Competition Custom Protocol Replay skill

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

1. Identify client and server roles, session boundaries, and reset conditions before decoding field semantics.
2. Recover framing, lengths, delimiters, sequence numbers, checksums, nonces, and state transitions before broad replay attempts.
3. Keep one canonical transcript of a successful exchange.
4. Change one field or one message at a time while replaying.
5. 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