Mirror the alphabet: aleph↔tav, bet↔shin, gimel↔resh, and in general the two positions sum to 23. בבל becomes ששך (bet→shin twice, lamed→kaf), which is the substitution Jeremiah uses.
Atbash is the oldest cipher in the tradition and the only one used inside the Hebrew Bible itself: Jeremiah writes ששך for Babylon and לב קמי for the Chaldeans, veiling the name of the power he is cursing. Kabbalistically it pairs first with last, so an atbash word is read from the far end of the alphabet, and the Zohar applies the method to divine names to show a name 'facing both ways'.
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