Proj X2: XOR Encryption in Python (Up to 40 pts. extra credit)

What You Need

Purpose

Understanding XOR

XOR operates on one bit at a time, with these results:

0 XOR 0 = 0
0 XOR 1 = 1
1 XOR 0 = 1
1 XOR 1 = 0


Characters are ASCII-encoded, like this:

A is 01000001
B is 01000010
C is 01000011
...

http://www.asciitable.com/

Consider A^B:

A is 01000001
B is 01000010
A^B= 00000011


However, A^s is printable:

A is 01000001
s is 01110011
A^B= 00110010


XOR in Python

nano xor1

#!/usr/bin/python

import sys

if len(sys.argv) != 4:
  print "Usage: ./xor1 infile outfile k"
  print "k is a one-character XOR key"
  print "For hexadecimal keys, use $'\\x01'"
  exit()

f = open(str(sys.argv[1]), "rb")
g = open(str(sys.argv[2]), "a")
k = ord(sys.argv[3])

try:
    byte = f.read(1)
    while byte != "":
        xbyte = ord(byte) ^ k
        g.write(chr(xbyte))
        byte = f.read(1)
finally:
    f.close()

g.close()

Save the file with Ctrl+X, Y, Enter. Next, we need to make the file executable.

In a Terminal window, execute this command:

chmod a+x xor1

Encrypting a Single Character

./xor1

To create a file named plain1 with the letter A in it, execute these commands :

echo -n A > plain1

cat plain1

The "cat plain1" command printed out the file, which appeared as a single A at the start of the next line, as shown below:

To encode the plain1 file with a key of s, execute these commands:

./xor1 plain1 cipher1 s

cat cipher1

Encrypting a Text File

nano plain2

Normal English text; written by YOUR NAME

Save the file with Ctrl+X, Y, Enter. To encrypt the file using a key of x, execute these commands:

./xor1 plain2 cipher2 x

cat cipher2

Decrypting a Text File

Execute these commands:

./xor1 cipher2 plain2r x

cat plain2r

Challenge A: Unknown Single-Digit Key (5 pts. extra credit)

To download a file in Chrome, right-click it and click "Save Link As...".

Use the form below to put your name on the WINNERS PAGE.

Your Name (without spaces): The complete decrypted text:

Challenge B: Unknown Single-Byte Key (10 pts. extra credit)

Hint This script shows how to loop through bytes:

Your Name (without spaces): The complete decrypted text:

Challenge C: Unknown Two-Byte Key (10 pts. extra credit)

This means you XOR it two bytes at a time, as shown below for a key of '\xf00d' and plaintext of 'AB':

Plaintext:  0xf00d = 0b1111000000001101
Key:        'AB"   = 0b0010000100010010
XOR:                 0b1101000100011111 = 0xd11f

Hint The decrypted text begins with the word Congratulations

Your Name (without spaces): The complete decrypted text:

Challenge D: Asynchronous XOR With Single-Byte Key (15 pts. extra credit)

Here is an example of the algorithm for a key of 'A' and plaintext of 'ABC':

Plaintext:                  'ABC' = 0b01000001 0b01000010 0b01000011
Key:                        'A'   = 0b01000001
XOR first byte with key:            0b00000000

Add result to key (mod 256):        0b01000001
Use result for next key:                       0b01000001
XOR next byte:                                 0b00000011

Add result to key (mod 256):                   0b01000100
Use result for next key:                                  0b01000100
XOR next byte:                                            0b00000111

Final ciphertext:                   0b00000000 0b00000011 0b00000111 = 0x000307

Hint The decrypted text contains the word Congratulations

Your Name (without spaces): The complete decrypted text:

Turning in your Project