FindAndOpen picoCTF 2023 Solution

Description

1. Step 1Extract the hint

   Observation

   The challenge pairs a PCAP with a password-protected ZIP. The password is inside the capture, reachable with strings and a base64 decode rather than brute force.

   The PCAP contains printable strings referencing the flag. Base64-decode them to reveal the password fragment. Decoding the captured Base64 yields plaintext like "This is the secret: picoCTF{R34DING_LOKd_fil56...9b}" so you can verify decoding worked before attempting the unzip.

   Optional alternate path: binwalk -e dump.pcap can carve embedded files out of the capture. It is rarely needed here (strings + base64 is enough) but is worth knowing for PCAPs that smuggle full archives over plaintext protocols.

   What didn't work first

   Tried: Opening the PCAP in Wireshark and scanning each packet manually looking for the password.

   Scrolling hundreds of packets by hand is slow and easy to get wrong. strings surfaces all the printable text at once, and grep filters it to the Base64-looking output. Wireshark earns its place on deeper analysis and is overkill for finding one embedded string.

   Tried: Treating the extracted string as the flag directly without Base64-decoding it.

   The payload in the PCAP is Base64-encoded, not raw plaintext. Pasting the encoded string as the ZIP password will fail with a wrong-password error. The correct step is to first decode it with 'base64 --decode' or CyberChef to get the actual password fragment.

   Learn more

   PCAP (Packet Capture) files store raw network traffic recorded by tools like Wireshark or tcpdump. Each packet is saved with its full headers and payload. When traffic is unencrypted, sensitive data - credentials, tokens, file contents - can be recovered simply by reading the payload bytes.

   The strings command extracts sequences of printable ASCII characters from any binary file, including PCAPs. Because packet payloads are just byte arrays, strings dump.pcap surfaces any human-readable text transmitted over the network - no protocol knowledge required. For more targeted analysis, Wireshark's "Follow TCP Stream" reassembles the full application-layer conversation, and its "Export Objects" feature can save transferred files (HTTP, FTP, SMB) directly to disk.

   Base64 is not encryption - it is an encoding scheme that represents binary data using 64 printable ASCII characters. Its characteristic alphabet (A-Z, a-z, 0-9, +, /) and padding = signs make it immediately recognizable. In this challenge the attacker encoded the password to avoid triggering simple keyword filters, but Base64 is trivially reversible with base64 --decode or CyberChef.

2. Step 2Unlock the archive

   Observation

   The decoded Base64 from the capture is a picoCTF{...} token. That is the ZIP password for extracting the real flag file.

   Use the recovered fragment as the password to unzip the provided archive, then open flag.txt for the complete flag.

   What didn't work first

   Tried: Using the full decoded string including the surrounding sentence as the ZIP password.

   The decoded output is something like 'This is the secret: picoCTF{...}'. Only the flag value itself (picoCTF{...}) is the ZIP password. Passing the entire sentence produces a wrong-password error. Strip the label text and use only the flag-shaped token.

   Tried: Attempting to crack the ZIP password with John the Ripper before finishing the PCAP analysis.

   The ZIP password is hidden in the PCAP, so brute-forcing is the hard path. This challenge is designed as a two-stage scavenger hunt - the PCAP contains the password for the ZIP. Finishing the PCAP extraction step first is always faster than dictionary or brute-force attacks.

   Learn more

   Password-protected ZIP archives use a symmetric cipher (traditionally PKZIP stream cipher; modern archives use AES-256) to encrypt file contents. The password must be supplied at extraction time. The unzip -P 'password' archive.zip flag passes the password non-interactively, which is useful in scripts.

   If you do not know the password, offline cracking tools like John the Ripper (zip2john archive.zip > hash.txt && john hash.txt) or hashcat can brute-force or dictionary-attack ZIP hashes. This challenge avoids brute force by hiding the password inside the PCAP - a scavenger-hunt technique common in multi-stage CTF forensics problems.

   A useful mental model: in real incident response, adversaries often exfiltrate data inside password-protected archives to evade DLP tools. Investigators recover the password from the same traffic channel (email, chat, another packet capture) and then decrypt the archive to see what was stolen.

Flag

Ensure you remove ellipses or extra text from the decoded string before using it as the ZIP password.