credstuff picoCTF 2022 Solution

Description

1. Step 1Map the username

   Observation

   The archive holds usernames.txt and passwords.txt, aligned line for line. So the line number of the target username in one file indexes straight to the right entry in the other.

   grep -n "cultiris" usernames.txt shows the account at line 378. Because both files are line-aligned, that same line number in passwords.txt holds the encrypted secret.

   bash

   grep -n cultiris usernames.txt

   Copied!

   bash

   paste usernames.txt passwords.txt | grep cultiris

   Copied!

   Expected output

   378:cultiris

   What didn't work first

   Tried: Run grep without -n to find the username and assume the password is on the same screen output.

   grep without -n returns the matching line and no line number, so you see the username but have no index to carry into passwords.txt. Without it you are counting rows by hand across a thousand entries, which will go wrong.

   Tried: Use paste usernames.txt passwords.txt | grep cultiris and try to decode the password with a base64 decoder instead of ROT13.

   The ciphertext is not valid base64: it contains curly braces and its character distribution does not match the base64 alphabet, so base64 -d errors or returns garbled bytes. The real clue is that the prefix has the same letter-count shape as picoCTF, which points at a fixed-shift Caesar rather than an encoding.

   Learn more

   Credential dumps (or "combo lists") typically ship as paired text files: line N in usernames.txt matches line N in passwords.txt. grep -n prints the line number alongside the match, which is what links the two files together.

   For one shot, just paste them: paste usernames.txt passwords.txt | grep cultiris joins the columns side-by-side and shows you the user and their cipher in one row. Same idea as zip() in Python. Saves you from manually sed -n 'Np''ing on the password file.

   The root cause of this challenge: passwords were stored in plaintext (or trivially reversible) on the server. Real systems store a salted slow hash (bcrypt, scrypt, Argon2). When the dump leaks, attackers must crack each hash one by one instead of reading off the password directly. Cracking workflow in Hash Cracking for CTFs.

2. Step 2Retrieve the password entry

   Observation

   grep -n puts the username on line 378, so sed pulls line 378 out of passwords.txt for the stored ciphertext.

   sed -n '378p' passwords.txt prints cvpbPGS{P7e1S_54I35_71Z3} - a substitution that still looks like the picoCTF format.

   Learn more

   How to recognize ROT13 visually: the ciphertext preserves word boundaries, punctuation, and case, and the prefix cvpbPGS has the same shape as picoCTF (4 lowercase + 3 uppercase). Each character is shifted by 13: p -> c, i -> v, c -> p, o -> b. Once you spot a flag-shaped string with the wrong letters, count the shift on one or two characters and ROT13 falls out. More cipher-spotting tactics in CTF Encodings.

   sed -n 'Np' is the shortest way to print line N. Alternatives: awk 'NR==N' or head -N file | tail -1.

3. Step 3Apply ROT13

   Observation

   The retrieved ciphertext keeps the flag shape, curly braces and all, and its prefix matches picoCTF for length and letter case. That is a fixed-shift Caesar, and the shift is 13.

   Running the line through caesar 13 (from bsdgames) or any ROT13 decoder transforms it back into plaintext, yielding the final flag.

   Learn more

   ROT13 is a Caesar cipher with a fixed shift of 13. The mathematical formulation is E(x) = (x + 13) mod 26 where x is the letter's zero-based position (A=0, B=1, ..., Z=25). Because 13 is exactly half of 26, applying ROT13 twice gives (x + 26) mod 26 = x, so the cipher is its own inverse.

   Worked example on the prefix 'cvpb' (where a=0, b=1, ..., z=25):
     c (= 2)  -> (2  + 13) mod 26 = 15 -> 'p'
     v (= 21) -> (21 + 13) mod 26 = 8  -> 'i'
     p (= 15) -> (15 + 13) mod 26 = 2  -> 'c'
     b (= 1)  -> (1  + 13) mod 26 = 14 -> 'o'
   Result: 'pico'  -> matches the expected flag prefix.

   On Linux, caesar 13 (from bsdgames) applies the shift. Other quick methods: tr 'A-Za-z' 'N-ZA-Mn-za-m', Python's codecs.encode(s, 'rot_13'), or any online ROT13 tool. Storing passwords in ROT13 is essentially storing them in plaintext - real password storage requires a slow hash (bcrypt, scrypt, Argon2) with a per-user salt.

Flag

Because the files are line-aligned, finding the username index immediately pinpoints the paired password.