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.
test
gives gvhg
x123 yes
gives c123b vh
gvhg
gives test
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
- text
to your list of dependencies in package.yaml.Data.Text
in the following
way:import qualified Data.Text as T
import Data.Text (Text)
Text
type e.g. decode :: Text -> Text
and refer to
Data.Text
combinators as e.g. T.pack
.Data.Text
.String
with Text
in Atbash.hs, i.e.:decode :: Text -> Text
decode cipherText = ...
encode :: Text -> Text
encode plainText = ...
This part is entirely optional.
Sign up to Exercism to learn and master Haskell with 107 exercises, and real human mentoring, all for free.