Re: [Tutor] Iterate over letters in a word
Hello Tutors! could be split into:
Hell o Tut ors!
and xor'd with beer
I think I understand how xor works (thanks to an earlier post) but I'm
not sure how to iterate over each letter in a string. What is the
recommended way to do this?
The xor bitwise operator works with numbers --- not directly with strings
--- so one of your tasks will probably be to take a chunk like:
beer
and turn it into some numeric bit pattern.
It turns out that this isn't too bad if we abuse the 'struct' module:
http://www.python.org/doc/lib/module-struct.html
The idea is to unpack four single characters as a single 4-byte integer.
For example:
##
struct.unpack('i', 'food')
(1685024614,)
##
This kind of transformation is reversable:
##
struct.pack('i', 1685024614)
'food'
##
Does this make sense?
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Re: [Tutor] Iterate over letters in a word
On 3/14/06, Danny Yoo [EMAIL PROTECTED] wrote: The idea is to unpack four single characters as a single 4-byte integer. That's really useful, thanks, as I was planning to iterate over each letter and call ord() This kind of transformation is reversable: Highly useful. Thanks very much indeed. Does this make sense? Absolutely. S. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterate over letters in a word
As a side note, remember that that xor-ing a key with a message is trivial to break (it's just a variation on the Vigenere cipher first published in 1568). So don't use if for any real applications. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Steve Nelson Sent: 14 March 2006 15:29 To: tutor@python.org Subject: [Tutor] Iterate over letters in a word Hello, I'm trying to work on some programs to help me understand ciphers and ultimately cryptography. I've understood so far, that a simple form of bit-level cryptography is to split the original message into chunks the same length as a 'key' and then do an xor. I'm trying to keep this really simple so I can understand from first principles - so eg: Hello Tutors! could be split into: Hell o Tut ors! and xor'd with beer I think I understand how xor works (thanks to an earlier post) but I'm not sure how to iterate over each letter in a string. What is the recommended way to do this? Thanks, S. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterate over letters in a word
On 3/14/06, Matthew Webber [EMAIL PROTECTED] wrote: As a side note, remember that that xor-ing a key with a message is trivial to break (it's just a variation on the Vigenere cipher first published in 1568). So don't use if for any real applications. Yes - at the moment this is just a way for me to begin to get my head around how cryptography works from anabsolutely ludicrously basic position. This all started because I couldn't get my head around the difference between an encryption algorithm and the key. I thought that by writing my own, I would work it out! S. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterate over letters in a word
On 3/14/06, Steve Nelson [EMAIL PROTECTED] wrote:
On 3/14/06, Danny Yoo [EMAIL PROTECTED] wrote:
The idea is to unpack four single characters as a single 4-byte integer.
That's really useful, thanks, as I was planning to iterate over each
letter and call ord()
Ok, so experimenting a little further, and looking at the
documentation, it seems that len(string) and calcsize (i) must be the
same. Is there a reason why 'i' is a 4 byte integer? Doesn't this
mean that this method wouldn't scale if I then decided I wanted to
use, eg, a 6 byte key instead of a four? Or do I misunderstand?
I am also struggling to understand why a 4 byte integer is so large?
mystring = Hello I am Steve
import struct
struct.unpack('i', mystring[0:4])
(1819043144,)
I can see that the largest number I can generate in a 1 byte integer
is 255 - which is (2^8)-1. Is the point that with a 4 byte number I
can actually get (2^32)-1 , ie 4294967295? This just seems like a
huge number!
I suppose I've answered my question... but any comments or
clarifications would help a lot.
S.
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Re: [Tutor] Iterate over letters in a word
As a side note, remember that that xor-ing a key with a message is trivial to break (it's just a variation on the Vigenere cipher first published in 1568). So don't use if for any real applications. Hi Matthew, Counterpoint: think of one-time pads. http://en.wikipedia.org/wiki/One-time_pad XOR itself is just a technique --- it's just a binary operation between two bit patterns --- but the use of one-time pads provides unbreakable encryption. As long as the xor-ing key is protected, and as long as the encrypting key pattern is as long as the message, it's theoretically unbreakable. If we reuse the same encrypting bit-pattern over and over, or make the XORing key less than random, then all bets are off, of course. *grin* There are different encryption schemes that, at its heart, use XOR. But XOR itself is not inherently insecure: it's our use of it that determines the quality of the result. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterate over letters in a word
Yes - at the moment this is just a way for me to begin to get my head around how cryptography works from anabsolutely ludicrously basic position. This all started because I couldn't get my head around the difference between an encryption algorithm and the key. I thought that by writing my own, I would work it out! Hi Steve, Yup; it really has to do with numbers. Once we manage that leap, then it's just a matter of writing functions that transform numbers to other numbers. When you want to get more in depth, you may find: http://mitpress.mit.edu/sicp/psets/ps3/readme.html useful; it's a fun homework assignment on the RSA cryptosystem. The support functions are in Scheme, but converting it to Python isn't too bad. Best of wishes! ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterate over letters in a word
Here's something you might find useful they've just started a series on cryptography on this site, you can read them or listen to a podcast. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterate over letters in a word
On 14/03/06, Adam [EMAIL PROTECTED] wrote: Here's something you might find useful they've just started a series on cryptography on this site, you can read them or listen to a podcast. D'oh! Would help if I actually stuck the link in http://www.grc.com/SecurityNow.htm#30 ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
