Crypto Protocol Diagram

Trail of BitsOfficialPublished by Trail of Bits

Skill
Content generator
Security
CC-BY-SA-4.0
Low risk
Has scripts

From the author

Extracts protocol message flow from source code, RFCs, academic papers, pseudocode, informal prose, ProVerif (.pv), or Tamarin (.spthy) models and generates Mermaid sequenceDiagrams with cryptographic annotations. Use when diagramming a crypto protocol, visualizing a handshake or key exchange flow, extracting message flow from a spec or RFC, diagramming a ProVerif or Tamarin model, or drawing sequence diagrams for TLS, Noise, Signal, X3DH, Double Ratchet, FROST, DH, or ECDH protocols.

— the description by Trail of Bits

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill extracts the semantic message flow and cryptographic operations of a protocol from various inputs, including source code, RFCs, academic papers, and formal models like ProVerif or Tamarin. It produces both a Mermaid sequence diagram file and an inline ASCII representation, ensuring that cryptographic transformations and protocol phases are clearly annotated for security review.

What it does

Parses source code, RFCs, papers, and formal models to extract protocol structure.
Identifies protocol participants and maps them to sequence diagram roles.
Traces message flows and cryptographic operations like DH, signing, and KDF.
Groups protocol steps into logical phases using visual blocks.
Generates a Mermaid sequence diagram file with cryptographic annotations.
Provides an inline ASCII sequence diagram for quick reference.

When to use it

When you need to visualize a cryptographic handshake or key exchange flow.
When auditing a protocol implementation against its specification.
When documenting a protocol from an RFC, academic paper, or formal model.
When you need to identify divergences between code and specification.

Advantages

Ensures protocol semantics are captured rather than just code structure.
Highlights cryptographic operations at each step for easier security analysis.
Supports both formal models and informal prose as input sources.
Provides standardized visual output for consistent documentation.

Usage examples

“Diagram the handshake flow from this TLS 1.3 implementation.”: The agent analyzes the provided source code, extracts the handshake message sequence, annotates the key exchange and signing operations, and outputs a Mermaid file and ASCII diagram.
“Create a sequence diagram for the X3DH protocol based on this specification.”: The agent reads the provided spec, identifies the participants (Alice, Bob, Server), maps the message flow, and generates a diagram showing the key bundle exchange and DH operations.

Install

Download the skill as a ZIP:
https://autogineer.com/agent-skills/trailofbits/crypto-protocol-diagram/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip crypto-protocol-diagram.zip -d .agents/skills

Documentation by Trail of Bits

Crypto Protocol Diagram

Produces a Mermaid sequenceDiagram (written to file) and an ASCII sequence diagram (printed inline) from either:

Source code implementing a cryptographic protocol, or
A specification — RFC, academic paper, pseudocode, informal prose, ProVerif (.pv), or Tamarin (.spthy) model.

Tools used: Read, Write, Grep, Glob, Bash, WebFetch (for URL specs).

Unlike the diagramming-code skill (which visualizes code structure), this skill extracts protocol semantics: who sends what to whom, what cryptographic transformations occur at each step, and what protocol phases exist.

For call graphs, class hierarchies, or module dependency maps, use the diagramming-code skill instead.

When to Use

User asks to diagram, visualize, or extract a cryptographic protocol
Input is source code implementing a handshake, key exchange, or multi-party protocol
Input is an RFC, academic paper, pseudocode, or formal model (ProVerif/Tamarin)
User names a specific protocol (TLS, Noise, Signal, X3DH, FROST)

When NOT to Use

User wants a call graph, class hierarchy, or module dependency map — use diagramming-code
User wants to formally verify a protocol — use mermaid-to-proverif (after generating the diagram)
Input has no cryptographic protocol semantics (no parties, no message exchange)

Rationalizations to Reject

Rationalization	Why It's Wrong	Required Action
"The protocol is simple, I can diagram from memory"	Memory-based diagrams miss steps and invert arrows	Read the source or spec systematically
"I'll skip the spec path since code exists"	Code may diverge from the spec — both paths catch different bugs	When both exist, run spec workflow first, then annotate code divergences
"Crypto annotations are optional decoration"	Without crypto annotations, the diagram is just a message flow — useless for security review	Annotate every cryptographic operation
"The abort path is obvious, no need for alt blocks"	Implicit abort handling hides missing error checks	Show every abort/error path with `alt` blocks
"I don't need to check the examples first"	The examples define the expected output quality bar	Study the relevant example before working on unfamiliar input
"ProVerif/Tamarin models are code, not specs"	Formal models are specifications — they describe intended behavior, not implementation	Use the spec workflow (S1–S5) for `.pv` and `.spthy` files

