

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


 
# Caeser cipher
# Julia 1.5.4
# author: manuelcaeiro | https://github.com/manuelcaeiro
 
function csrcipher(text, key)
    ciphtext = ""
    for l in text
        numl = Int(l)
        ciphnuml = numl + key
        if numl in 65:90
            if ciphnuml > 90
                rotciphnuml = ciphnuml - 26
                ciphtext = ciphtext * Char(rotciphnuml)
            else
                ciphtext = ciphtext * Char(ciphnuml)
            end
        elseif numl in 97:122
            if ciphnuml > 122
                rotciphnuml = ciphnuml - 26
                ciphtext = ciphtext * Char(rotciphnuml)
            else
                ciphtext = ciphtext * Char(ciphnuml)
            end
        else
            ciphtext = ciphtext * Char(numl)
        end
    end
    return ciphtext
end
 
text = "Magic Encryption"; key = 13
csrcipher(text, key)
 