Atbash Cipher
Atbash Cipher

Atbash Cipher



Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.

The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards. The first letter is replaced with the last letter, the second with the second-last, and so on.

An Atbash cipher for the Latin alphabet would be as follows:

Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: zyxwvutsrqponmlkjihgfedcba

It is a very weak cipher because it only has one possible key, and it is a simple mono-alphabetic substitution cipher. However, this may not have been an issue in the cipher's time.

Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, leaving numbers unchanged, and punctuation is excluded. This is to make it harder to guess things based on word boundaries. All text will be encoded as lowercase letters.


  • Encoding test gives gvhg
  • Encoding x123 yes gives c123b vh
  • Decoding gvhg gives test
  • Decoding gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt gives thequickbrownfoxjumpsoverthelazydog

You need to implement the decode and encode functions, which decode and encode a String using an Atbash cipher. You can use the provided signature if you are unsure about the types, but don't let it restrict your creativity.

This exercise works with textual data. For historical reasons, Haskell's String type is synonymous with [Char], a list of characters. For more efficient handling of textual data, the Text type can be used.

As an optional extension to this exercise, you can

import qualified Data.Text as T
import           Data.Text (Text)
  • use the Text type e.g. decode :: Text -> Text and refer to Data.Text combinators as e.g. T.pack.
  • look up the documentation for Data.Text.
  • replace all occurrences of String with Text in Atbash.hs, i.e.:
decode :: Text -> Text
decode cipherText = ...

encode :: Text -> Text
encode plainText = ...

This part is entirely optional.


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