

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


 
# based on Rot-13 solution: http://rosettacode.org/wiki/Rot-13#R
ceasar <- function(x, key)
{
  # if key is negative, wrap to be positive
  if (key < 0) {
    key <- 26 + key 
  }
 
  old <- paste(letters, LETTERS, collapse="", sep="")
  new <- paste(substr(old, key * 2 + 1, 52), substr(old, 1, key * 2), sep="")
  chartr(old, new, x)
}
 
# simple examples from description
print(ceasar("hi",2))
print(ceasar("hi",20))
 
# more advanced example
key <- 3
plaintext <- "The five boxing wizards jump quickly."
cyphertext <- ceasar(plaintext, key)
decrypted <- ceasar(cyphertext, -key)
 
print(paste("    Plain Text: ", plaintext, sep=""))
print(paste("   Cypher Text: ", cyphertext, sep=""))
print(paste("Decrypted Text: ", decrypted, sep=""))
 
 
 