Cifrado cesar, bastante viejo supongo :P

caesar.py

#!/usr/bin/python2

import sys

def main(argv):
	if (len(sys.argv) != 2):
		sys.exit('Uso: caesar.py <entero>')
		
	plaintext = list(raw_input('Mensage: '))
	alphabet = list('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
	k = int(sys.argv[1])
	cipher = ''
		
	for c in plaintext:
		if c in alphabet:
			cipher += alphabet[(alphabet.index(c)+k)%(len(alphabet))]
		
	print 'El mensaje encriptado es: ' + cipher
	

if __name__ == "__main__":
	main(sys.argv[1:])