sigil

Claude can sign, but never see.

sigil is a local signing tool and Claude Code integration that lets agentic coding tools use private keys without ever putting key material in the model's context window.

Status: pre-alpha. The MCP server, CLI, unlock flow, ward hooks, and policy engine (static checks) all work end-to-end. Out-of-band confirmation, rolling-window value caps, and EIP-712 domain allowlists are not yet implemented. Until they land — and until the supply-chain attestations promised for v0.1.0 ship — do not use this with real funds yet. Build plan lives in the tracking issue.

What it is

One MCP server process, four bins, three runtime deps:

1. sigil-mcp — the only thing that runs. Claude Code spawns it per session via your mcpServers config; it dies when Claude exits. Holds unlocked keys in process memory (zeroized on shutdown, sigil lock, or unlock-failure; mlock against swap is planned). Keys at rest are encrypted with XChaCha20-Poly1305 and an Argon2id-derived key. Signs over stdio using a DIY MCP wire protocol (~200 lines, no SDK dep). Claude never sees key material — only opaque handles like eth:executor.
2. sigil — control CLI. init, status, portal add/list/remove, unlock, lock.
3. sigil-hook-pre / sigil-hook-post — Claude Code hook binaries that block reads of common key paths and redact key-shaped strings from tool output.

sigil-mcp boots locked: empty in-memory handle table, no keys loaded. Sign methods return DAEMON_LOCKED (-32003) with a "run sigil unlock" message until you push the passphrase in from a separate terminal via sigil unlock. That CLI connects to a Unix socket at ~/.sigil/control.sock (0600) that sigil-mcp opens at startup. After unlock, signs work for the rest of the session; sigil lock zeroizes the table without killing the process.

Sign methods exposed today: EIP-191 personal_sign, EIP-1559 + legacy transactions, EIP-712 typed data.

What it isn't

• Not a hardware wallet replacement. If you can use a Ledger or YubiKey, do that.
• Not a custody solution. It runs on your laptop or VPS and protects you from one specific class of failure: leaking key material through an LLM agent.
• A first cut of bounding signing authority via the policy engine — but not the full thing. v1 covers static checks (chain ID, destination allowlist, per-tx value cap, function-selector allowlist, on/off toggles for personal_sign and EIP-712). Rolling-window caps, EIP-712 domain allowlists, and out-of-band human confirmation are tracked in #3 + #4 and will land incrementally.

Install

npm install -g sigild

This drops four binaries on your $PATH: sigil, sigil-mcp, sigil-hook-pre, sigil-hook-post. (The package name on npm is sigild for legacy reasons; the bins do not include a daemon any more.)

Requires Node 22+.

Quick start

# 1. Wire sigil into Claude Code (project-scoped). Pass --user to do it globally.
sigil init

# 2a. Generate a fresh key inside sigil (no plaintext ever hits disk):
sigil portal new eth:bot
# → prompts for a passphrase, mints a fresh secp256k1 key, prints the
#   address, writes ~/.sigil/keys/eth:bot.sigil + permissive policy.
#
# 2b. OR import an existing private key from a file:
#     Accepts either 32 raw bytes or 64 hex chars (with optional 0x prefix).
sigil portal add eth:bot --key-file ./private.hex
# → same as above but seeded from the file. Source file is deleted by
#   default (pass --no-remove-source to keep it).
#
# Either form: pass --strict to start with a locked-down policy template
# you fill in before any sign succeeds.

# 3. Open Claude Code. It spawns sigil-mcp automatically via your MCP config.
#    sigil-mcp boots locked — the first sign attempt will return DAEMON_LOCKED.

# 4. In a separate terminal, push the passphrase to the running sigil-mcp.
sigil unlock
# → prompts for the passphrase, decrypts every keyfile in ~/.sigil/keys/

# 5. Use Claude Code. The four sigil_* tools will work for the rest of the session.

# Optional: re-lock without restarting Claude.
sigil lock

If you close Claude Code, sigil-mcp exits and its memory is wiped. Open a new session and sigil unlock again — the encrypted keyfiles on disk persist.

