FE04 - 100pts - NCS Competition 2021 Writeup

Briefing
Download the file and filter down to the username according to criteria below. The username you are looking for has x as the 3rd character, followed immediately by a number from 2 to 6, it has a Z character in it and the last character is S. When you have the username, submit it as the flag. Contents: 50k-users.txt
Challenge Files:
​fe04.zip​
Solution
1.This is a simple data processing challenge that can be solved with a little Python and a basic regular expression.
2.The regular expression is ..x[2-6].*S$ and can be built and tested easily using RegExr. The first two dots match any character, next the x matches an x, [2-6] matches a number from 2 to 6 inclusive, a dot again can match any character and the star * modifies the dot so that an unlimited number of any character can be matched. Finally, the S matches an S and the $ ensures that this marks the end of the string being checked. We will handle the "it has a Z character in it" criteria later.
3.We can test every username in the 50k-users.txt file against the regular expression we built using Python. We compile the regular expression and then use filter to test it against every username. We then filter all the matches again by checking if they contain a Z, thus fullying applying the challenge criteria. The final matching username is printed and is the flag.
Flag
YXx52hsi3ZQ5b9rS

FH01 - 500pts

FM01 - 250pts

Last modified 1yr ago

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Contents

Briefing

Solution

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