hacker.org Forum Index
RegisterSearchFAQMemberlistUsergroupsLog in
Didactic XOR Cipher 3
Goto page 1, 2, 3  Next
 
Reply to topic    hacker.org Forum Index » Challenges View previous topic
View next topic
Didactic XOR Cipher 3
Author Message
MerickOWA



Joined: 07 Apr 2008
Posts: 182
Location: HkRkoz al KuwaiT 2019 HaCkEr 101

Post Didactic XOR Cipher 3 Reply with quote
Having a bit of confusion understanding the 'x' parameter.

Is this a constant? Or does this change? Or is this question part of the challenge? Wink

Edit: I've come to the conclusion that either 'x' isn't constant or theres more to the challenge than it states Wink
Thu Oct 09, 2008 3:33 pm View user's profile Send private message
adum



Joined: 19 Apr 2007
Posts: 390

Post Reply with quote
'x' is a constant. it is 'b' that changes after each enciphered byte. i'll try to think of a better way to explain this one. since it has been solved, at least i know it's doable however =)

adum
Thu Oct 09, 2008 4:49 pm View user's profile Send private message Visit poster's website
MerickOWA



Joined: 07 Apr 2008
Posts: 182
Location: HkRkoz al KuwaiT 2019 HaCkEr 101

Post Reply with quote
Hrm, thats how i interpreted it the first time. I think my assumptions about what kind of data this is might be incorrect then.

No combination of b and x produced anything I recognized anyway Wink
Thu Oct 09, 2008 8:17 pm View user's profile Send private message
Shirkit



Joined: 09 Oct 2008
Posts: 2

Post Reply with quote
Hint: OR AND NOT...
Thu Oct 09, 2008 9:35 pm View user's profile Send private message
gfoot



Joined: 05 Sep 2007
Posts: 269
Location: Brighton, UK

Post Reply with quote
I decoded part of it, but not the answer. It does look a bit broken... some challenges might be intentionally easy to partially decode, but from the description of this one I wouldn't have expected that.

I have found that the Didactic Long XOR challenge has a similar fault, although I solved that one. I wondered whether it was a typo in the ciphertext.
Thu Oct 09, 2008 11:27 pm View user's profile Send private message
adum



Joined: 19 Apr 2007
Posts: 390

Post Reply with quote
since tails solved the XOR 3, i don't think there's a mistake.

i think what happens is that because this 'cipher' is so weak, you can get values for x that decode some of the text but not others. for a real cipher like AES, it's an all-or-nothing proposition.

adum
Thu Oct 09, 2008 11:41 pm View user's profile Send private message Visit poster's website
MerickOWA



Joined: 07 Apr 2008
Posts: 182
Location: HkRkoz al KuwaiT 2019 HaCkEr 101

Post Reply with quote
gfoot wrote:
I decoded part of it, but not the answer. It does look a bit broken... some challenges might be intentionally easy to partially decode, but from the description of this one I wouldn't have expected that.

I have found that the Didactic Long XOR challenge has a similar fault, although I solved that one. I wondered whether it was a typo in the ciphertext.


Yea i see what you mean.

Edit: I've almost figured out whats wrong with Didactic Long XOR challenge. I'll send adum a pm when i figure it all out.

Edit: Solved Didactic Long. Not going to post exactly whats wrong with it, as it might make the problem more interesting Wink
Fri Oct 10, 2008 1:22 am View user's profile Send private message
adum



Joined: 19 Apr 2007
Posts: 390

Post Reply with quote
whoops, i messed up! the didactic long and XOR 3 should be fixed now. sorry about that!

thanks to merrickowa and tails for the corrections.

the _really_ sick part is that people solved these even in the broken state. especially on XOR 3, that is just insane.

thanks,
adum
Fri Oct 10, 2008 5:35 am View user's profile Send private message Visit poster's website
tails



Joined: 10 Jun 2008
Posts: 191
Location: Tokyo

Post Reply with quote
I didn't notice these posts!

adum wrote:
since tails solved the XOR 3, i don't think there's a mistake.

