rashi explains that this based on atbash substitution. (Take a pen and paper and try it: it works.)

If Jeremiah encodes the name Bavel in this prophecy against the city, it may be because the book was written under Babylonian rule.