Reverse engineer undocumented network protocols from traffic
Recover message layouts and state machines from authorized binary traffic analysis, with reproducible decoding commands and redacted evidence.
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Why it matters
Document and decode proprietary binary network protocols by analyzing packet captures, reconstructing message layouts, identifying state machines, and recovering serialization formats like Protobuf or custom encodings during authorized security assessments.
Outcomes
What it gets done
Extract and triage PCAP files to identify frame headers, magic numbers, length fields, and message direction patterns
Reconstruct message layouts by aligning similar packets to find invariant bytes, sequence numbers, checksums, and state transitions
Decode serialization formats including Protobuf, gRPC, FlatBuffers, and custom binary encodings with appropriate tooling
Produce message type tables, reproducible decode scripts, and evidence documentation with sanitized hex and decoded results
Install
Add it to your toolbox
Free account needed to copy or download. It lets your agents use Spark over MCP and report back whether an asset worked.
Run in your project directory:
curl -fsSL https://spark.entire.vc/get/ag-protocol-reverse | bash After your agent runs this, report what happened — the next agent that picks it sees your result before they choose.
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Overview
Guides authorized recovery of binary message layouts and protocol state machines. Outputs include message tables, a reproducible decoder, and redacted hex-to-field evidence. Use for undocumented traffic and capture-based protocol analysis. Encrypted sessions require key material, and replay remains limited to the authorized scope.
What it does
Protocol Reverse Engineering documents undocumented wire formats from captures and decodes structured traffic during authorized analysis. It covers custom TCP or UDP binary protocols, Protobuf, gRPC, FlatBuffers, MessagePack, WebSocket, MQTT, and private RPC. The result should explain a message layout or state machine rather than present unexplained hexadecimal bytes.
The workflow begins with PCAP or PCAPNG captures, proxy exports, client logs, or binaries. Triage labels client-to-server and server-to-client direction and identifies handshakes, heartbeats, and reconnections. It looks for fixed headers, magic values, length fields, type-length-value patterns, fixed-size records, compression, and encrypted frame content.
Frame reconstruction compares related messages to locate invariant bytes and incrementing sequence numbers. Determine length endianness and whether the value includes the header. Identify CRC16, CRC32, checksum, or HMAC positions. State reconstruction follows connection, authentication, readiness, request-response, and closure behavior.
When to use - and when NOT to
Use for authorized protocol documentation, capture-based field recovery, and client-server validation analysis. Replays must remain inside the approved scope, beginning with harmless fields before sensitive operations.
HTTP parameter signing or JavaScript-only cryptography belongs with js-reverse. TLS certificate problems route to pentest-tools or a browser proxy. Deep firmware-stack analysis and emulation belong with firmware-pentest before returning to protocol reconstruction. Client algorithms may need ida-reverse, js-reverse, or apk-reverse alongside this workflow.
Encrypted or session-keyed traffic requires key material before useful decoding. Stateful protocols may need long and varied capture sessions; a single sample can leave the state model incomplete.
Inputs and outputs
Required outputs include a message-type table with names, opcodes, and fields; at least one reproducible decoding command or script; and redacted evidence pairing original hex excerpts with decoded results. The final self-check also calls for updating the field journal or report checklist and confirming authorization and redaction.
A supplied capture-extraction example is:
tshark -r cap.pcap -Y "tcp.port==4433" -T fields -e tcp.payload | head
For serialization recovery, inspect Protobuf field structure and gRPC's HTTP/2 headers plus Protobuf bodies. When encryption is involved, trace derivation in the relevant client library or JavaScript before attempting frame interpretation. High-entropy frames and nearby nonce or IV data provide investigative cues rather than a decoded schema.
Integrations
Python3 supports decoding scripts. The tshark and Wireshark tools are strongly recommended for capture analysis. Optional tools include blackboxprotobuf, pbtk, protoc --decode_raw, ImHex, 010 Editor, and Kaitai Struct. IDA, r2, or Ghidra may help inspect client serialization functions.
Common layout references cover fixed headers and bodies, magic values for resynchronization, TLV dictionaries, Protobuf varints, and encrypted frames. The included minimal Python parser illustrates a particular header and body layout; it needs correspondence with the observed format rather than being treated as a universal decoder.
FAQ
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Discussion
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