@domandigital/synthetic

The signed synthetic-submission protocol for Doman Digital client forms. DD Checks (dd-checks) submits a real enquiry through a client's real form handler on a schedule; Turnstile blocks synthetic browsers, so the handler accepts a signed request that skips only the Turnstile token.

Web Crypto only, no dependencies: it runs in Cloudflare Workers, Node 22+ and the browser-side signing helpers in dd-checks. Tests run in both Node and workerd.

The protocol is specified in the "DD Checks: plan" document on DOM-487, section 3. This package is that section as code.

Install

pnpm add @domandigital/synthetic

In a form handler

import {
  verifyRequest, keysFromEnv, turnstileConfigCheck, assertCanaryRecipient,
  syntheticReport, sendBeacon, upstashReplayGuard,
} from "@domandigital/synthetic";

export async function POST(request: Request) {
  const verified = await verifyRequest(request, {
    keys: keysFromEnv(env),                 // empty => synthetic path off
    host: "www.example.co.uk",              // the public host
    path: "/api/enquiry",
    contentTypes: ["application/json"],
    replayGuard: upstashReplayGuard({ url: env.UPSTASH_REDIS_REST_URL, token: env.UPSTASH_REDIS_REST_TOKEN }),
  });

  if (!verified.ok && verified.rejected) {
    // Handle as an ordinary request (Turnstile still applies) and tell dd-checks.
    ctx.waitUntil(sendBeacon(reporter, { form: "enquiry", code: "synthetic_rejected", reason: verified.reason }));
  }

  const synthetic = verified.ok ? verified.synthetic : null; // frozen; the only source of truth
  // 1. same-origin  2. schema validation
  // 3. synthetic ? turnstileConfigCheck({...}) : verify the Turnstile token
  // 4. the real rate limiter, identifier `synthetic:<form>` when synthetic
  // 5. full mode only: DB write (record_origin='synthetic', synthetic_run_id, purge_after),
  //    staff email redirected to canaryAddress("s", runId); submitter email to
  //    canaryAddress("u", runId) on the daily run only. assertCanaryRecipient() first.
  // 6. skip SMS, CRM, marketing lists, trackLead; report them as "skipped".
  // 7. respond with { ..., synthetic: syntheticReport(synthetic, stages) } only when synthetic
}

Rules the code enforces for you:

• Off by default. Without DD_SYNTHETIC_SECRET (and DD_SYNTHETIC_KID) every request is ordinary.
• Shape before signature. Method, host, path and content type are checked before the body is hashed.
• A rejected request is an ordinary request, never a failure. verify never throws.
• The replay guard is required and runs after the signature, so a forged request cannot burn a real run id.
• The body and query are never read for synthetic. The SyntheticContext is built only from verified headers and is frozen.

Replay guard options: upstashReplayGuard (SET synth:<runId> NX EX 300), or your own function over a unique constraint on synthetic_run_id:

replayGuard: async (runId) => {
  try { await db.insert(syntheticRuns).values({ syntheticRunId: runId }); return true; }
  catch (e) { if (isUniqueViolation(e)) return false; throw e; }
}

Rotation

A site holds at most two keys: DD_SYNTHETIC_SECRET + DD_SYNTHETIC_KID, and DD_SYNTHETIC_SECRET_NEXT + DD_SYNTHETIC_KID_NEXT. Set the new key as _NEXT with a new kid, switch the platform over, promote it, then remove the old one. An unknown kid is rejected. The secret is 32 random bytes, base64url:

node -e "console.log(require('node:crypto').randomBytes(32).toString('base64url'))"

Signing (dd-checks)

const { headers, runId } = await sign({
  key: { kid: "example-2026-10", secret },
  url: "https://www.example.co.uk/api/enquiry",
  contentType: "application/json",
  body: rawBodyString,
  client: "example", form: "enquiry", mode: "probe",
});

runId is 16 characters of [a-z2-7], generated fresh for every call from the CSPRNG. It is the replay nonce and goes into the canary address, so every run is unique and no idempotency key on the client's side can dedupe one.

The wire format

Headers: X-DD-Synth: v1, X-DD-Synth-Kid, -Ts (unix seconds), -Run, -Mode (probe or full), -Client, -Form, -Sig: v1=<hex>.

The signature is HMAC-SHA256 with the 32-byte secret over:

DD-SYNTH-V1\n<kid>\n<ts>\n<runId>\n<mode>\n<client>\n<form>\nPOST\n<host>\n<path>\n<content-type>\n<hex sha256(raw body)>

host is lower-case and includes a port only if the URL has one. path has the query string removed. content-type is the header exactly as sent (a multipart boundary is part of it). Window: ±120 s. Comparison: constant time.

Test vectors are published in test-vectors.json (@domandigital/synthetic/test-vectors.json): a test secret and, per case, the inputs, the canonical string, the body hash and the expected signature. They are generated with node:crypto by scripts/gen-vectors.mjs, independent of this package's code. Any other implementation (a Python checker, a Go service) must reproduce them.

Recipient lock

assertCanaryRecipient(submitterEmail);   // throws SyntheticRecipientError

Only @canary.domandigital.co.uk passes, as one plain address. Display names, lists, subdomains and look-alike domains all fail. Call it before anything is sent.

Turnstile config self-check

turnstileConfigCheck({ siteKey, secretKey, expectedHostname, publicHost }) replaces the token on a verified synthetic request. It returns { ok, outcome }, with outcome one of:

Hostnames compare lower-case with scheme, port, path and trailing dot removed; www. is kept, because it is a different host. Staging hosts accept the test keys, since staging browser journeys use Cloudflare's always-pass keys.

Beacons and purge receipts

const reporter = { key, client: "example", waitUntil: ctx.waitUntil.bind(ctx) };
sendBeacon(reporter, { form: "enquiry", code: "accepted" });
sendPurgeReceipt(reporter, { form: "enquiry", deleted: 24, remaining: 0 });

Both are signed with the site's key (-Mode: beacon / receipt), post to https://checks.domandigital.co.uk/beacon and /purge-receipt, are fire-and-forget, and never throw or reject: a bad argument is dropped. A beacon holds codes only (accepted, hostname_mismatch, invalid-input-secret, misconfigured, service_unavailable, delivery_failed, captcha_failed, synthetic_rejected plus a reason), and never personal data.

Keeping the headers out of Sentry and logs

Sentry.init({ beforeSend: scrubSentryEvent, beforeSendTransaction: scrubSentryEvent });
logger.info(scrubText(headerDump));
const safe = scrubHeaders(request.headers);

Development

pnpm --filter @domandigital/synthetic test    # Node, then workerd