CLI reference

sigil init [--user]
  Project scope: writes the ward hooks to <cwd>/.claude/settings.json
  and the MCP server registration to <cwd>/.mcp.json.
  --user: writes hooks to ~/.claude/settings.json and the MCP server
  registration to ~/.claude.json. (Claude Code CLI reads MCP configs
  from .mcp.json / ~/.claude.json — not from settings.json.)
  Idempotent — preserves your unrelated settings, and on upgrade
  migrates any stale mcpServers.sigil entry out of settings.json.

sigil portal new <handle> [--strict]
  Generate a fresh secp256k1 key inside sigil, encrypt with your
  passphrase, write it to ~/.sigil/keys/<handle>.sigil (mode 0600).
  No plaintext key ever lands on disk. Use this when you want a clean
  hot wallet for a bot (vs importing an existing key from a file).
  Also writes ~/.sigil/policy/<handle>.toml — permissive by default,
  or --strict for a locked-down template.

sigil portal add <handle> --key-file <path> [--no-remove-source] [--strict]
  Import an existing private key. Encrypts it with your passphrase
  and stores at ~/.sigil/keys/<handle>.sigil (mode 0600). Handle
  format is <kind>:<name> where kind is "eth". The source key file
  is deleted by default — pass --no-remove-source to keep it.
  Also writes ~/.sigil/policy/<handle>.toml — permissive by default
  (signs anything), or --strict for a locked-down template you fill
  in before signs succeed.

sigil policy show <handle>
  Print the current policy file for a portal. Validates schema; exits
  1 if the file is missing or malformed.

sigil policy init <handle> [--strict]
  Provision a policy file for an existing portal whose policy is
  missing (e.g. a keyfile from an older sigil version, or one you
  manually deleted). Refuses to overwrite — edit the file directly
  or remove it first. Defaults to permissive; --strict writes the
  locked-down template.

sigil portal list
  List the encrypted keyfiles on disk with their derived addresses.
  Requires the passphrase.

sigil portal remove <handle>
  Delete a keyfile from disk.

sigil unlock
  Prompt for the passphrase and push it to the running sigil-mcp over
  the control socket. After unlock, sign calls succeed for the rest
  of the Claude session. Fails if sigil-mcp is not running (start a
  Claude Code session first) or if already unlocked.

sigil lock
  Tell sigil-mcp to zeroize and clear its in-memory keys. Re-unlock
  with sigil unlock — sigil-mcp keeps running.

sigil status
  Report whether sigil-mcp is running (probes ~/.sigil/control.sock),
  its PID, whether it's unlocked, what portals it has loaded, and
  how many keyfiles exist on disk. Does not require the passphrase.

Set SIGIL_HOME to override ~/.sigil. Set SIGIL_CONTROL_SOCK to override the control socket path.

Multi-window behaviour

Each Claude Code window spawns its own sigil-mcp. They share the on-disk keyfiles + audit log but have separate in-memory handle tables — you sigil unlock once per window. (The first MCP to start owns control.sock; further sessions will get their own socket once flock-based per-instance sockets land in Phase C of #23. Until then, only the first window's sigil-mcp is reachable from the CLI.)

OS-keychain integration (planned, v0.3) will make unlock zero-touch for users who set it up.

Policy engine

Once a portal is unlocked, signing authority over its key is real. To bound the blast radius of a successful prompt injection, every portal has a policy file at ~/.sigil/policy/<handle>.toml. Two modes:

Permissive (default for sigil portal add): no rules. Sign anything the agent asks. The key isolation guarantees still hold — your key never enters the agent's context — but the unlocked portal can be made to sign whatever an attacker can get the agent to ask for. Useful for: testnet bots, demo flows, anyone who only cares about the context-window protection.

