Overthewire Bandit Level 11 -> 12

Solution for the Overthewire.org Bandit level 11 -> 12

Level Goal

The password for the next level is stored in the file data.txt, where all lowercase (a-z) and uppercase (A-Z) letters have been rotated by 13 positions

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Walkthrough

Login to the server using the password obtained from the previous level Bandit level 10 -> 11.

username: bandit11

ssh bandit11@bandit.labs.overthewire.org -p 2220

The file data.txt is located in the home directory.

bandit11@bandit:~$ ls
data.txt 

We cat the contents of the file and can see that it does not make any sense when we try to read.

bandit11@bandit:~$ cat data.txt
Gur cnffjbeq vf 5Gr8L4qetPEsPk8htqjhRK8XSP6x2RHh

Going back to the description it mentions characters being rotated, this immediately bring ROT13 to mind.
In a ROT13 cipher characters are rotated 13 places this means

a becomes n
a is 1st letter of the alphabet.
Rotate 13 places 1 + 13 = 14.
14th letter of alphabet is n.

t becomes g
t is the 20th letter of the alphabet.
Rotate 13 places 20 + 13 = 33.
Since 33 is greater then 26 (26 letters in alphabet) we take 26 from 33 to bring it back into the alphabet range.
33 - 26 = 7
7th letter of the alphabet is g.

Full ROT13 mappings

Character	ROT13	Character	ROT13	Character
a	->	n	->	a
b	->	o	->	b
c	->	p	->	c
d	->	q	->	d
e	->	r	->	e
f	->	s	->	f
g	->	t	->	g
h	->	u	->	h
i	->	v	->	i
j	->	w	->	j
k	->	x	->	k
l	->	y	->	l
m	->	z	->	m

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Now with an understanding of ROT13 we can move onto the level.

We can use a linux command line tool tr (translate) to translate the rotated characters back to their original posistion.

bandit11@bandit:~$ cat data.txt
Gur cnffjbeq vf 5Gr8L4qetPEsPk8htqjhRK8XSP6x2RHh

bandit11@bandit:~$ cat data.txt | tr a-zA-Z n-za-mN-ZA-M
The password is 5Te8Y4##########################

Or we can one of many online tools like CyberChef that have graphical interfaces to solve the ROT13 ciphertext.