Atbash Cipher Decoder

Jeremiah 25:12 refers plainly to "the king of Babylon", and the Hebrew spells Babel as bet-bet-lamed. Fourteen verses later, 25:26 refers to the king of Sheshach - a kingdom nobody has ever located.

Run those Hebrew letters through Atbash and Sheshach is Babel. The Jewish Publication Society translation on Sefaria footnotes the word directly: "Sheshach A cipher for Babel 'Babylon.'" Why one verse names the empire and the other encodes it, we can't tell you, and neither can the footnote.

Key Takeaways

• Atbash folds the alphabet in half: first letter to last, second to second-last, and on to the middle.
• Hebrew has 22 letters, so position n becomes 23 - n.
• Jeremiah 25:26 uses Sheshach where the plain reading is Babel; Sefaria's translation footnotes it as a cipher.
• The name is the rule: A-T, B-Sh - aleph-tav, bet-shin - the first two pairs.
• It has no key, so there's nothing to guess. Knowing the method is knowing the message.

How does Atbash work?

Fold the alphabet in half and swap. First letter becomes last, second becomes second-last, and you keep going until you meet in the middle.

In Hebrew's 22 letters that's a clean bit of arithmetic: a letter at position n moves to position 23 - n. Aleph is 1, so it goes to 22, which is tav. Bet is 2, so it goes to 21, which is shin. Those first two swaps are where the name comes from: Aleph-Tav, Bet-Shin. Atbash.

English works the same way with 26 letters and the rule 27 - n. A becomes Z, B becomes Y, and M and N swap in the middle. Type anything into the box above and watch it happen, or compare it with A1Z26, which numbers the same alphabet without reversing it.

Does the Babel to Sheshach swap actually check out?

It does, and you can do it on your fingers in about twenty seconds.

Babel in Hebrew is three letters: bet, bet, lamed - you can read them in Jeremiah 25:12, which spells the name out in the clear. Bet sits at position 2 of the standard 22-letter order and lamed at 12, and you can count that order off yourself.

Letter	Position	23 - n	Becomes
bet	2	21	shin
bet	2	21	shin
lamed	12	11	kaf

Shin, shin, kaf. Which is Sheshach, written with the final form of kaf that Hebrew uses at the end of a word.

We ran that mapping ourselves against the 22-letter alphabet before writing it down, because "an ancient text contains a secret code" is exactly the sort of claim that gets repeated without anyone checking. This one holds.

Why would a prophet bother?

We don't know, and the text doesn't say, so treat what follows as the usual guesses rather than the record.

The obvious reading is caution: naming the empire that has just conquered you carries risks that a substituted word does not. The other reading is literary, because a reader who spots the swap gets a small jolt of recognition that a plain noun wouldn't deliver.

What's worth noticing is what the cipher is not doing. It isn't hiding the message from a determined reader, because Atbash has no key and anyone who knows the trick reads it instantly. It's more like writing a name in a way that needs one extra step, which is a different job from secrecy.

Why is Atbash the weakest cipher we have?

Because there's no key. That's the whole answer.

A Caesar shift has 25 possible settings, which is a small number but it's a number. Atbash has one arrangement, fixed by the alphabet itself. Knowing the method is knowing the message, and it's its own inverse, so encoding and decoding are the same operation.

That makes it useless for secrecy and rather good for something else: it's the simplest possible demonstration that substitution preserves structure. Run a long paragraph through it and the letter shapes change while the letter pattern doesn't. Double letters stay double, word lengths stay identical, and the frequency profile just gets relabelled. That's the property frequency analysis attacks: al-Kindi set the method out in a treatise of around 850 AD (Simon Singh), and it's why the Caesar cipher fell to it too.

Frequently asked questions

What does Atbash mean?

The name is the first two swaps of the Hebrew version: Aleph-Tav and Bet-Shin. Aleph is the first letter and tav the last; bet is second and shin second-last. The name is a worked example of the rule.

Where does Atbash appear in the Bible?

Jeremiah 25:26 refers to "the king of Sheshach". The Jewish Publication Society translation on Sefaria footnotes the word as "A cipher for Babel 'Babylon.'" Jeremiah 25:12, in the same chapter, spells Babel in the clear, and the Hebrew letters of one map to the other under Atbash.

Is Atbash secure?

No. It has no key at all, so there is nothing to guess. One fixed arrangement, publicly known, and it is its own inverse. Treat it as a puzzle or a demonstration rather than as protection.

How do you do Atbash in English?

Subtract the letter's position from 27. A is position 1, so it becomes position 26, which is Z. B becomes Y, C becomes X, and M and N swap in the middle. Applying it twice returns your original text.

Is Atbash the same as ROT13?

No, though both are their own inverse. ROT13 shifts every letter 13 places along the alphabet and keeps the order intact. Atbash reverses the alphabet, so the letters run backwards rather than sliding.

Paste something into the box above and watch a paragraph turn inside out. Then try the rest of the ciphers and see which ones actually hide anything.

Sources

• Jeremiah 25:26, Jewish Publication Society translation, Sefaria - retrieved 30 July 2026, https://www.sefaria.org/Jeremiah.25.26
• Jeremiah 25:12 (Hebrew, for the spelling of Babel), Sefaria - retrieved 30 July 2026, https://www.sefaria.org/Jeremiah.25.12
• Simon Singh, Arab Code Breakers (for al-Kindi and frequency analysis) - retrieved 30 July 2026, https://simonsingh.net/media/articles/maths-and-science/arab-code-breakers/