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Custom Protocol Replay

Reconstructs custom protocol handshakes and builds minimal replay harnesses for CTF sandbox work.

Source Download skill
Repository
zhaoxuya520/reverse-skill
Stars
43,969
Licence
MIT
Files
3 files

What it does

This skill gives an AI agent a specialized method for analysing an unknown binary or text communication protocol inside a CTF (capture-the-flag) sandbox. The agent maps the session's message order, works out framing, lengths, sequence numbers and checksums, and then reduces a captured conversation to the smallest sequence of messages the server accepts. It is a downstream step: it is meant to run only after a separate parent skill, ctf-sandbox-orchestrator, has set up sandbox assumptions. Replies are in Simplified Chinese unless English is requested.

Great for

  • Security students working on CTF protocol-reversing challenges in an authorised sandbox
  • Researchers who need to reproduce an accepted message sequence from a captured custom protocol session
  • Teams running internal competition labs that already use the ctf-sandbox-orchestrator skill

Skip it if

  • Attacking live or third-party systems: the skill is written for sandboxed competition targets and does not establish authorisation, which is risky outside a lab
  • Generic packet capture analysis with no stateful replay need: the skill itself says to switch to the broader PCAP skill
  • Users who do not already have the parent ctf-sandbox-orchestrator skill, since the files tell the agent to return there first

Try asking

  • “Decode this unknown binary protocol from my captured session and recover its framing and checksum rules.”
  • “Build a minimal replay harness that reaches the accepted branch in this CTF challenge.”
  • “Work out which message order satisfies the server's sequence-number check.”
Step-by-step guide: how to use Custom Protocol ReplayInstall, first run, examples, tips and FAQs

Install in 3 steps

  1. 1

    Download the skill

    The ZIP contains the 3 original files, the MIT licence and an attribution note.

    competition-custom-protocol-replay.zip
  2. 2

    Add it to your agent

    Claude Code (all projects)

    mkdir -p ~/.claude/skills && unzip competition-custom-protocol-replay.zip -d ~/.claude/skills/

    Claude Code / Agent SDK (this project only)

    mkdir -p .claude/skills && unzip competition-custom-protocol-replay.zip -d .claude/skills/

    Claude apps (web/desktop): Settings → Capabilities → Skills → Upload skill, and choose the ZIP.

  3. 3

    Ask for the task

    No special command needed — the agent reads the skill's description and loads it when your request matches. Try one of the prompts above.

Get better results

  • Run the parent ctf-sandbox-orchestrator skill first so sandbox assumptions and scope are set.
  • Provide a clean captured transcript of one successful exchange as the starting point.
  • Confirm the target is an authorised lab before asking for replay work.

What's inside

  • agents/openai.yaml
  • references/custom-protocol-replay.md
  • SKILL.md

Redistributed unmodified from zhaoxuya520/reverse-skill under the MIT licence. The assessment was drafted with AI from the skill's files on 2026-10-29. Skills can instruct your agent to run code — read SKILL.md and any scripts before installing.