Solution #

When I execute the given command, I get this output:

 1$ nc mercury.picoctf.net 21135
 2112 
 3105 
 499 
 5111 
 667 
 784 
 870 
 9123 
10103 
1148 
1248 
13100 
1495 
15107 
1649 
17116 
18116 
19121 
2033 
2195 
22110 
2349 
2499 
2551 
2695 
27107 
2849 
29116 
30116 
31121 
3233 
3395 
3497 
35102 
36100 
3753 
38102 
39100 
4097 
4152 
42125 
4310

This seems to be a list of ASCII codes. I can use a converter from here to convert them to ASCII characters. I get this output:

1picoCTF{g00d_k1tty!_n1c3_k1tty!_afd5fda4}

Acknowledgement #

Thank you, CMU security and privacy experts, for creating this challenge.