Read and write binary file in Python

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…

Accepted Answer

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.

Read and write binary file in Python

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