## Didactic XOR Cipher 3

### Didactic XOR Cipher 3

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?

Edit: I've come to the conclusion that either 'x' isn't constant or theres more to the challenge than it states

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

Edit: I've come to the conclusion that either 'x' isn't constant or theres more to the challenge than it states

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.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.

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

Please count me out next time!adum wrote:since tails solved the XOR 3, i don't think there's a mistake.

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.

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

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.tails wrote:I didn't notice these posts!

Please count me out next time!adum wrote:since tails solved the XOR 3, i don't think there's a mistake.

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.

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

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.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.

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 ><

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

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.

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.tails wrote:And the _really_ sick part is that gfoot solved it even in the broken state, that is just insane.

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.