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, wolfrage8...@gmail.com > <wolfrage8...@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? _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
