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picoCTF Nice netcat

This writeup is about solving the Nice netcat challenge on picoCTF.

The description tells us that “There is a nice program that you can talk to by using this command in a shell: $ nc mercury.picoctf.net 22902, but it doesn’t speak English…”

So let’s open a terminal window and listen to what that program has to say:

$ nc mercury.picoctf.net 22902
112 
105 
99 
111 
67 
84 
70 
123 
103 
48 
48 
100 
...

Oh, what a bunch of numbers. But what do they mean? Any ideas? They are the ASCII representation of characters. ASCII stands for American Standard Code for Information Interchange’. Fortunately there is a Python command to convert this numbers back to characters. So a small python script will do the whole work for us:

#!/usr/bin/env python3
import sys

for line in sys.stdin:
  print(chr(int(line)), end='')

Save the file as decode.py and set it executable with chmod u+x decode.py.

All we have to do now is piping the output from netcat to the python script:

$ nc mercury.picoctf.net 22902 | ./decode.py
picoCTF{flag}

Congratulations. You’ve got another flag.

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