Competition Custom Protocol Replay
Use this skill only as a downstream specialization after $ctf-sandbox-orchestrator is already active and has established sandbox assumptions, node ownership, and evidence priorities. If that has not happened yet, return to $ctf-sandbox-orchestrator first.
Use this skill when the hard part is not merely naming the protocol, but reproducing the exact message order and state needed for acceptance.
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Quick Start
- Identify client and server roles, session boundaries, and reset conditions before decoding field semantics.
- Recover framing, lengths, delimiters, sequence numbers, checksums, nonces, and state transitions before broad replay attempts.
- Keep one canonical transcript of a successful exchange.
- Change one field or one message at a time while replaying.
- Reproduce the smallest accepted conversation that proves the decisive branch.
Workflow
1. Map The Session State Machine
- Identify handshake, negotiation, authentication, keepalive, command, and teardown phases.
- Record which fields are static, which are derived, and which depend on prior messages.
- Keep message order, direction, and timing tied to the same session identity.
2. Recover Framing And Integrity
- Reconstruct lengths, delimiters, type bytes, checksums, MACs, counters, compression, or encryption boundaries.
- Distinguish transport framing from application-level framing.
- Note exactly where server acceptance changes when one field or step is mutated.
3. Build The Minimal Replay Harness
- Reduce the path to the smallest transcript that reaches the accepted state, parser branch, command effect, or artifact.
- Preserve both the original captured sequence and the replayed minimal sequence.
- If the problem is mainly generic PCAP or stream decoding with no stateful replay requirement, switch back to the broader PCAP skill.
Read This Reference
- Load
references/custom-protocol-replay.md for the state-machine checklist, transcript checklist, and evidence packaging.
What To Preserve
- Canonical transcript, message types, field boundaries, checksums, counters, and session identifiers
- Original capture slices and the replay harness inputs that produce acceptance
- The exact mutation that flips the protocol from rejected to accepted, or vice versa