Workflow

Protocol Diagram Progress:
- [ ] Step 0: Determine input type (code / spec / both)
- [ ] Step 1 (code) or S1–S5 (spec): Extract protocol structure
- [ ] Step 6: Generate sequenceDiagram
- [ ] Step 7: Verify and deliver

Step 0: Determine Input Type

Before doing anything else, classify the input:

Signal	Input type
Source file extensions (`.py`, `.rs`, `.go`, `.ts`, `.js`, `.cpp`, `.c`)	Code
Function/class definitions, import statements	Code
RFC-style section headers (`§`, `Section X.Y`, `MUST`/`SHALL` keywords)	Spec
`Algorithm`/`Protocol`/`Figure` labels, mathematical notation	Spec
ProVerif file (`.pv`) with `process`, `let`, `in`/`out`	Spec
Tamarin file (`.spthy`) with `rule`, `--[...]->`	Spec
Plain prose or numbered steps describing a protocol	Spec
Both source files and a spec document	Both (annotate divergences with `⚠️`)

Code only → skip to Step 1 below
Spec only → skip to Spec Workflow (S1–S5) below
Both → run Spec Workflow first, then use the code-reading steps to verify the implementation against the spec diagram and annotate any divergences with ⚠️
Ambiguous → ask the user: "Is this a source code file, a specification document, or both?"

Step 1: Locate Protocol Entry Points

Grep for function names, type names, and comments that reveal the protocol:

# Find handshake, session, round, phase entry points
rg -l "handshake|session_init|round[_0-9]|setup|keygen|send_msg|recv_msg" {targetDir}

# Find crypto primitives in use
rg "sign|verify|encrypt|decrypt|dh|ecdh|kdf|hkdf|hmac|hash|commit|reveal|share" \
    {targetDir} --type-add 'src:*.{py,rs,go,ts,js,cpp,c}' -t src -l

Start reading from the highest-level orchestration function — the one that calls into handshake phases or the main protocol loop.

Step 2: Identify Parties and Roles

Extract participant names from:

Struct/class names: Client, Server, Initiator, Responder, Prover, Verifier, Dealer, Party, Coordinator
Function parameter names that carry state for a role
Comments declaring the protocol role
Test fixtures that set up two-party or N-party scenarios

Map these to Mermaid participant declarations. Use short, readable aliases:

participant I as Initiator
participant R as Responder

Step 3: Trace Message Flow

Follow state transitions and network sends/receives. Look for patterns like:

Pattern	Meaning
`send(msg)` / `recv()`	Direct message exchange
`serialize` + `transmit`	Structured message sent
Return value passed to other party's function	Logical message (in-process)
`round1_output` → `round2_input`	Round-based MPC step
Struct fields named `ephemeral_key`, `ciphertext`, `mac`, `tag`	Message contents

For in-process protocol implementations (where both parties run in the same process), treat function call boundaries as logical message sends when they represent what would be a network boundary in deployment.

Step 4: Annotate Cryptographic Operations

At each protocol step, identify and label:

Operation	Diagram annotation
Key generation	`Note over A: keygen(params) → pk, sk`
DH / ECDH	`Note over A,B: DH(sk_A, pk_B)`
KDF / HKDF	`Note over A: HKDF(ikm, salt, info)`
Signing	`Note over A: Sign(sk, msg) → σ`
Verification	`Note over B: Verify(pk, msg, σ)`
Encryption	`Note over A: Enc(key, plaintext) → ct`
Decryption	`Note over B: Dec(key, ct) → plaintext`
Commitment	`Note over A: Commit(value, rand) → C`
Hash	`Note over A: H(data) → digest`
Secret sharing	`Note over D: Share(secret, t, n) → {s_i}`
Threshold combine	`Note over C: Combine({s_i}) → secret`

Keep annotations concise — use mathematical shorthand, not code.

