competition-zip-archive skill

Competition ZIP Archive

Use this skill only as a downstream specialization after $ctf-sandbox-orchestrator is active and has established sandbox assumptions, evidence priorities, and the analysis project root. If that has not happened yet, return to $ctf-sandbox-orchestrator first.

Use this skill when the decisive path is an encrypted ZIP/PKZIP archive rather than an upload parser or a generic crypto blob. Prefer the legacy ZipCrypto known-plaintext path when the challenge gives a predictable file, format header, template, or other recoverable plaintext. Do not begin with blind password brute force.

Reply in Simplified Chinese unless the user explicitly requests English. Keep commands and tool output in their original form.

Quick Start

1. Preserve the original archive, compute a hash, and work on a copy under the analysis project's work// directory.
2. Confirm the actual archive format and list entries before attempting a password attack.
3. Determine whether the entry uses legacy ZipCrypto. bkcrack does not recover WinZip AES or other modern encryption.
4. Build an exact known-plaintext candidate. The attack needs at least 12 known plaintext bytes, including at least 8 contiguous bytes.
5. Recover the internal keys with bkcrack, then create an unencrypted copy and extract it.
6. Preserve the command, entry names, known-plaintext source, recovered keys, output hash, and final flag as evidence.

Tool Setup

Use the normal tool index and bootstrap path before guessing an executable location:

powershell -NoProfile -ExecutionPolicy Bypass -File skills/scripts/refresh-tool-index.ps1
powershell -NoProfile -ExecutionPolicy Bypass -File skills/scripts/bootstrap-reverse.ps1 -Capability bkcrack

On Kali, use the equivalent capability command:

bash kali/scripts/bootstrap-reverse.sh bkcrack

The Windows capability is pinned to the v1.8.1 release and verifies the GitHub asset digest. If the tool is installed manually, refresh skills/tool-index.md afterward.

Workflow

1. Establish Archive Truth

Keep the original immutable and record:

sha256sum challenge.zip
file challenge.zip
bkcrack -L challenge.zip

On Windows, use Get-FileHash in place of sha256sum. bkcrack -L shows entry names, compression methods, and encryption status. Do not infer the encryption type from the .zip extension alone.

Check the entry that will provide the known plaintext. A useful candidate is a stored or otherwise predictable file such as a PNG, PDF, text template, or challenge-generated configuration. A local ZIP header (PK\x03\x04) is metadata for the archive entry, not plaintext inside the encrypted member, so it is not by itself a useful known-plaintext sample.

2. Recover Keys With Known Plaintext

When the ciphertext entry is flag.txt and a matching plaintext entry is available in known.zip:

bkcrack -C challenge.zip -c flag.txt -P known.zip -p flag.txt

For raw ciphertext and plaintext files:

bkcrack -c cipherfile -p plainfile

The plaintext must match the bytes represented in the encrypted entry. If the entry was deflated, an uncompressed copy of the file is not automatically the right input; use a matching ZIP fixture or the exact compressed bytes.

If the known bytes begin at an offset, add -o . If only 8-11 bytes are contiguous, combine them with other known bytes using sparse hints:

bkcrack -c cipherfile -p plainfile -x 25 4b4f -x 30 21

The successful run yields three internal ZipCrypto keys. Record them exactly as printed; they are not the original password.

3. Unlock And Validate

Use the recovered keys to make a new archive, leaving the source untouched:

bkcrack -C challenge.zip -k K0 K1 K2 -D unlocked.zip
7z t unlocked.zip
7z x unlocked.zip -ounpacked

Replace K0 K1 K2 with the hexadecimal values printed by bkcrack. Validate the output with the archive test command and a hash or exact flag comparison. If only one raw member is needed, -d can write its deciphered bytes; deflated raw data may need the inflate.py helper shipped with bkcrack.

4. Re-route When Preconditions Fail

• WinZip AES or another modern encryption mode: stop this path and identify the challenge-specific primitive.
• No reliable known plaintext: inspect filenames, metadata, compression choices, challenge source, and other entries before considering password recovery.
• Known plaintext shorter than the requirement: locate more contiguous bytes or use evidence-backed sparse offsets.
• The problem is an application upload/parser chain: hand off to $competition-file-parser-chain.
• The problem is a generic ciphertext or custom cipher after archive extraction: hand off to $competition-crypto-mobile.

Evidence To Preserve

• Original and working-copy paths plus SHA-256 hashes
• bkcrack -L output and the selected encrypted member
• Exact plaintext fixture, compression method, offsets, and sparse byte hints
• Key recovery command and the three recovered internal keys
• unlocked.zip validation output, extraction path, and final artifact hash

Read references/zip-archive.md for the decision table and evidence checklist.