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:
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^s= 00110010
The result is the hexadecimal value
0x32, or the numeral 2.
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.
Troubleshooting
If you are using Windows, it's difficult to enter a non-ASCII key, and it also adds carriage return characters after line feeds when writing, so use this code instead:
#!/usr/bin/python import sys if len(sys.argv) != 4: print "Usage: ./xor1 infile outfile keynum" print "k is a number in base 10" exit() f = open(str(sys.argv[1]), "rb") g = open(str(sys.argv[2]), "ab") k = int(sys.argv[3]) out = "" try: byte = f.read(1) while byte != "": xbyte = ord(byte) ^ k out += chr(xbyte) byte = f.read(1) finally: f.close() g.write(out) g.close()
In a Terminal window, execute this command:
chmod a+x xor1
./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:
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:
Execute these commands:
./xor1 cipher2 plain2r x
cat plain2r
The file is restored to readable
text,
as shown below:
To get the ciphertext, execute this command:
curl https://samsclass.info/124/proj14/xorchal1-cipher > xorchal1-cipher
Windows users: right-click on this link and "Save As":
https://samsclass.info/124/proj14/xorchal1-cipher
Decrypt the file to find the message. Then use the form below to get your name onto the Winners Page.
To get the ciphertext, execute this command:
curl https://samsclass.info/124/proj14/xorchal2-cipher > xorchal2-cipher
Windows users: right-click on this link and "Save As":
https://samsclass.info/124/proj14/xorchal2-cipher
Decrypt the file to find the message. Then use the form below to get your name onto the Winners Page.
To get the ciphertext, execute this command:
curl https://samsclass.info/124/proj14/xorchal3-cipher > xorchal3-cipher
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.
Read the message written inside the image. Use the form below to get your name onto the Winners Page.
To get the ciphertext, execute this command:
curl https://samsclass.info/124/proj14/xorchal4-cipher > xorchal4-cipher
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.
Use the form below to get your name onto the Winners Page.