Step 5: Identify Protocol Phases

Group message steps into named phases using rect or Note blocks:

Common phases to detect:

Setup / Key Generation: party key creation, trusted setup, parameter gen
Handshake / Init: ephemeral key exchange, nonce exchange, version negotiation
Authentication: identity proof, certificate exchange, signature verification
Key Derivation: session key derivation from shared secrets
Data Transfer / Main Protocol: encrypted application data exchange
Finalization / Teardown: session close, MAC verification, abort handling

Detect abort/error paths and show them with alt blocks.

Spec Workflow (S1–S5)

Use this path when the input is a specification document rather than source code. After completing S1–S5, continue with Step 6 (Generate sequenceDiagram) and Step 7 (Verify and deliver) from the code workflow above.

Step S1: Ingest the Spec

Obtain the full spec text:

File path provided → read with the Read tool
URL provided → fetch with WebFetch
Pasted inline → work directly from conversation context

Then identify the spec format and read references/spec-parsing-patterns.md for format-specific extraction guidance:

Format	Signals
RFC	`RFC XXXX`, `MUST`/`SHALL`/`SHOULD`, ABNF grammars, section-numbered prose
Academic paper / pseudocode	`Algorithm X`, `Protocol X`, `Figure X`, numbered steps, `←`/`→` in math mode
Informal prose	Numbered lists, "A sends B ...", plain English descriptions
ProVerif (`.pv`)	`process`, `let`, `in(ch, x)`, `out(ch, msg)`, `!` (replication)
Tamarin (`.spthy`)	`rule`, `--[ ]->`, `Fr(~x)`, `!Pk(A, pk)`, `In(m)`, `Out(m)`

If the spec references a known named protocol (TLS, Noise, Signal, X3DH, Double Ratchet, FROST), also read references/protocol-patterns.md to use its canonical flow as a skeleton and fill in spec-specific details.

Step S2: Extract Parties and Roles

Identify all protocol participants. Look for:

Named roles in prose or pseudocode: Alice, Bob, Client, Server, Initiator, Responder, Prover, Verifier, Dealer, Party_i, Coordinator, Signer
Section headers: "Parties", "Roles", "Participants", "Setup", "Notation"
ProVerif: process names at top level (let ClientProc(...), let ServerProc(...))
Tamarin: rule names and fact arguments (e.g. !Pk($A, pk) — $A is a party)

Map each role to a Mermaid participant declaration. Use short IDs with descriptive aliases (see naming conventions in references/mermaid-sequence-syntax.md).

Step S3: Extract Message Flow

Trace what each party sends to whom and in what order. Extraction patterns by format:

RFC / informal prose:

Arrow notation: A → B: msg, A -> B
Sentence patterns: "A sends B ...", "B responds with ...", "A transmits ...", "upon receiving X, B sends Y"
Numbered steps: extract in order, inferring sender/receiver from context

Pseudocode:

Function signatures with explicit sender/receiver parameters
send(party, msg) / receive(party) calls
Return values passed as inputs to the other party's function in the next step

ProVerif (.pv):

out(ch, msg) — send on channel ch
in(ch, x) — receive on channel ch, bind to x
Match out/in pairs on the same channel to identify message flows
! (replication) signals a role that handles multiple sessions

Tamarin (.spthy):

In(m) premise — receive message m
Out(m) conclusion — send message m
Rule name and ordering of rules reveal protocol rounds
Fr(~x) — fresh random value generated by a party
--[ Label ]-> facts — security annotations, not messages

Preserve the ordering and round structure. Group concurrent sends (broadcast) using par blocks in the final diagram.

Step S4: Extract Cryptographic Operations

For each protocol step, identify the cryptographic operations performed and which party performs them:

Spec notation	Operation	Diagram annotation
`keygen()`, `Gen(1^λ)`	Key generation	`Note over A: keygen() → pk, sk`
`DH(a, B)`, `g^ab`	DH / ECDH	`Note over A,B: DH(sk_A, pk_B)`
`KDF(ikm)`, `HKDF(...)`	Key derivation	`Note over A: HKDF(ikm, salt, info) → k`
`Sign(sk, m)`, `σ ← Sign`	Signing	`Note over A: Sign(sk, msg) → σ`
`Verify(pk, m, σ)`	Verification	`Note over B: Verify(pk, msg, σ)`
`Enc(k, m)`, `{m}_k`	Encryption	`Note over A: Enc(k, plaintext) → ct`
`Dec(k, c)`	Decryption	`Note over B: Dec(k, ct) → plaintext`
`H(m)`, `hash(m)`	Hash	`Note over A: H(data) → digest`
`Commit(v, r)`, `com`	Commitment	`Note over A: Commit(value, rand) → C`
ProVerif `senc(m, k)`	Symmetric encryption	`Note over A: Enc(k, m) → ct`
ProVerif `pk(sk)`	Public key derivation	`Note over A: pk = pk(sk)`
ProVerif `sign(m, sk)`	Signing	`Note over A: Sign(sk, m) → σ`

