sunshineCTF - Time Warp

Oh no! A t3mp0ral anoma1y has di5rup7ed the timeline! Y0u’ll have to 4nswer the qu3stion5 before we ask them!
nc tw.sunshinectf.org 4101

Solution:

When you connect to the server it gives you a prompt saying
“I’m going to give you some numbers between 0 and 999. Repeat them back to me in 30 seconds or less!”
so clearly this is a scripting challenge.

I manually input a few numbers to see how it responded and the server sent an error along with the correct number upon failure so a script that keeps trying to connect and updating the input file was necessary. Initially I was manually making a small input file for testing and if there were only correct numbers but not every correct number then the connection would halt and eventually close meaning we needed a placevalue number in our file (which i chose as 1) although I should have chose -1 because the number range was [0,999].
Here is my solution

#!/bin/bash
while :
do

    nc tw.sunshinectf.org 4101 < in.txt > out.txt
   
    #sometimes the first line returned was the next needed number 
    #and sometimes it was the error message so valA and valB handle both of those cases
   
    valA=$(tail -2 out.txt | head -1 | wc -c) 
    valB=$(tail -1 out.txt | wc -c)
    
    #gets only the line count value of the wc function and not also the trailing filename string
    inLen=$(wc -l in.txt | awk '{print $1}') 
    
    #the size of both values must be fewer than 5 bytes so we check to make sure one of the 
    #two is the actual next number we need. 
    
    #If it is then we take our original input file (which should have a trailing placeholder 1 at the moment)
    #and write to a temporary file with everything except for that 1. Next it will append the value we need
    #which is stored in the out.txt file (which is the server response file) onto the temporary file.
    
    #Finally it will append 1 to the temporary file again and then change the file name to in.txt.
   
    if [ $valA -lt 5 ]; then
        head -n -1 in.txt > temp.txt
        tail -2 out.txt | head -1 >> temp.txt
        echo 1 >> temp.txt
        mv temp.txt in.txt
        cat in.txt
    elif [ $valB -lt 5 ]; then
        head -n -1 in.txt > temp.txt
        tail -1 out.txt >> temp.txt
        echo 1 >> temp.txt
        mv temp.txt in.txt
        cat in.txt
    fi

done

After running the script there is a point where the 1 no longer gets replaced and the connection halts. Soon it closes and the bottom of our out.txt file contains the lines
“Wow! You did it!
As reward for fixing the timestream, here’s the flag:
sun{derotser_enilemit_1001130519}”

Challenge:

Description:

Solution: