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reverse-eng-protocol

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Reverse a custom network protocol from captured traffic or the client binary: recover the frame layout, message-type dictionary, field meanings, and state machine. Load for custom TCP/UDP binary protocols, Protobuf/gRPC without reflection, FlatBuffers/MessagePack, WebSocket/MQTT/private RPC framing, PCAP-driven format recovery, length-prefixed or TLV frames, magic bytes, CRC/checksum fields, or encrypted frame headers.

skills/reverse-engineering/reverse-eng-protocol/SKILL.md

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Oct 14, 2026
License
MIT

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# Custom protocol reverse engineering

## When it applies
The target speaks something that isn't plain HTTP/JSON: a custom binary TCP/UDP protocol,
Protobuf/gRPC with no reflection, FlatBuffers/MessagePack, WebSocket or MQTT frames, or a
private RPC format — and you have a PCAP, a proxy export, client logs, or the client binary.
(For pure HTTP parameter signing in JS, use `reverse-eng-js`; for gRPC's HTTP/2 attack surface
itself, `api-grpc`.) Target and any replay testing must be in `scope.txt`
(`tradecraft-scope-roe`) — replay only against systems you're authorized to touch.

## Why it works
Protocols are machines, and machines are regular: fixed headers, magic bytes, length fields,
monotonic sequence numbers, and repeated type-length-value structures. Align many samples of
the same message type and the invariant bytes identify themselves; the variable ones are your
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