

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


def caesar_encode(plaintext, shift)
	uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	lowercase = "abcdefghijklmnopqrstuvwxyz"
 
	cipher = ""
	for char in plaintext
		if char in uppercase
			cipher += uppercase[uppercase[char] - (26 - shift)]
		else if char in lowercase
			cipher += lowercase[lowercase[char] - (26 - shift)]
		else
			cipher += char
		end
	end
 
	return cipher
end
 
def caesar_decode(cipher, shift)
	uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	lowercase = "abcdefghijklmnopqrstuvwxyz"
 
	plaintext = ""
	for char in cipher
		if char in uppercase
			plaintext += uppercase[uppercase[char] - shift]
		else if char in lowercase
			plaintext += lowercase[lowercase[char] - shift]
		else
			plaintext += char
		end
	end
 
	return plaintext
end