

Task

Implement a Caesar cipher, both encoding and decoding. 

The key is an integer from 1 to 25.

This cipher rotates (either towards left or right) the letters of the alphabet (A to Z).

The encoding replaces each letter with the 1st to 25th next letter in the alphabet (wrapping Z to A).

So key 2 encrypts "HI" to "JK", but key 20 encrypts "HI" to "BC".

This simple "mono-alphabetic substitution cipher" provides almost no security, because an attacker who has the encoded message can either use frequency analysis to guess the key, or just try all 25 keys.

Caesar cipher is identical to Vigenère cipher with a key of length 1. 

Also, Rot-13 is identical to Caesar cipher with key 13.



Related tasks

 Rot-13
 Substitution Cipher
 Vigenère Cipher/Cryptanalysis


  [ dup upper != ]              is lower? ( c --> b )
 
  [ dup lower != ]              is upper? ( c --> b )
 
  [ swap $ "" unrot witheach
      [ dup lower? iff
          [ over +
            dup lower? not if
              [ 26 - ] ]
        else
          [ dup upper? if
            [ over +
              dup upper? not if
                [ 26 - ] ] ]
        rot swap join swap ]
    drop ]                      is caesar ( $ --> $ )
 
  [ 26 swap - ]                 is decode ( n --> n )
 
  $ "Fere libenter homines id quod volunt credunt."
  dup              echo$ cr
  23 caesar dup    echo$ cr
  23 decode caesar echo$ cr