Security Blue Team VIP CTF #1 – “Twin” Write-up

The first CTF created by Security Blue Team was initially for subscribers only, but was made available to the public for a short time at the end of February 2020. While it covered network traffic analysis, password cracking, steganography, forensics, and some general knowledge challenges I didn’t have as much time as I would have liked to spend, so concentrated on the aspects that were most interesting to me personally.

Of the five “general knowledge” questions, four were multiple choice. This write-up covers the one general knowledge challenge which required a bit of command-line work – Twin. You can find the rest of my write-ups for Security Blue Team VIP CTF #1 here.

After downloading and extracting the archive we are indeed presented with 4400 files, totalling 88000 lines.

ls | wc -l
cat * | wc -l

Still, we can find the duplicate line by chaining together a few Linux command-line tools: cat, sort, and uniq. First we cat all 4400 files out, and sort all 88000 files into alphabetical order. Then use uniq with the -c flag to count the occurrences of each line. This should be 1 in every case except for our flag. Next use sort again, this time with the -n and -r flags so that we sort in numerical order, which is then reversed so that our duplicate line appears at the top of the list. Optionally, use head to restrict the output to the first 10 lines.

cat * | sort | uniq -c | sort -nr | head

2 VXZ5eWdiY1BHU3tnajFhNV9wNGFfYTNpM2VfbzNfZjNjNGU0NzNxfQ==

Given the == at the end of the line, the output looks like base64. Let’s feed it to CyberChef and see what we can do.

UvyygbcPGS{gj1a5_p4a_a3i3e_o3_f3c4e473q}

The From Base64 operation gave us human-readable text, but we still don’t have our flag in the correct format. It looks like a substitution cipher, so let’s try the Rot13 function.

That’s much better. We have our flag!

HilltopCTF{tw1n5_c4n_n3v3r_b3_s3p4r473d}

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