[Tutor] string to binary and back... Python 3

[wolfrage8765 at gmail.com]

  On Fri, Jul 20, 2012 at 12:33 AM, Dave Angel <d at davea.name> wrote:

> On 07/19/2012 05:55 PM, Prasad, Ramit wrote:
> >> I am not sure how to answer that question because all files are binary,
> >> but the files that I will parse have an encoding that allows them to be
> >> read in a non-binary output. But my program will not use the in a
> >> non-binary way, that is why I plan to open them with the 'b' mode to
> >> open them as binary with no encoding assumed by python. I just not have
> >> tested this new technique that you gave me on a binary file yet as I was
> >> still implementing it for strings.
> > As far as I know, even in binary mode, python will convert the
> > binary data to read and write strings. So there is no reason
> > this technique would not work for binary. Note, I was able to use
> > the string representation of a PDF file to write another PDF file.
> > So you do not need to worry about the conversion of binary to strings.
> > All you need to do is convert the string to int, encrypt, decrypt,
> > convert back to string, and write out again.
> >
> > Note Python3 being Unicode might change things a bit. Not sure if
> > you will need to convert to bytes or some_string.decode('ascii').
>
> In Python 3, if you open the file  with "b"  (as Jordan has said), it
> creates a bytes object.  No use of strings needed or wanted.  And no
> assumptions of ascii, except for the output of the % operator on a hex
> conversion.
>
>
> myfile = open(filename, "b")
> data = myfile.read(size)
>
> At that point, convert it to hex with:
>    hexdata = binascii.hexlify(data)
> then convert that to an integer:
>    numdata = int(hexdata, 16)
>
> At that point, it's ready to xor with the one-time key, which had better
> be the appropriate size to match the data length.
>
> newhexdata = bytes("%x" % numdata, "ascii")
> newdata = binascii.unhexlify(newhexdata)
>
> If the file is bigger than the key, you have to get a new key. If the
> keys are chosen with a range of 2**200, then you'd read and convert the
> file 25 bytes at a time.
>
Thanks I will give this a try. Can you explian a little further for me what
exactly this:

newhexdata = bytes("%x" % numdata, "ascii")
line is doing? I don't quite understand the use of the "%x" % on numdata.

>
>
> --
>
> DaveA
>
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