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.

import binascii
 
def strxor(a, b): # xor two strings (trims the longer input)
  return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b)])
 
def hexxor(a, b): # xor two hex strings (trims the longer input)
  ha = a.decode('hex')
  hb = b.decode('hex')
  return "".join([chr(ord(x) ^ ord(y)).encode('hex') for (x, y) in zip(ha, hb)])
 
def bitxor(a, b): # xor two bit strings (trims the longer input)
  return "".join([str(int(x)^int(y)) for (x, y) in zip(a, b)])
 
def str2bin(ss):
  """
    Transform a string (e.g. 'Hello') into a string of bits
  """
  bs = ''
  for c in ss:
    bs = bs + bin(ord(c))[2:].zfill(8)
  return bs
 
def hex2bin(hs):
  """
    Transform a hex string (e.g. 'a2') into a string of bits (e.g.10100010)
  """
  bs = ''
  for c in hs:
    bs = bs + bin(int(c,16))[2:].zfill(4)
  return bs
 
def bin2hex(bs):
  """
    Transform a bit string into a hex string
  """
  return hex(int(bs,2))[2:-1]
 
def byte2bin(bval):
  """
    Transform a byte (8-bit) value into a bitstring
  """
  return bin(bval)[2:].zfill(8)
 
def str2int(ss):
  """
    Transform a string (e.g. 'Hello') into a (long) integer by converting
    first to a bistream
  """
  bs = str2bin(ss)
  li = int(bs, 2)
  return li
 
def int2hexstring(bval):
  """
    Transform an int value into a hexstring (even number of characters)
  """
  hs = hex(bval)[2:]
  lh = len(hs)
  return hs.zfill(lh + lh%2)
 
def bin2str(bs):
  """
    Transform a binary srting into an ASCII string
  """
  n = int(bs, 2)
  return binascii.unhexlify('%x' % n)

Find the plaintext messages for the following ciphertexts knowing that the cipher is the XOR operation (ciphertext = plaintext XOR key) and the key is “abcdefghijkl”.

C1 = "000100010001000000001100000000110001011100000111000010100000100100011101000001010001100100000101"

C2 = "02030F07100A061C060B1909"