Strict (opt in with --strict): every sign request is checked. Generated template:

mode = "strict"

chain_ids = [1]                           # allowed chain IDs
allow_to = []                             # allowed destination addresses (lowercase 0x)
max_value_wei = "0"                       # per-tx cap, in wei, as decimal string
allowed_selectors = []                    # 4-byte function selectors, e.g. "0xa9059cbb"

allow_message_signing = false             # EIP-191 personal_sign (e.g. SIWE)
allow_typed_data = false                  # EIP-712 (Permit, OpenSea — can be financial)

A failed rule throws POLICY_DENIED (-32001) back to the agent with the human-readable reason ("tx denied — value X exceeds max_value_wei Y"), and the deny is appended to the hash-chained audit log alongside allows. Denies are forensically the more interesting half — they're the prompt-injection canary.

What's deferred to follow-up PRs (still in #3): rolling-window value caps (e.g. 1 ETH/day per portal), EIP-712 domain + primary-type allowlists, decoded-calldata arg checks, and the require_confirm_above_wei outcome that hooks into the OOB push gate (#4).

Supply chain posture

Key-management libraries die from supply chain compromise, not from clever attacks on the code. Given the npm ecosystem in 2026 (Mini Shai-Hulud, Axios, pgserve, TanStack), sigil commits to:

• Zero install scripts. No postinstall, preinstall, prepare. CI-enforced (planned: a CI guard that fails if any dep adds one).
• Three runtime deps, all version-pinned (no caret ranges):
  • @noble/ciphers for XChaCha20-Poly1305
  • @noble/hashes for Argon2id, keccak256, sha2, HMAC
  • @noble/secp256k1 for ECDSA All by paulmillr, audited, zero transitive deps.
• No MCP SDK. The official @modelcontextprotocol/sdk pulls 92 transitive deps (ajv, hono, cors, cross-spawn, etc) — unacceptable surface. We implement the MCP wire protocol directly in ~200 lines.
• No Bun. Plain Node only. Bun is currently being weaponized by Mini Shai-Hulud as an evasion layer; we will not give that pattern any cover.
• Provenance attestations on every npm publish. Starting v0.0.4, releases are built by a GitHub Actions workflow under OIDC trusted-publisher auth, signed with a Sigstore attestation. No long-lived npm token; tampered or out-of-band publishes fail signature verification.
• CycloneDX SBOM attached to every GitHub Release. Full transitive dep tree enumerated at release time.
• Install-scripts CI guard. Every PR fails if any package in the resolved tree declares preinstall / install / postinstall. .npmrc already has ignore-scripts=true so these never actually run for us; the guard catches new transitive deps that might run for a user without our .npmrc.
• Still planned for v0.1.0:
  • Signed standalone binaries from GitHub Releases for users who'd rather not touch npm
  • Action SHA pinning rotation via Dependabot

Verifying a release

You can confirm a sigild tarball was built by the public workflow at the commit it claims to come from:

# Validates every package in your install tree:
npm audit signatures

# Inspect the attestation for a specific sigild version:
npm view sigild@<version> dist.attestations
# → shows the workflow filename, the commit SHA, and the Sigstore signing cert

What the attestation tells you: this tarball was built by cdrn/sigil's .github/workflows/release.yml, at a specific commit on main, at a specific time. It does not tell you that commit is non-malicious — for that, read the diff between the version you trust and the version you're upgrading to. But it does mean an attacker who steals an npm token can't publish a malicious sigild under our name; they'd need to compromise the GitHub repo + push a tag, which leaves an audit trail.

Every release also publishes a CycloneDX SBOM as a GitHub Release asset, enumerating every package (direct + transitive) in the install tree at the version pinned by package-lock.json:

# Download + inspect the SBOM for a specific release:
gh release download v0.0.4 --repo cdrn/sigil --pattern '*.cdx.json'
# → produces sigild-v0.0.4.cdx.json — feed to syft/grype/etc. for vuln scan

Threat model

See THREAT_MODEL.md. Read it before trusting this with anything.

Development

git clone https://github.com/cdrn/sigil
cd sigil
npm install      # respects .npmrc ignore-scripts=true
npm test         # builds + runs ~330 tests; should finish in under 10s

See CONTRIBUTING.md for the PR-per-layer workflow.

License

Apache License 2.0. See LICENSE.