Please count me out next time! Very Happy

As MerickOWA and gfoot should have done, 'x' could be easily found anyway. So, with some try-and-errors, it was not so insane to solve it once you discover how it was broken. 'b' doesn't matter; it has only 256 possibilities. Wink

But it's funny that I cannot solve 'Broken XOR 3' so far ... Laughing
Fri Oct 10, 2008 10:02 am View user's profile Send private message
canine



Joined: 14 Sep 2008
Posts: 189

Post Reply with quote
tails wrote:
I didn't notice these posts!

adum wrote:
since tails solved the XOR 3, i don't think there's a mistake.

Please count me out next time! Very Happy

As MerickOWA and gfoot should have done, 'x' could be easily found anyway. So, with some try-and-errors, it was not so insane to solve it once you discover how it was broken. 'b' doesn't matter; it has only 256 possibilities. Wink

But it's funny that I cannot solve 'Broken XOR 3' so far ... Laughing


Sure, but if I'm understanding the problem correctly, and there is a significant chance that I'm not, b can be any 8 bit number. So if b can range from 0-255, and so can x, then we have 65536 possibilities, too many to sift through to find the answer.

If I am missing something, or someone has a hint for me, it would be greatly appreciated.
Fri Oct 10, 2008 12:04 pm View user's profile Send private message
MerickOWA



Joined: 07 Apr 2008
Posts: 182
Location: HkRkoz al KuwaiT 2019 HaCkEr 101

Post Reply with quote
canine wrote:


Sure, but if I'm understanding the problem correctly, and there is a significant chance that I'm not, b can be any 8 bit number. So if b can range from 0-255, and so can x, then we have 65536 possibilities, too many to sift through to find the answer.

If I am missing something, or someone has a hint for me, it would be greatly appreciated.


Actually, its not too bad to do. I attempted writing a program that went through all 65536 possibilities and then did a quick check of the string to see if it had good data.

So far tho, either my program is wrong, or my assumptions about the data is wrong because I haven't been successful on decrypting it ><
Fri Oct 10, 2008 2:14 pm View user's profile Send private message
adum



Joined: 19 Apr 2007
Posts: 390

Post Reply with quote
okay, i messed up again! i was modding by 255 instead of 256. whoops. didactic XOR 3 and broken 3 are now fixed... sorry

adum
Fri Oct 10, 2008 4:18 pm View user's profile Send private message Visit poster's website
tails



Joined: 10 Jun 2008
Posts: 191
Location: Tokyo

Post Reply with quote
adum wrote:
okay, i messed up again! i was modding by 255 instead of 256. whoops. didactic XOR 3 and broken 3 are now fixed... sorry


And the _really_ sick part is that gfoot solved it even in the broken state, that is just insane. Very Happy
Fri Oct 10, 2008 4:26 pm View user's profile Send private message
gfoot



Joined: 05 Sep 2007
Posts: 269
Location: Brighton, UK

Post Reply with quote
canine wrote:
If I am missing something, or someone has a hint for me, it would be greatly appreciated.

It's not a large search space, and it's easy to scan the results for things that might be correct. I think most of us tripped up on the fact that none of the results actually look correct to begin with!

That said, I used a state-of-the-art pattern-recognition device coupled to optical sensors to scan the search space more efficiently, reducing it to O(n log n) rather than O(n^2).

Unfortunately the pattern-recognition device is slower than brute-forcing it, and relatively expensive to run, but (much like Mortal Coil) it's more fun that way. Smile
Fri Oct 10, 2008 4:38 pm View user's profile Send private message
gfoot



Joined: 05 Sep 2007
Posts: 269
Location: Brighton, UK

Post Reply with quote
tails wrote:
And the _really_ sick part is that gfoot solved it even in the broken state, that is just insane. Very Happy

Well, the funny thing is, I was trying a 255 modulus on the old broken version, because I wasn't sure what was wrong with it (especially what could cause me to decrypt the end of the message perfectly but not the start). I'd left my solver with the wrong modulus, though, when the fixed challenge went up, and didn't notice until I'd already solved it.

No doubt there'll be another variant of the challenge up soon, where you also need to figure out the modulus. The interaction between x and m has interesting effects.
Fri Oct 10, 2008 4:54 pm View user's profile Send private message
Display posts from previous:    
Reply to topic    hacker.org Forum Index » Challenges All times are GMT
Goto page 1, 2, 3  Next
Page 1 of 3

 
Jump to: 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group
Design by Freestyle XL / Flowers Online.