Imported from trailofbits/skills (plugins/trailmark/skills/crypto-protocol-diagram/SKILL.md) via skills.sh. Install upstream with npx skills add trailofbits/skills --skill crypto-protocol-diagram. Copyright stays with the author.

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

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:

• 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:

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:

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:

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:

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