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

[Jordan]

A question I have for the group before I respond is a option that I saw
that I had earlier was to ord() each element of a string and then bin()
that number. But since bin() produces a string I could not figure out
the correct way to attach two bin() outputs back together again due to
the leading 'b' and even if I use lstrip('b') I was not sure if that
would be correct?
My next hesitation is can the same or at least similar techniques be
applied to a file? I want to be able to work on both files and strings.

On 07/19/2012 01:22 PM, eryksun wrote:
> On Thu, Jul 19, 2012 at 1:41 AM, wolfrage8765 at gmail.com
> <wolfrage8765 at gmail.com> wrote:
>> I was comparing them but I think I understand how to compare them well, now
>> I want to convert them both to binary so that I can XOR them together. Thank
>> you for your time and help Dave, now I need to reply to Ramit.
> A bytes object is a container of 8-bit numbers (i.e. range 0 to 255).
> If you index it, you'll get an int that supports the XOR operation:
>
>>>> b1 = b'a'
>>>> b2 = b'b'
>>>> b1[0]
> 97
>>>> b2[0]
> 98
>>>> bin(b1[0])
> '0b1100001'
>>>> bin(b2[0])
> '0b1100010'
>>>> bin(b1[0] ^ b2[0])
> '0b11'
>
> You can use the int method  "from_bytes" to XOR two bitstrings stored
> as Python bytes:
>
>>>> b3 = b'aaaa'
>>>> b4 = b'bbbb'
>>>> bin(int.from_bytes(b3, 'big') ^ int.from_bytes(b4, 'big'))
> '0b11000000110000001100000011'
>
> The computation is done between int objects, not strings. Creating a
> string using "bin" is just for presentation.
>
> P.S.:
>
> Instead of "bin" you can use the "format" command to have more
> control, such as for zero padding. The integer format code "b" is for
> a binary representation. Preceding it by a number starting with zero
> will pad with zeros to the given number of characters (e.g. 032 will
> prepend zeros to make the result at least 32 characters long):
The control sounds good and I may need that latter (To adjust things to
a fixed length), but for the purpose of XORing a message padding a key
with zeros would not be desirable if Eve was able to get her hands on
the source code.
>>>> r = int.from_bytes(b3, 'big') ^ int.from_bytes(b4, 'big')
>>>> format(r, "032b")
> '00000011000000110000001100000011'
>
> Instead of hard coding the length (e.g. "032"), you can use the length
> of the input bitstrings to calculate the size of the result:
That sounds good.
>>>> size = 8 * max(len(b3), len(b4))
>>>> format(r, "0%db" % size)
> '00000011000000110000001100000011'
Is this output the output for size rather than the two variables joined
together?