22.py

'''
In cryptography, a Caesar cipher is a very simple encryption techniques in which each letter in the plain text is replaced by a
letter some fixed number of positions down the alphabet. For example,
with a shift of 3, A would be replaced by D, B would become E, and so on.
The method is named after Julius Caesar, who used it to communicate with his generals. 
ROT-13 ("rotate by 13 places") is a widely used example of a Caesar cipher where the shift is 13.
In Python, the key for ROT-13 may be represented by means of the following dictionary:

key = {'a':'n', 'b':'o', 'c':'p', 'd':'q', 'e':'r', 'f':'s', 'g':'t', 'h':'u', 
       'i':'v', 'j':'w', 'k':'x', 'l':'y', 'm':'z', 'n':'a', 'o':'b', 'p':'c', 
       'q':'d', 'r':'e', 's':'f', 't':'g', 'u':'h', 'v':'i', 'w':'j', 'x':'k',
       'y':'l', 'z':'m', 'A':'N', 'B':'O', 'C':'P', 'D':'Q', 'E':'R', 'F':'S', 
       'G':'T', 'H':'U', 'I':'V', 'J':'W', 'K':'X', 'L':'Y', 'M':'Z', 'N':'A', 
       'O':'B', 'P':'C', 'Q':'D', 'R':'E', 'S':'F', 'T':'G', 'U':'H', 'V':'I', 
       'W':'J', 'X':'K', 'Y':'L', 'Z':'M'}
Your task in this exercise is to implement an encoder/decoder of ROT-13.
Once you're done, you will be able to read the following secret message:

Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!
Note that since English has 26 characters, your ROT-13 program will be able to both encode and decode texts written in English.
'''


def rot_decoder(x):
    new =[]
    d = {'a':'n', 'b':'o', 'c':'p', 'd':'q', 'e':'r', 'f':'s', 'g':'t', 'h':'u', 
       'i':'v', 'j':'w', 'k':'x', 'l':'y', 'm':'z', 'n':'a', 'o':'b', 'p':'c', 
       'q':'d', 'r':'e', 's':'f', 't':'g', 'u':'h', 'v':'i', 'w':'j', 'x':'k',
       'y':'l', 'z':'m', 'A':'N', 'B':'O', 'C':'P', 'D':'Q', 'E':'R', 'F':'S', 
       'G':'T', 'H':'U', 'I':'V', 'J':'W', 'K':'X', 'L':'Y', 'M':'Z', 'N':'A', 
       'O':'B', 'P':'C', 'Q':'D', 'R':'E', 'S':'F', 'T':'G', 'U':'H', 'V':'I', 
       'W':'J', 'X':'K', 'Y':'L', 'Z':'M'}
    for i in x:
        new.append(d.get(i,i))
    print ''.join(new)
    
rot_decoder('Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!')

# Our decoder function can also encode the message since we have 26 characters. But in case if isn't you can use below strategy. 
def rot_encoder(x):
    key_inverse = { v:k for k,v in d.items()} 
    for i in x:
        new.append(d.get(i,i))
    print ''.join(new)

rot_decoder('Caesar cipher? I much prefer Caesar salad!')