credstuff picoCTF 2022 Solution

Description

1. Step 1

   Map the username

   Observation

   I noticed the archive contained two separate files, usernames.txt and passwords.txt, aligned line-by-line, which suggested that finding the line number of cultiris in the username file would directly index the correct password entry in the other file.

   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 just the matching line content with no line number, so you see 'cultiris' but have no index to carry over to passwords.txt. Without the line number you have no way to pick the right row from passwords.txt unless you count manually, which is error-prone across 1000+ entries.

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

   The ciphertext 'cvpbPGS{P7e1S_54I35_71Z3}' is not valid base64 because it contains curly braces and the character distribution does not match base64 alphabet. base64 -d returns an error or garbled bytes. The correct clue is that the prefix 'cvpbPGS' has the same letter-count shape as 'picoCTF', pointing to a fixed-shift Caesar cipher rather than an encoding scheme.

   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 2

   Retrieve the password entry

   Observation

   I noticed that grep -n returned line 378 for cultiris, which told me exactly which line to extract from passwords.txt using sed -n '378p' to get 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 3

   Apply ROT13

   Observation

   I noticed the retrieved ciphertext cvpbPGS{P7e1S_54I35_71Z3} preserved the flag-shaped structure with curly braces and had the prefix cvpbPGS matching the length and case pattern of picoCTF, which indicated a fixed-shift Caesar cipher and a shift of 13 (ROT13) was needed to decode it.

   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.