SecurityFest 2017 - 2bright

In ancient times, giants ruled the world. Thought long gone, some giants has once again appeard, and might have even been here all the time. And though old still shine bright - maybe too bright? Note: Flag does not follow the format

Solves: 8

Download: https://github.com/jbzteam/CTF/blob/master/SecurityFest2017/2bright/2bright.tar.gz

By importing the given file on Audacity using 8 bit pcm as encoding and 1 as channel we can better understand the structure of the file. By zooming on about half the file we can notice the following waveform:

wave This suggests that in some way each byte at position i has been “encrypted” with some key dependent on i.

After some trials, we noticed that by transforming the data in the following way, something useful results.

for(int i=0;i<512;i++){
    result[i] = data[i] ^ (512-i);
} 

The obtained waveform is the following:

wave_manipulated

By removing all bytes at position x*64-1 and writing the result in binary, 3 bytes per line, replacing zeros with spaces, we obtain the following ascii art:

 1111111  1     1   111111   
   1     1  1         
   1     1  1         
   1     1  1         
   1111111  11111     
   1     1  1         
   1     1  1         
   1     1  1         
   1     1  1         
   1     1   111111   
                             
           1111111  1        
           1        1        
           1        1        
           1        1        
           1111     1        
           1        1        
           1        1        
           1        1        
           1        1        
           1        1111111  
                             
  11111             
   1  1     1            
   1  1     1            
   1  1          11      
 1111111  1  111     11      
   1  1     1            
   1  1     1    11      
   1  1     1    11      
   1  1     1            
   1   11111             
                             
           1111111  11111    
           1       1     1   
           1       1     1   
           1       1     1   
           1111    1111111   
           1       1     1   
           1       1     1   
           1       1     1   
           1       1     1   
           1       1     1   
                             
 111111   1        
   1     1  1        
   1     1  1        
   1     1  1        
   111111   1        
   1 1      1        
   1  1     1        
   1   1    1        
   1    1   1        
 1     1  1111111  
                             
 11111   1     1   
  1     1  1     1   
  1     1  1     1   
  1        1     1   
  1  111   1111111   
  1     1  1     1   
  1     1  1     1   
  1     1  1     1   
  1     1  1     1   
 11111   1     1   
                             
 1111111            111111   
                   1  
                   1  
                   1  
               1111   
                   1  
                   1  
                   1  
                   1  
             111111   
                             
 1     1  111111   
  1  1     1  1     1  
1   1  1     1  1     1  
1   1   11111   1     1  
1  1     1     111111   
1  1     1     1 1      
 1 1     1     1  1     
 1 1     1     1   1    
  11     1     1    1   
    1     1     1

THE FLAG: FAIRLIGHT 30YR

It is actually accepted if written without spaces.

Actually, the given file contains other stuff, that we couldn’t understand. Also, a given hint said that we needed to use a debugger, we don’t know why.

And now, the solution in 1 line :D

od -j 6145 -N 512 -t u1 -An -w1 -v 2bright | awk 'BEGIN {i=0} { c=xor($1,(512-NR+1))%256; if(c>127)c-=256; if(NR%64!=0)printf "%c",c }' | xxd -b -c 3 | cut -b 10-38 |tr 0 ''

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