Atbash is a monoalphabetic substitution cipher originally used for the Hebrew alphabet, where the first letter is replaced by the last, the second by the second-to-last, and so on. Because it is its own inverse, encoding and decoding are the same operation — apply twice and you're back where you started.
Atbash dates back to around 500 BCE and appears several times in the Hebrew Bible, where it was used as a simple code. Because the mapping is fixed, it offers no real security — a frequency analysis cracks it instantly. Still, it's a useful demonstration of symmetric substitution and a fun pattern to spot in puzzles and capture-the-flag challenges. The name "Atbash" itself is a mnemonic: aleph-taw-bet-shin — the first letter paired with the last, the second with the second-to-last.