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.
In cryptography, a Caesar cipher, also known as Caesar’s cipher, the shift cipher, Caesar’s code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.[1]
The encryption step performed by a Caesar cipher is often incorporated as part of more complex schemes, such as the Vigenère cipher, and still has modern application in the ROT13 system. As with all single-alphabet substitution ciphers, the Caesar cipher is easily broken and in modern practice offers essentially no communications security.
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.
Examples
import string
lowercase_normal = list(string.ascii_lowercase)
lowercase_encode = [chr(((ord(c)+13-ord('a'))%26)+ord('a')) for c in lowercase_normal]
uppercase_normal = list(string.ascii_uppercase)
uppercase_encode = [chr(((ord(c)+13-ord('A'))%26)+ord('A')) for c in uppercase_normal]
def ROT_13_encoder(passed_string):
encoder_map=dict(zip(lowercase_normal,lowercase_encode)).copy()
encoder_map.update(dict(zip(uppercase_normal,uppercase_encode)))
output_string = ''
for c in passed_string:
output_string+=encoder_map.get(c,c)
return output_string
def ROT_13_decoder(passed_string):
decoder_map=dict(zip(lowercase_encode,lowercase_normal)).copy()
decoder_map.update(dict(zip(uppercase_encode,uppercase_normal)))
output_string = ''
for c in passed_string:
output_string+=decoder_map.get(c,c)
return output_string
print(ROT_13_encoder(ROT_13_decoder('Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!')))
print(ROT_13_decoder('Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!'))
def ROT_13(string):
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'}
code = ''
for i in string:
try:
code += key[i]
except:
code += i # for space characters
return code
print(ROT_13('Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!'))
a_string = 'Pnrfne pvcure? V zhpu cersre Pnrfne fnynq!'
def cipher(encoded_string):
encoded_string = encoded_string.lower()
decrypted = ' '
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',
}
for i in encoded_string:
if i not in key.values():
decrypted += i
else:
for value in key:
if value == i:
decrypted += key[i]
return decrypted
print cipher(a_string)
