Caesar Cipher
March 11, 2014
I was asked about a caesar cipher the other day, and suggested that my questioner search for caesar cipher here at Programming Praxis. To my surprise, I discovered that we have never done a caesar cipher (we did the ROT13 cipher, but not a general-purpose caesar cipher), so I apologized to my friend and wrote today’s exercise.
A caeser cipher, named after Julius Caesar, who either invented the cipher or was an early user of it, is a simple substitution cipher in which letters are substituted at a fixed distance along the alphabet, which cycles; children’s magic decoder rings implement a caesar cipher. Non-alphabetic characters are passed unchanged. For instance, the plaintext PROGRAMMINGPRAXIS is rendered as the ciphertext SURJUDPPLQJSUDALV with a shift of 3 positions.
Your task is to write functions that encipher and decipher text using a caesar cipher. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
Programming Praxis 
What should be the cipher key ?
the key is the shift distance
Here’s a solution in Julia. There’s also a one-liner, encipherOneLiner.
function encipher(s, key) function shift(c) if isalpha(c) char(mod((int(uppercase(c)) - 65 + key), 26) + 65) else c end end map(shift, s) end function decipher(s, key) encipher(s,-key) end function encipherOneLiner(s, key) map(c->isalpha(c)?char(mod((int(uppercase(c))-65+key),26)+65):c,s) endThe Caesar cipher shows up in Graham Hutton’s *great* book on Haskell.
Here’s a version similar to @Daniel’s:
import Data.Char (chr, isAsciiUpper, ord)
shift :: Int -> Char -> Char
shift n c | isAsciiUpper c = chr $ a + ((n + (ord c – a) `mod` 26))
| otherwise = c
where
a = ord ‘A’
caesar :: Int -> String -> String
caesar n = map $ shift n
main :: IO ()
main = mapM_ putStrLn [p, c, p’]
where
p = "PROGRAMMING PRAXIS"
c = caesar 3 p
p’ = caesar (-3) c
Apologies for the crap formatting. Apparently I’m really out of practice!
Racket / Rackjure
(define (caesar str n) (define A (char->integer #\A)) (define a (char->integer #\a)) (list->string (for/list ([c (in-string str)]) (cond [(char<=? #\A c #\Z) (~> c char->integer (- A) (+ n) (mod 26) (+ A) integer->char)] [(char<=? #\a c #\z) (~> c char->integer (- a) (+ n) (mod 26) (+ a) integer->char)] [else c]))))Full writeup: jverkamp.com
python solution:
class SubstitutionCypher: def __init__(self, mapping): self.encode_map = mapping self.decode_map = {v:k for k,v in mapping.items()} def encode(self, cleartext): return cleartext.translate(self.encode_map) def decode(self, cyphertext): return cyphertext.translate(self.decode_map) class CaesarCypher(SubstitutionCypher): from string import ascii_uppercase def __init__(self, key, alphabet=ascii_uppercase): key %= len(alphabet) shifted = alphabet[key:] + alphabet[:key] super().__init__(str.maketrans(alphabet, shifted)) CaesarCypher(3).encode('PROGRAMMING PRAXIS') # => SURJUDPPLQJ SUDALV CaesarCypher(-3).encode('SURJUDPPLQJ SUDALV') # => PROGRAMMING PRAXISTo bring the cypher up to the current century, we can use an alphabet containing all the uppercase Unicode characters.
import unicodedata as ud alphabet = ''.join(c for c in map(chr, range(0x10FFFF)) if c.isupper()) CaesarCypher(432, alphabet=alphabet).encode('PROGRAMMING PRAXIS') # => ՇՉՆԾՉԸՄՄՀՅԾ ՇՉԸՏՀՊ CaesarCypher(-432, alphabet=alphabet).encode('ՇՉՆԾՉԸՄՄՀՅԾ ՇՉԸՏՀՊ') # => PROGRAMMING PRAXIS{code}
def CeaserC(word,offset=1):
#final=[chr(ord(i)+offset) for i in word]
return (”.join([chr(ord(i)+offset) for i in word]))
def Ceaserback(word,offset=1):
#final=[chr(ord(i)-offset) for i in word]
return (”.join([chr(ord(i)-offset) for i in word]))
print (CeaserC(“thisisatest”,1))
print (Ceaserback(“uijtjtbuftu”))
{/code}
Here’s a Java solution:
import java.util.HashMap; public class CaesarCipher { private static final char[] ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray(); private HashMap<Character, Character> cryptTable; private HashMap<Character, Character> decryptTable; public CaesarCipher(final int shift) { this.cryptTable = buildTranslationTable(ALPHABET, shift); this.decryptTable = buildTranslationTable(ALPHABET, -shift); } public String crypt(final String plainText) { return translate(this.cryptTable, plainText); } public String decrypt(final String encryptedText) { return translate(this.decryptTable, encryptedText); } private HashMap<Character, Character> buildTranslationTable(final char[] alphabet, final int shift) { HashMap<Character, Character> translationTable = new HashMap<>(alphabet.length); for (int i = 0; i < alphabet.length; i++) { Character plain = alphabet[i]; Character translated = alphabet[mod((i + shift), alphabet.length)]; translationTable.put(plain, translated); } return translationTable; } private String translate(final HashMap<Character, Character> translationTable, final String text) { StringBuilder translatedText = new StringBuilder(text.length()); for (int i = 0; i < text.length(); i++) { Character plain = text.charAt(i); Character translated = translationTable.get(plain); if (translated != null) { translatedText.append(translated); } else { translatedText.append(plain); } } return translatedText.toString(); } private int mod(int a, int b) { int r = a % b; if (r < 0) { r += b; } return r; } public static void main(String[] args) { CaesarCipher cipher = new CaesarCipher(3); System.out.println(cipher.crypt("PROGRAMMINGPRAXIS")); // -> SURJUDPPLQJSUDALV System.out.println(cipher.decrypt("SURJUDPPLQJSUDALV")); // -> PROGRAMMINGPRAXIS } }