Identify security conditions and abort paths:

Prose: "if verification fails, abort", "only if ...", "reject if ..."
Pseudocode: assert, require, if ... abort
ProVerif: if m = expected then ... else 0
Tamarin: contradicting facts or restriction lemmas

These become alt blocks in the final diagram.

Step S5: Flag Spec Ambiguities

Before moving to Step 6, check for gaps:

Unclear message ordering: infer from round structure or section order; annotate with ⚠️ ordering inferred from spec structure
Implied parties: if a party's role is implied but unnamed, give it a descriptive name and note the inference
Missing steps: if the spec omits a step that the canonical pattern for this protocol requires, annotate: ⚠️ spec omits [step] — canonical protocol requires it
Underspecified crypto: if the spec says "encrypt" without specifying the scheme, annotate: ⚠️ encryption scheme not specified
ProVerif/Tamarin: private channels (c declared with new c or as a private free name) represent out-of-band channels — note them

Step 6: Generate sequenceDiagram

Produce Mermaid syntax following the rules in references/mermaid-sequence-syntax.md.

Completeness over brevity. Show every distinct message type. Omit repeated loop iterations (use loop blocks instead), but never omit a distinct protocol step.

Correctness over aesthetics. The diagram must match what the code actually does. If the code diverges from a known spec, annotate the divergence:

Note over A,B: ⚠️ spec requires MAC here — implementation omits it

Step 7: Verify and Deliver

Before delivering:

[ ] Every participant declared actually sends or receives at least one message
[ ] Arrows point in the correct direction (sender → receiver)
[ ] Cryptographic operations are on the correct party (the one computing them)
[ ] If protocol phases are used, no arrows appear outside a phase block
[ ] alt blocks cover known abort/error paths
[ ] Diagram renders without syntax errors (check references/mermaid-sequence-syntax.md for common pitfalls)
[ ] If spec divergence found, annotated with ⚠️

Write the diagram to a file. Choose a filename derived from the protocol name, e.g. noise-xx-handshake.md or x3dh-key-agreement.md. Write a Markdown file with this structure:

# <Protocol Name> Sequence Diagram

\`\`\`mermaid
sequenceDiagram
    ...
\`\`\`

## Protocol Summary

- **Parties:** ...
- **Round complexity:** ...
- **Key primitives:** ...
- **Authentication:** ...
- **Forward secrecy:** ...
- **Notable:** [spec deviations or security observations, or "none"]

After writing the file, print an ASCII sequence diagram inline in the response, followed by the Protocol Summary. State the output filename so the user knows where to find the Mermaid source.

Follow all drawing conventions in references/ascii-sequence-diagram.md, including the inline output format.

Decision Tree

── Input is a spec document (not code)?
│  └─ Step S1: identify format, read references/spec-parsing-patterns.md
│
── Input is source code (not a spec)?
│  └─ Step 1: grep for handshake/round/send/recv entry points
│
── Both spec and code provided?
│  └─ Run Spec Workflow (S1–S5) first to build canonical diagram,
│     then read code and annotate divergences with ⚠️
│
── Spec is a known protocol (TLS, Noise, Signal, X3DH, FROST)?
│  └─ Read references/protocol-patterns.md and use canonical flow as skeleton
│
── Spec is ProVerif (.pv) or Tamarin (.spthy)?
│  └─ Read references/spec-parsing-patterns.md → Formal Models section
│
── Spec message ordering is ambiguous?
│  └─ Infer from round/section structure, annotate with ⚠️
│
── Can't identify parties from spec?
│  └─ Check "Parties"/"Notation" sections; for ProVerif read process names;
│     for Tamarin read rule names and fact arguments
│
── Don't know which code files implement the protocol?
│  └─ Step 1: grep for handshake/round/send/recv entry points
│
── Can't identify parties from struct names?
│  └─ Read test files — test setup reveals roles
│
── Protocol runs in-process (no network calls)?
│  └─ Treat function argument passing at role boundaries as messages
│
── MPC / threshold protocol with N parties?
│  └─ Read references/protocol-patterns.md → MPC section
│
── Mermaid syntax error?
│  └─ Read references/mermaid-sequence-syntax.md → Common Pitfalls
│
└─ ASCII drawing conventions?
   └─ Read references/ascii-sequence-diagram.md

