for loop c++ vs python

Question

Could you please help me converting this c++ code into python: I am trying to XOR the data

C++ :

void Encrypt(void data, Dword size)
{
    if(size > 0)
        for(DWORD i = size - 1; i > 0; i--)
            ((LPBYTE)data)[i] ^= ((LPBYTE)data)[i - 1];
}

I assume you mean void* data.

Antimony
– Antimony

2012-06-28 04:48:00 +00:00
Commented Jun 28, 2012 at 4:48 — Antimony
– Antimony, Commented Jun 28, 2012 at 4:48

Antimony · Accepted Answer · 2012-06-28 04:46:18Z

1

def Encrypt(data, size):
    for i in range(size-1, 0, -1):
        data[i] = data[i] ^ data[i-1]

Though this isn't quite pythonic. You'd probably want to remove the explicit size argument and just use len(data)

answered Jun 28, 2012 at 4:46

Antimony

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2 Comments

Roger Over a year ago

Thanks Antimony, but for some reason i am getting this error "TypeError: unsupported operand type(s) for ^: 'str' and 'str'" Any thoughts?

Antimony Over a year ago

That's because your running it on strings, for which xor doesn't make sense. You can convert it to numerical bytes by saying map(ord, s) where s is your string.

Jonathon Reinhart · Accepted Answer · 2012-06-28 04:46:54Z

0

To do this in python, you probably want to use the bytearray class:

def encrypt(data):
    n = len(data)
    for i in range(n-1, 0, -1):
        data[i] ^= data[i-1]      # for this to work, data has to be mutable

f = open('somefile.bin', 'rb')
buf = bytearray(f.read())
f.close()

encrypt(buf)

Note the comment, that you cannot pass a string object, because strings in python are immutable. bytearray on the other hand, is not.

answered Jun 28, 2012 at 4:46

Jonathon Reinhart

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2 Comments

Roger Over a year ago

Thanks Jonathon, this seems to work but how come the output data size is doubled?

Roger Over a year ago

It works perfectly now. Appreciate your time and sharing your knowledge. You are both brilliant!

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