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Loyal_book.md

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Problem Statement

Every book is unique in its own way! Go and read the story of a brave man!

We're provided with a zip file books.zip

$ unzip books.zip -d books/
Archive:  books.zip
  inflating: books/0001.txt          
  inflating: books/0002.txt          
  inflating: books/0003.txt          
  inflating: books/0004.txt          
  inflating: books/0005.txt          
  inflating: books/0006.txt          
  inflating: books/0007.txt          
  inflating: books/0008.txt          
  inflating: books/0009.txt          
  inflating: books/0010.txt

So, all these txt are like huge novels, and I currently had no idea what to do. As I started reading some a little bit, I found that they were all same in the starting.Hmmm. Let's find if something is different.

$ sort books/0001.txt books/0002.txt | uniq -u
glimpse last summer at the Palais-Royal. Some of 
glimpse last summer DC at the Palais-Royal. Some of

I can see above that there's DC extra in 0002.txt. Still not sure where to go. Let's check again for other file.

$ sort books/0001.txt books/0003.txt | uniq -u
benches ranged along TFthe walls, and in the centre of 
benches ranged along the walls, and in the centre of

So here, TF is an extra word in the file 0003.txt as compared to 0001.txt, rest is same. So, so far, we've got DCTF. Now we know exactly what to do :D

But when I did this for 0001.txt and 0010.txt, I got more than 2 lines with different text, and sort command just messes up with the order, so can't use that to get the flag.

Why not use diff then, right? Let's write a loop to traverse all txt files and create a dump of diff of all files with 0001.txt.

$ for file in books/*; do diff books/0001.txt $file | grep -v "^---" | grep -v "^[0-9c0-9]" >> dump; done
$ wc -l dump
32 dump
$ # We'll take two lines at a time and find the words which are different, narrowing down our search
$ for i in {1..16}; do head -"$((2*i))" dump | tail -2 | grep -oE '\w+\S+' | sort | uniq -u; done

DC
TFthe
have
h{ave
7ba61
wa0cc5ds
was
aa3966nd
and
fob7c64r
for
a81c3
Morcfcdbeau,
Moreau,
1b1d0
9e3de
5ad11
89268
bf0e6
18ff7
1f08}

So, we can easily figure out the flag now.

Flag: DCTF{7ba610cc5da3966b7c64a81c3cfcdb1b1d09e3de5ad1189268bf0e618ff71f08}