Proj 13: XOR Encryption in Python (10 pts. + 40 extra credit)

What You Need

A Kali Linux machine, real or virtual. You could also use OS X, or Windows with Python installed.

Purpose

Encrypt and decrypt files using XOR in Python.

Understanding XOR

Exclusive OR (XOR) is a fundamental mathematical operation used in many encryption algorithms.

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
For our purposes, we'll use the Python ^ operator, which acts on a whole byte at a time.

Characters are ASCII-encoded, like this:

A is 01000001
B is 01000010
C is 01000011
...
A whole table of ASCII values is here:

http://www.asciitable.com/

Consider A^B:

A is 01000001
B is 01000010
A^B= 00000011
That is character 3, an unprintable end-of-text mark.

However, A^s is printable:

A is 01000001
s is 01110011
A^B= 00110010
The result is the hexadecimal value 0x32, or the numeral 2.

XOR in Python

In Kali Linux, in a Terminal window, execute this command: 
nano xor1
In nano, enter the code shown below: 
#!/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

In a Terminal window, execute this command: 
./xor1
You see the help message, explaining how to use the program, as shown below.

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

echo -n A > plain1

cat plain1
The "echo -n" command created a file named plain1 which contains a single letter A, without a carriage return at the end of the file.

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
The result is 2, as shown below:

Encrypting a Text File

In Kali Linux, in a Terminal window, execute this command: 
nano plain2
In nano, enter the code shown below, replacing "YOUR NAME" with your own name: 
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
The result is strange unreadable characters, as shown below:

Decrypting a Text File

To decrypt a file, simply repeat the XOR operation with the same key. It will reverse itself.

Execute these commands: 

./xor1 cipher2 plain2r x

cat plain2r
The file is restored to readable text, as shown below:

Capturing a Screen Image

Make sure YOUR NAME is visible, as shown above.

Click on the host system's taskbar, at the bottom of the screen.

Press the PrntScrn key to capture the whole desktop. Open Paint and paste in the image.

Save the image as "Proj 13a from YOUR NAME".

YOU MUST SEND IN A WHOLE-DESKTOP IMAGE FOR FULL CREDIT

Challenge 1: Decrypting a Text File Without the Key (10 pts. extra credit)

This one is pretty simple: the key is a capital letter, from A to Z.

To get the ciphertext, execute this command: 

wget https://samsclass.info/124/proj14/xorchal1-cipher --no-check-certificate
Windows users: right-click on this link and "Save As":

https://samsclass.info/124/proj14/xorchal1-cipher

Decrypt the file. When you get it, capture a whole-desktop image showing the contents, which begin with "Congratulations", as shown below:

Save the image as "Proj 13b from YOUR NAME".

Challenge 2: Decrypting a Text File Without the Key (10 pts. extra credit)

The key is a single byte from \x00 to \xff.

To get the ciphertext, execute this command: 

wget https://samsclass.info/124/proj14/xorchal2-cipher --no-check-certificate
Windows users: right-click on this link and "Save As":

https://samsclass.info/124/proj14/xorchal2-cipher

Decrypt the file. When you get it, capture a whole-desktop image showing the contents, which begin with "Congratulations", as shown below:

Save the image as "Proj 13c from YOUR NAME".

Challenge 3: Decrypting an Image File Without the Key (10 pts. extra credit)

The key is a single byte from \x00 to \xff.

To get the ciphertext, execute this command: 

wget https://samsclass.info/124/proj14/xorchal3-cipher --no-check-certificate
Windows users: right-click on this link and "Save As":

https://samsclass.info/124/proj14/xorchal3-cipher

This is an image file. When decrypted, it starts with a PNG file header, as shown below:

Notice that the first 4 bytes are 89, 50, 4E, 47; the 2nd through 4th byte spell out PNG in ASCII.

Decrypt the file. When you get it, change its filename extension to PNG and open it in an image viewer or Web browser.

Capture a whole-desktop image showing the visible image.

Save the image as "Proj 13d from YOUR NAME".

Challenge 4: Decrypting an Image File Without the Key (10 pts. extra credit)

The key is two bytes from \x0000 to \xffff.

To get the ciphertext, execute this command: 

wget https://samsclass.info/124/proj14/xorchal4-cipher --no-check-certificate
Windows users: right-click on this link and "Save As":

https://samsclass.info/124/proj14/xorchal4-cipher

This is an image file. When decrypted, it starts with a JPEG file header, as shown below:

Notice that the first 2 bytes are FFD8, and some later bytes contain the ASCII characters JFIF.

Decrypt the file. When you get it, change its filename extension to JPG and open it in an image viewer or Web browser.

Capture a whole-desktop image showing the visible image.

Save the image as "Proj 13e from YOUR NAME".

Turning in Your Project

Send the images to cnit.124@gmail.com with a subject of "Proj 13 from YOUR NAME".

Last revised: 5-24-17