Python is an easy to learn interactive programming language. Throughout this lab, we'll be using Python 2.7.

You can run Python code in two ways:

• by writing a source file and running it (eventually, with some command line arguments)
• by entering commands in interactive mode

To start interactive mode, simply open a terminal window and run python:

linux$ python

You can now write Python code just as you would in an actual program:

>>> print "Hello, SASC"

To quit interactive mode either press Ctrl+D or use:

>>> quit()

Interactive mode is useful when you want to quickly try something out (for example, testing decryption for a certain key) without going through the hassle of writing a new source file and running it.

In Python, indentation is important; it is used to organize code in blocks. For example, the following C code:

int foo(int a, int b) {
    int x;
    if (a > b) {
        x = a;
    } else {
        x = b;
    }
    return x
}

is (approximately) the same as the following in Python:

def foo(a, b):
    if a > b:
        x = a
    else:
        x = b
    return x

In this first recipe we'll see how to encrypt a single letter using Caesar's cipher.

alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
def caesar_enc(letter):
  if letter < 'A' or letter > 'Z':
    print 'Invalid letter'
    return
  else:
    return alphabet[(ord(letter) - ord('A') + 3) % len(alphabet)]

Create a new file named caesar.py containing the code above. To test the code, open the interpreter and try the following:

linux$ python
>>> from caesar import *
>>> print alphabet
>>> alphabet[0]
>>> ord('A')
>>> len(alphabet)
>>> ord('D') - ord('A')
>>> 28 % 26
>>> -1 % 26
>>> caesar_enc('D')
>>> caesar_enc('Z')
>>> caesar_enc('B')

Add a caesar_dec function to caesar.py, which decrypts a single letter encrypted using Caesar's cipher.

We'll now expand our function to take string as input. First, a little intro to Python for loops. Some examples:

for i in [1, 2, 3]:
    print i

In this first example, i iterates through the values in list [1, 2, 3]. The code is similar to the classical C code:

for (int i = 1; i <= 3; i++) {
    printf("%d\n", i);
}

To create lists of integers use the range function:

for i in range(3):
    print i

Execute the code above in the interpreter. Using the range function with two parameters, change the above to print integers 1 through 10.

In Python, for can iterate through multiple types of objects, including strings. Try the below:

for letter in 'ABCD':
    print letter

We will now use the for instruction to expand our caesar_enc function:

alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
def caesar_enc_string(plaintext):
    ciphertext = ''
    for letter in plaintext:
        ciphertext = ciphertext + caesar_enc(letter)
    return ciphertext
 

Test the above by starting a new interpreter; this time, we'll skip the from caesar import * part by running the source file before starting interactive mode:

linux$ python -i caesar.py
>>> test = 'HELLO'
>>> test + 'WORLD'
>>> caesar_enc_string(test)

Another way to run things, which can be very useful in general is to use a main() function and write your program script as follows:

'test_caesar.py'

import sys
import random
import string
import operator
 
alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
 
def caesar_enc(letter):
  if letter < 'A' or letter > 'Z':
    print 'Invalid letter'
    return
  else:
    return alphabet[(ord(letter) - ord('A') + 3) % len(alphabet)]
 
def caesar_enc_string(plaintext):
    ciphertext = ''
    for letter in plaintext:
        ciphertext = ciphertext + caesar_enc(letter)
    return ciphertext
 
def main():
  m = 'BINEATIVENIT'
  c = caesar_enc_string(m)
  print c
 
 
if __name__ == "__main__":
  main()

Then you can simply run the program, or type the following in a terminal:

python test_caesar.py

Add the corresponding caesar_dec_string function.

Python allows passing default values to parameters:

def foo(a, b = 3):
    print a, b

>>> foo(1)
>>> foo(1, 2)

We can use default parameter values to expand our caesar_enc function to take the key as an additional parameter, without breaking compatibility with our previous code.

def caesar_enc(letter, k = 3):
    if letter < 'A' or letter > 'Z':
        print 'Invalid letter'
        return None
    else:
        return alphabet[(ord(letter) - ord('A') + k) % len(alphabet)]
 
def caesar_enc_string(plaintext, k = 3):
    ciphertext = ''
    for letter in plaintext:
        ciphertext = ciphertext + caesar_enc(letter, k)
    return ciphertext

To test the new functions, try the below:

linux$ python -i caesar.py
>>> caesar_enc_string('HELLO')
>>> caesar_enc_string('HELLO', 0)
>>> caesar_enc_string('HELLO', 1)

Using default parameters, expand your shift cipher decryption functions to support arbitrary keys.

Alice sends Bob the following ciphertexts:

LDPWKHORUGBRXUJRG
XNTRGZKKGZUDMNNSGDQFNCRADENQDLD
DTZXMFQQSTYRFPJDTZWXJQKFSDLWFAJSNRFLJ
SIOMBUFFHINNUEYNBYHUGYIZNBYFILXSIOLAIXCHPUCH
ERZRZOREGURFNOONGUQNLGBXRRCVGUBYL
CJIJPMTJPMAVOCZMVIYTJPMHJOCZM
DTZXMFQQSTYRZWIJW
ZPVTIBMMOPUDPNNJUBEVMUFSZ
FVBZOHSSUVAZALHS
KAGETMXXZAFSUHQRMXEQFQEFUYAZKMSMUZEFKAGDZQUSTNAGD
MCIGVOZZBCHRSGWFSOBMHVWBUHVOHPSZCBUGHCMCIFBSWUVPCIF

Charlie manages to capture the ciphertexts and he finds that the cipher used for encryption is the shift cipher (each message possibly encrypted with a different key). Can you decrypt the messages ?

Charlie also knows that the plaintext consists only of the English letters A to Z (all capitals, no punctuation).

Alice sends Bob another ciphertext, but much longer this time:

Download message file

Charlie needs to decrypt this as well. Some colleagues tell him this is encrypted using the substitution cipher, and that again the plaintext consists only of the English letters A to Z (all capitals, no punctuation). Try to help Charlie to decrypt this.

Hint: use the frequency analysis mechanisms we discussed in class. Note that the frequency of each letter does not map precisely. In particular, the most frequent two letters do match well with the given table, but the others are sometimes mixed. However, Charlie knows that the most frequent bi-grams are the following (from most frequent to less frequent): TH, HE, IN, OR, HA, ET, AN, EA, IS, OU, HI, ER, ST, RE, ND

With this information, can you tell what the ciphertext is about?