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        riddles >> cs >> Breaking o t p
        
(Message started by: R0B1N on Mar 21^st, 2012, 4:43am)

Title: Breaking o t p
Post by R0B1N on Mar 21^st, 2012, 4:43am

Hi

I do have 11 strings XOR encrypted with same key.
Why i can break them ? at least one of them. appreciate any input

Logic:
For each of the possible string i and j find mssg_i xor mssg_j store in xors

go a grid look up to identify possible candidates for space

I'm damn new to python and below is the work so far. (uses a ^ ' ' = A , B ^ ' ' =
b etc ) (Sorry just couldnt get them indented properly)

Code:

for i in range(11):
xors.append([])
for j in range(11):
msg_i_xor_msg_j = strxor(msgs[i].decode('hex'),msgs[j].decode('hex'))
xors[i].append(msg_i_xor_msg_j)

Code:

for i in range(11):
print "==="
for j in range(11):
#for msg in xors:
out = ""
for char in xors[i][j]:
#if ord(char) >=32 and 126>=ord(char):
if (ord(char) >=65 and 90>=ord(char)) or (ord(char)>=97 and 122>= ord(char)):
out += char
else:
out += "_"
print out

Title: Re: Breaking o t p
Post by Grimbal on Mar 21^st, 2012, 5:35am

Suppose
msg[i] are the crypted messages you have,
key is the key,
clear[i] are the clear messages: msg[i] = clear[i] xor key.

Then (msg[i] xor msg[j]) = (clear[i] xor clear[j]). You get rid of the key, but it is still the xor of 2 words. It gives nothing readable.

Title: Re: Breaking o t p
Post by R0B1N on Mar 22^nd, 2012, 11:30am

Here is what i am doing

=> I am assuming all 11 msg[i]'s are [a-zA-Z] or ' '(space)

=> observation 'A' XOR ' ' = 'a' and 'a' XOR ' ' = 'A'

=>if msg[1] XOR msg[2] is a character at position k then either msg[1] or msg[2] has a space at position k
if all or most of msg[1] ^ msg[j] has space at position k then it is safe enough to conclude msg[1] has space at position k .
[hide]Find all spaces to get key[/hide]
http://en.wikipedia.org/wiki/Venona_project

Title: Re: Breaking o t p
Post by SMQ on Mar 28^th, 2012, 5:17am

on 03/21/12 at 04:43:56, R0B1N wrote:

Hi
I do have 11 strings XOR encrypted with same key.
Why i can break them ? at least one of them. appreciate any input

Posting assignments from Stanford's online cryptography class, are we? ;-) I'd suggest the class discussion forums are a more appropriate venue...

--SMQ

Title: Re: Breaking o t p
Post by R0B1N on May 1^st, 2012, 4:41am

on 03/28/12 at 05:17:57, SMQ wrote:

Posting assignments from Stanford's online cryptography class, are we? ;-) I'd suggest the class discussion forums are a more appropriate venue...

--SMQ

Yes It is from coursera.
It was hard to discuss anything in the coursera forum without spoiling the fun for others
I didn't want search engines to find it out before the deadline . Hence did not mention it :-[

Title: Re: Breaking o t p
Post by Barukh on May 7^th, 2012, 10:18pm

on 03/28/12 at 05:17:57, SMQ wrote:

Posting assignments from Stanford's online cryptography class, are we? ;-)

I am attending this class, and find it very interesting. The lecturer is excellent, albeit problems are a bit too easy, in my opinion.

Anybody else enrolled? Impression?