Atbash

IPhenomenon

Write the 26 letters in a row. Write them a second time in reverse underneath. Read vertically.

A B C D E F G H I J K L M

Z Y X W V U T S R Q P O N

The word LOOM becomes OLLN. The word BRASS becomes YIZHH. Nothing is shifted; everything is folded.

The fold has a visible side effect: letters in the middle of the alphabet swap with near neighbours. M and N trade places, L and O trade places. The further a letter sits from the centre, the further it travels — A walks the full 25 places to Z while M only walks one.

IIKey space

Zero. There is one mapping and everyone who knows the method has it.

This is worth pausing on, because the term 'key' has just been shown to be optional. A cipher can be perfectly well defined, fully reversible, and completely public. Its only protection is that most people have never heard of it.

A cipher whose secret is 'nobody has read the manual' has a key space measured in ignorance.

Modern equivalents of the same mistake: hiding an admin panel at an unlisted URL, or trusting an unpublished API endpoint. The atbash keyspace and the atbash threat model both survive into the present day.

IIIEnciphering

• Write the plaintext in capitals.
• Write the alphabet A-Z, then the same alphabet reversed beneath it.
• For each plaintext letter, find it in the top row and take the letter below it.
• Leave spaces and punctuation untouched.
• Register the result. Because the cipher is self-inverse, the same procedure decrypts it — there is no second table to learn.

IVDeciphering

• Apply the same fold. Atbash(Atbash(x)) = x by construction.
• Confirm the result reads as language before trusting it — the fold is unambiguous, so a failure means you mis-copied a letter rather than mis-keyed.
• If the text is short and ambiguous, check the middle of the alphabet first: M/N and L/O swaps are easy to verify by eye.

VWeakness

It is one substitution alphabet. Every monoalphabetic attack applies with no modification: count letters, match the profile, read the message.

The fold leaks structure. Letters near the centre of the alphabet barely move, so a word containing M and N in quantity keeps much of its shape. MNMN encrypts to NMNM — an attacker does not need to know the mapping to recognise that.

Self-inverse is not strength. Being its own inverse is an elegant property (the same tool both locks and unlocks) but it says nothing about security. ROT13 has the same property and is worth the same amount.

It is trivially recognisable. The joint appearance of many A/Z and B/Y pairs in a ciphertext is a strong hint that a mirror is in play, which is precisely what a real cryptanalyst looks for: not the message, but the method.

VIPractice

·History note

The name is a portmanteau of the Hebrew letter names Aleph, Tav, Bet and Shin — the first, last, second and last-but-one letters of the Hebrew alphabet, which is the fold written out. The technique appears in the Hebrew Bible (Jeremiah 25:26 and 51:41), in the Talmudic and Kabbalistic literature, and in the medieval Atbash commentary tradition. It also appears independently in the Roman notae system and in the Greek alphabet, which suggests that folding an alphabet is one of those ideas that occurs to people more than once.

·Further reading

Atbash survives in two places. The first is scripture: the Book of Jeremiah contains Sheshach (ששך) for Babel and Lev Kamai (לב קמי) for Chaldea, both produced by this exact fold, which is why the name comes from the Hebrew letters Aleph-Tav-Bet-Shin — the first, last, second and second-to-last letters.

The second is ROT13, which is the Caesar version of the same idea: a fixed, public, honest transformation used to hide information from the incurious rather than from an attacker.

Next on the loom: scytale leaves substitution behind entirely and rearranges the message instead of renaming its letters.

+Weave into