Reverse engineer undocumented network protocols from traffic
Decode custom TCP/UDP protocols, Protobuf, gRPC, and binary formats from packet captures with frame layout reconstruction and state machine mapping.
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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
Protocol Reverse Engineering
This skill systematically reverse engineers undocumented binary network protocols from packet captures. It reconstructs message frame layouts, state machines, and serialization formats for custom TCP/UDP, Protobuf, gRPC, FlatBuffers, MessagePack, WebSocket, MQTT, and private RPC implementations, including client-server validation, sequence numbers, and encrypted frame headers. Use this when documenting undocumented wire protocols from captures, decoding structured traffic during authorized analysis, or restoring PCAP/PCAPNG fields and state machines. It handles custom binary protocols, Protobuf/gRPC recovery, and encrypted frame analysis for security research and penetration testing within authorized scope.
What it does
This skill provides a systematic workflow for reverse engineering undocumented network protocols from packet captures and binary traffic. The workflow covers reconstruction of message frame layouts, state machines, serialization formats (Protobuf, gRPC, FlatBuffers, MessagePack), and encryption schemes for custom TCP/UDP protocols, WebSocket, MQTT, and private RPC implementations.
When to use - and when NOT to
Use this skill when documenting undocumented wire protocols from captures, decoding structured traffic during authorized analysis, or reconstructing custom binary protocols including Protobuf/gRPC/FlatBuffers/MessagePack, WebSocket/MQTT/private RPC, and PCAP/PCAPNG field and state-machine restoration. It handles client-server validation, sequence numbers, and encrypted frame headers.
Do NOT use this skill for HTTP parameter signing or JavaScript encryption (use js-reverse/ instead), TLS certificate issues (use pentest-tools/ or browser proxy), or firmware protocol stack deep-dive with emulation (use firmware-pentest/ first, then return to this skill).
Inputs and outputs
You provide packet captures (PCAP/PCAPNG), proxy exports, client logs, or binary samples. The workflow walks through four phases:
Phase 1 - Collection and Triage:
□ 拿到样本：PCAP / 代理导出 / 客户端日志 / 二进制
□ 标记方向：C→S / S→C；是否有握手、心跳、重连
□ 固定头？魔数？长度字段？TLV？定长？
□ 是否压缩（zlib/gzip/lz4）或加密（AES/ChaCha 帧内）
□ tshark -r cap.pcap -T fields -e frame.number -e ip.src -e tcp.payload
Phase 2 - Frame Layout Reconstruction: Align multiple messages of the same type, identify invariant bytes and auto-incrementing sequence numbers, determine length fields (big-endian/little-endian, header-inclusive or not), locate checksums (CRC16/32, checksum, HMAC), and map the state machine (Connect → Auth → Ready → Request/Response → Close).
Phase 3 - Serialization and Encryption: Recover Protobuf schemas using blackboxprotobuf/pbtk/protoc --decode_raw, decode gRPC (HTTP/2 headers + protobuf body), extract encryption keys from client binaries (so/dll/JS) in coordination with ida-reverse/js-reverse/apk-reverse skills, and replay within authorized scope only.
Phase 4 - Deliverables:
MUST 产出：
- 消息类型表（name / opcode / fields）
- 至少 1 条可复现的解码命令或脚本
- Evidence：原始 hex 摘录 + 解码结果（脱敏）
The workflow specifies required deliverables: a message-type table (name/opcode/fields), at least one reproducible decode command or script, and evidence (original hex excerpts plus decoded results, sanitized).
Integrations
The workflow references tshark/Wireshark (strongly recommended for PCAP parsing), Python3 (required for decode scripts), blackboxprotobuf (optional, for unknown protobuf), ImHex/010 Editor (optional, for structure templates), and IDA/r2/Ghidra (as needed for client serialization functions). The workflow coordinates with ida-reverse, js-reverse, and apk-reverse skills when client algorithm extraction is required, and with pentest-tools/api-security for exploit replay.
Who it's for
This workflow is designed for documenting undocumented wire protocols from captures and decoding structured traffic during authorized analysis. Encrypted or session-keyed protocols require key material first; stateful protocols may require long, varied capture sessions.
FAQ
Common questions
Discussion
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