Question

The following code does not seem to read/write binary form correctly. It should read a binary file, bit-wise XOR the data and write it back to file. There are not any syntax errors but the data does not verify and I have tested the source data via another tool to confirm the xor key.

Update: per feedback in the comments, this is most likely due to the endianness of the system I was testing on.

xortools.py:

def four_byte_xor(buf, key):
    out = ''
    for i in range(0,len(buf)/4):
        c = struct.unpack("=I", buf[(i*4):(i*4)+4])[0]
        c ^= key
        out += struct.pack("=I", c)
    return out

Call to xortools.py:

from xortools import four_byte_xor
in_buf = open('infile.bin','rb').read()
out_buf = open('outfile.bin','wb')
out_buf.write(four_byte_xor(in_buf, 0x01010101))
out_buf.close()

It appears that I need to read bytes per answer. How would the function above incorporate into the following as the function above manipulate multiple bytes? Or Does it not matter? Do I need to use struct?

with open("myfile", "rb") as f:
    byte = f.read(1)
    while byte:
        # Do stuff with byte.
        byte = f.read(1)

For an example the following file has 4 repeating bytes, 01020304:

The data is XOR’d with a key of 01020304 which zeros the original bytes:

Here is an attempt with the original function, in this case 05010501 is the result which is incorrect:

Answer 1

Try this function:

def four_byte_xor(buf, key):
    outl = []
    for i in range(0, len(buf), 4):
        chunk = buf[i:i+4]
        v = struct.unpack(b"=I", chunk)[0]
        v ^= key
        outl.append(struct.pack(b"=I", v))
    return b"".join(outl)

I’m not sure you’re actually taking the input by 4 bytes, but I didn’t try to decipher it. This assumes your input is divisible by 4.

Edit, new function based in new input:

def four_byte_xor(buf, key):
    key = struct.pack(b">I", key)
    buf = bytearray(buf)
    for offset in range(0, len(buf), 4):
        for i, byte in enumerate(key):
            buf[offset + i] = chr(buf[offset + i] ^ ord(byte))
    return str(buf)

This could probably be improved, but it does provide the proper output.