

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


 
REM *** By changing the key and pattern, an encryption system that is difficult to break can be achieved. ***
 
text$ = "You are encouraged to solve this task according to the task description, using any language you may know."
key$ = "desoxirribonucleic acid" // With a single character you get a Caesar encryption. With more characters the key is much more difficult to discover.
CIPHER = 1 : DECIPHER = -1
 
print text$ : print
encrypted$ = criptex$(text$, key$, CIPHER)
a = open("cryptedText.txt", "w") : if a then print #a encrypted$ : close #a end if
print encrypted$ : print
print criptex$(encrypted$, key$, DECIPHER)
 
sub criptex$(text$, key$, mode)
	local i, k, delta, longtext, longPattern, longkey, pattern$, res$
 
	pattern$ = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890 .,;:()-"
 
	longPattern = len(pattern$)
	longtext = len(text$)
	longkey = len(key$)
 
	for i = 1 to longtext
		k = k + 1 : if k > longkey k = 1
		delta = instr(pattern$, mid$(text$, i, 1))
		delta = delta + (mode * instr(pattern$, mid$(key$, k, 1)))
		if delta > longPattern then
			delta = delta - longPattern
		elseif delta < 1 then
			delta = longPattern + delta
		end if
		res$ = res$ + mid$(pattern$, delta, 1)
	next i
 
	return res$
end sub