Examples

Code path — examples/simple-handshake/:

protocol.py — two-party authenticated key exchange (X25519 DH + Ed25519 signing + HKDF + ChaCha20-Poly1305)
expected-output.md — exact ASCII diagram and Mermaid file the skill should produce for that protocol

Spec path (ProVerif) — examples/simple-proverif/:

model.pv — HMAC challenge-response authentication modeled in ProVerif
expected-output.md — step-by-step extraction walkthrough (parties, message flow, crypto ops) and the exact ASCII diagram and Mermaid file the skill should produce

Study the relevant example before working on an unfamiliar input.

Supporting Documentation

references/spec-parsing-patterns.md — Extraction rules for RFC, academic paper/pseudocode, informal prose, ProVerif, and Tamarin input formats; read during Step S1
references/mermaid-sequence-syntax.md — Participant syntax, arrow types, activations, grouping blocks, escaping rules, and common rendering pitfalls
references/protocol-patterns.md — Canonical message flows for TLS 1.3, Noise, X3DH, Double Ratchet, Shamir secret sharing, commit-reveal, and generic MPC rounds; use as a reference when comparing implementation against spec
references/ascii-sequence-diagram.md — Column layout, arrow conventions, self-loops, phase labels, and inline output format for the ASCII diagram

Files

ATTRIBUTION.mdAdded by AutoGineer949 B
LICENSELicence file added by AutoGineer19.7 KiB
SKILL.md18.4 KiB
agents/openai.yaml244 B
assets/trail-of-bits-mark.svg3.0 KiB
examples/simple-handshake/expected-output.md4.8 KiB
examples/simple-handshake/protocol.pyScript5.7 KiB
examples/simple-proverif/expected-output.md4.0 KiB
examples/simple-proverif/model.pv1.8 KiB
references/ascii-sequence-diagram.md2.3 KiB
references/mermaid-sequence-syntax.md7.5 KiB
references/protocol-patterns.md10.9 KiB
references/spec-parsing-patterns.md11.6 KiB

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Requirements and compatibility

Written for: Claude Code

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Names Claude Code tools, Needs Python

Security

What our scanner noticed· Low risk

Scripts that your agent may run

This skill includes scripts that your agent may run. Read them before you use it.

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Published by

Trail of Bits

Publisher

Trail of Bits (Official)

Source

trailofbits/skills · plugins/trailmark/skills/crypto-protocol-diagram · commit e4dd0bb

Licence

Creative Commons Attribution-ShareAlike 4.0 International (redistribution allowed)

LICENSE (added by AutoGineer from LICENSE)
ATTRIBUTION.md (written by AutoGineer)

Synced

2026-11-09

AutoGineer is not affiliated with or endorsed by Trail of Bits or Trail of Bits.

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The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download crypto-protocol-diagram.zip

From the author

AutoGineer summary

What it does

When to use it

Advantages

Usage examples

Install

Documentation by Trail of Bits

Crypto Protocol Diagram

When to Use

When NOT to Use

Rationalizations to Reject

Workflow

Step 0: Determine Input Type

Step 1: Locate Protocol Entry Points

Step 2: Identify Parties and Roles

Step 3: Trace Message Flow

Step 4: Annotate Cryptographic Operations

Step 5: Identify Protocol Phases

Spec Workflow (S1–S5)

Step S1: Ingest the Spec

Step S2: Extract Parties and Roles

Step S3: Extract Message Flow

Step S4: Extract Cryptographic Operations

Step S5: Flag Spec Ambiguities

Step 6: Generate sequenceDiagram

Step 7: Verify and Deliver

Decision Tree

Examples

Supporting Documentation

Files

LICENSE

Requirements and compatibility

Security

Attribution and licence

Related skills

Security Audit

Sast Configuration

Firmware Analyst

Policy Enforcer

Backend Development Security Auditor

Memory Forensics

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