Path: ...!eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: Richard Heathfield <rjh@cpax.org.uk>
Newsgroups: sci.crypt
Subject: Re: CHAL2X
Date: Sun, 26 Sep 2021 06:59:11 +0100
Organization: Fix this later
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On 25/09/2021 23:53, Max wrote:
> On 25.09.21 23:14, Richard Heathfield wrote:
>> On 25/09/2021 22:03, Max wrote:
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>>> Also, this difference stays the same for two consecutive characters.
>>
>> I have no idea why that might be. My algorithm did nothing special for 
>> two consecutive characters. I conclude that you've done your Bright 
>> Eve[1] thing again and have not discerned my algorithm, although 
>> you're obviously quite close.
>>
>> [1] Not *that* bright. :-) This was solvable using standard techniques 
>> to be found in any cryppie's kitchen drawer, and I have no doubt 
>> that's how you did it. The one I *was* going to post was more of a 
>> bitch but I ran out of wossname, motivation to code it up.
>>
> 
> 
> Of course. Just by the book (this time the "and"s, not the "the"s). Yet, 
> do you really think, it is possible to find the algorithm before finding 
> the plaintext? While some person smarter than me might just eyeball the 
> CHATX algorithm from the ciphertext, I don't see how this could be done 
> here.

You think I'm being a bit tight? You're probably right. I think on 
reflection(!) I should have given it to you when you talked about 
nybbles being mirror images, even though nybbles had no part of the 
algorithm.

It was pure bit reversal of the middle six bits of each octet:

   while((ch = getchar()) != EOF)
   {
     int i;
     bi = (unsigned char)ch;
     bo = (unsigned char)ch;
     for(i = 1; i < 7; i++)
     {
       int j = 7 - i;
       if(BIT_QRY(p, i))
       {
         BIT_CLR(q, j);
       }
       else
       {
         BIT_SET(q, j);
       }
     }
     putchar(bo);
   }


I left the high bit alone so that ASCII would stay ASCII, and so I left 
the low bit alone for symmetry. I'd have posted the ciphertext as ASCII 
were it not for the fact that I was getting a lot of backspaces in the 
ciphertext, which I thought might confuse the issue.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within