LNURL-auth (LUD-04)

Use this skill for authentication only. It signs a service-provided k1 challenge with a secp256k1 linking key and sends the signature to the service. It never creates an invoice, makes a payment, or requires a Lightning node or wallet.

Inputs

• An lnurl1... string from a Sign in with Lightning link, QR code, or page.
• Optional --dry-run when the callback must be inspected before submission.
• Optional --single-key when one linking key must be shared across domains.

Do not invent or alter the lnurl1... value. Ask for a fresh challenge when a service reports that the challenge was already used.

Run

The bundled helper uses only Node.js built-ins and is located at:

<skill_dir>/scripts/lnurl_auth.js

Inspect the decoded URL and signature without authenticating:

node <skill_dir>/scripts/lnurl_auth.js "<lnurl1...>" --dry-run --json

After the user confirms that the service and callback are expected, submit the authentication request:

node <skill_dir>/scripts/lnurl_auth.js "<lnurl1...>" --json

The helper requires Node.js 20.19 or newer. The only network requests are the optional challenge GET and the final callback GET. The callback host comes from the decoded LNURL unless the user explicitly supplies --callback.

Protocol

1. Decode the bech32 lnurl1... value into a service URL.
2. Read the 32-byte hexadecimal k1 challenge, fetching it with a GET when it is not already in the URL.
3. Derive a stable per-domain linking key from the local master secret using HMAC-SHA256, unless --single-key is selected.
4. Sign the raw k1 bytes with secp256k1 and encode the signature as DER.
5. GET the callback with the existing query parameters plus sig and key.

The master secret is generated once at ~/.config/lnurl-auth/master.key (or at LNURL_AUTH_KEYFILE) and is written with mode 0600. Never print, paste, or send the master secret.

Results and failures

• JSON output contains the decoded service URL, domain, k1, compressed linking public key, callback URL, HTTP status, and service response.
• Exit 0: the service returned status: OK.
• Exit 1: malformed input, invalid key/challenge, or a network/client error.
• Exit 2: missing input or an unknown command-line option.
• Exit 3: the service returned status: ERROR.
• Exit 4: the service returned a non-200 or non-JSON response.

Do not retry a submitted challenge. A signature verification error usually means the service received a different key or the challenge was modified.