Question

Some cryptography and a little computer security.

You find an odd DeLorean parked around the corner. Realizing that Doc Brown has left the

keys in the ignition, you decide that a quick trip back to Rome in 40 BC is in order. A few mint

condition coins, and possibly a picture of Vincengetorix will be highly remunerative.

As usual, the car fails and you are stranded.

All is not lost, Julius Caesar recruits you for his secret service as a cryptographer. Falerian

wine is palatable, you have acquired a taste for garum (don't ask), and there are more than

enough other attractions to make your life comfortable.

Fortunately, you have your laptop, running linux with a python interpreter, and by the use of

lemon juice, copper denarii and silver sestarii you are able to rig up a battery to keep it running.

1. Write a program that takes a key (a number from 1 to 26) an input le name, and an

output le name from the command line and uses the key to encrypt it with a Caesar

cipher. A Caesar cipher performs modular addition of the key and the letter. If the key is

1 then a->b, b->c, ..., y->z, z->

a . Ignore all the things that aren't the letters from a to z

(simply pass them through) and you will want to put the letters into lower case. Use the

le message1.txt and a key of 7 to encrypt it. (You can do this one pretty easily in bash

too.)

2. Cato and Junius Brutus are using the cipher to encrypt their communications. In Latin as

well as that vulgar Germanic language Englisc the letter 'e' is the most common letter. 't'

is the second most common. Write a program to count the numbers of each letter in the

message. (i.e. how common is each symbol) Then use that to find the key for the message.

Use the le ceaser1.txt for input.

3. Caesar ciphers form a group. Therefore repeated encryption with any key will recover the

message. Use the results from your answer to the second question to nd the message.

The proper decryption will be when 'e' is the most common letter and 't' is the second

most common. Use the le ceaser2.txt for input

Answer 1

import sys

def countChar(data, ch):
string = ''
for line in data:
string += line
count = 0
for c in string:
if c == ch:
count += 1
return count

def countChars(data):
for i in range(0, 26):
count = countChar(data, chr(ord('a') + i))
print('count of ' + chr(ord('a') + i) + ' is ' + str(count))

def Main():
key = sys.argv[1]
inputFile = sys.argv[2]
outputFile = sys.argv[3]
with open(inputFile, 'r') as f:
data = f.readlines()
data = [x.strip('\n') for x in data]
countChars(data)
out = open(outputFile, 'w')
outputData = []
for line in data:
outline = ''
for ch in line:
if ch.isupper():
ch = ch.lower()
if ch.isalpha():
ch = chr(ord('a') + (ord(ch) - ord('a') + key) % 26)
outline = outline + ch
outline = outline + '\n'
outputData.append(outline)
out.writelines(outputData)
out.close()

Main()

-----------------------------------------------------------------------------------------------------
SAMPLE OUTPUT:

Input.txt :
'Who will watch the watchmen.' Juvenal
'Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin.' John Von Neumann
'The price of freedom is eternal vigilance.' Thomas Jefferson
'And so it often happens that an apparently ingenious idea is in fact a weakness which the scientific cryptographer seizes on for his solution.' Herbert Yardley

Output.txt:
'dov dpss dhajo aol dhajotlu.' qbcluhs
'hufvul dov jvuzpklyz hypaotlapjhs tlaovkz vm wyvkbjpun yhukvt kpnpaz pz, vm jvbyzl, pu h zahal vm zpu.' qvou cvu ulbthuu
'aol wypjl vm myllkvt pz lalyuhs cpnpshujl.' aovthz qlmmlyzvu
'huk zv pa vmalu ohwwluz aoha hu hwwhyluasf punlupvbz pklh pz pu mhja h dlhrulzz dopjo aol zjpluapmpj jyfwavnyhwoly zlpglz vu mvy opz zvsbapvu.' olyilya fhykslf