From SMS to RCS: A Technical Guide for Developers Integrating Secure Messaging into Signature Flows

Stop waiting for signed documents: bring secure, auditable messaging into your signature flows with RCS

If your business still relies on slow paper or plain SMS for signature requests and audit trails, you face delays, compliance risk, and a poor user experience. As of 2026, RCS (Rich Communication Services) has matured into a reliable channel for interactive signature requests — but integrating it correctly requires new patterns: capability checks, rich payloads, fallbacks to SMS, delivery receipts, webhook-driven audit logging, and mobile SDK deep links. This guide is a hands-on developer reference for implementing RCS in e-signature workflows with production-ready APIs, message payload examples, webhook patterns, security controls, and fallback strategies.

Why RCS matters for e-signature flows in 2026

RCS upgrades mobile messaging with structured cards, buttons, file transfers, suggested replies, and — increasingly — end-to-end encryption (E2EE). Since the GSMA's Universal Profile 3.0 and multi-vendor implementations in late 2024–2025, adoption accelerated. By early 2026 many CPaaS platforms and carriers support:

• Capability lookups to check if a number supports RCS
• Rich cards and suggested actions that host signing links or start in-app flows
• Read and delivery receipts enabling a richer audit trail
• Improved privacy via MLS-based E2EE in supported networks and client stacks

For signing workflows, these features mean faster conversions, better fraud signals, and more robust server-side evidence to meet ESIGN, UETA, and eIDAS-style requirements when paired with a compliant audit log and cryptographic proof of the signed artifact.

High-level integration pattern

Use this inverted-pyramid flow as your template: capability check → send RCS rich message → monitor receipts/webhook events → present signing UI (web or app) → record final signature artifact and evidentiary metadata into the audit log. Implement SMS fallback if RCS isn’t available or the user is on an unsupported device.

1. Capability lookup — ask your messaging provider if the recipient supports RCS in real time.
2. Rich request — send an RCS card with a secure, short-lived signing deep link or file attachment.
3. Receipt monitoring — handle delivery and read receipts via webhooks and map them to the audit log.
4. Verification step — bind the mobile channel to identity via OTP, device binding, or KBA as required.
5. Complete signing — collect the signature, hash the final document, and record cryptographic evidence.
6. Fallbacks — if RCS fails, fallback to SMS or email with the same short-lived link and same logging semantics.

Core technical building blocks

1) Capability lookup (pre-send)

Do not assume RCS availability. Use your messaging provider's capabilities or a carrier Number Lookup API. The typical response will contain rcsSupported, richFeatures (cards, suggestedReplies, fileTransfer), and E2EE support flags.

// Example pseudo-API: GET /capabilities?to=%2B14155550100
{
  "to": "+14155550100",
  "rcsSupported": true,
  "richFeatures": ["richCard","suggestedReplies","fileTransfer"],
  "e2ee": "partial" // possible values: none/partial/full
}

Implement these rules:

• If rcsSupported = true, use RCS with enhanced UX (cards, deep links).
• If e2ee = full, you can rely on stronger transport confidentiality for evidence — but you still must authenticate the user.
• If rcsSupported = false, route to your SMS fallback (and consider user preference for email/app).

2) Composing the RCS payload: message patterns that work for signing

Design your RCS messages to minimize friction and maximize traceability. Use rich cards with a single, clearly labeled CTA and a secure deep link that carries a one-time action token. Avoid embedding full documents in the message — instead, reference a hosted document (with hashed integrity checks).

// Example RCS rich card payload (JSON) via CPaaS
POST /messages
{
  "type": "rcs",
  "to": "+14155550100",
  "from": "AcmeSign",
  "content": {
    "richCard": {
      "title": "Sign: Sales Agreement #4521",
      "description": "Tap to review and e-sign. Link expires in 15 min.",
      "media": { "type": "image", "url": "https://files.example.com/doc-preview/4521.png" },
      "action": {
        "label": "Review & Sign",
        "deepLink": "acmeapp://sign?token=eyJhbGci...",
        "webLink": "https://sign.example.com/start?token=eyJhbGci..."
      },
      "suggestedReplies": ["I have a question","Sign later"]
    }
  },
  "metadata": {
    "documentId": "4521",
    "docHash": "sha256:3a7bd3...",
    "expiry": "2026-01-17T18:30:00Z"
  },
  "fallback": { "type": "sms", "text": "Sign your Sales Agreement: https://sign.example.com/start?token=eyJ..." }
}

Key design notes:

• Short lived tokens: Tokens must be single-use and expire quickly (e.g., 10–15 minutes) to limit replay and link-sharing risk.
• Include docHash: Always include a SHA-256 hash of the document in the message metadata; record the same hash when the document is signed.
• Deep links + web fallback: Provide both app deep link and web URL; the messaging client may prefer one over the other.
• Suggested replies: Useful for quick status (e.g., "Sign now", "Request phone call"). Each reply generates an event you can log.

3) Webhooks and delivery / read receipts

Webhooks are the backbone of your audit log. Subscribe to message events from your provider: sent, delivered, read, clicked, replied, failed. Record every event with server timestamps and provider-provided message IDs.

// Example webhook payload (POST to your /webhooks/messages)
{
  "event": "message.received",
  "messageId": "msg_abc123",
  "to": "+14155550100",
  "from": "AcmeSign",
  "channel": "rcs",
  "type": "delivered",
  "timestamp": "2026-01-17T17:45:12.123Z",
  "details": { "rcsDeliveryState": "DELIVERED" }
}

// Follow-up: read
{
  "event": "message.read",
  "messageId": "msg_abc123",
  "timestamp": "2026-01-17T17:46:05.456Z",
  "details": {}
}

Implementation tips:

• Verify webhook authenticity using provider HMAC signature headers or mutual TLS.
• Keep raw provider payloads as part of your evidence bundle to prove the sequence of events.
• Map provider statuses into a normalized schema: queued, sent, delivered, read, clicked, replied, failed.

4) Audit log schema: what you must store

A legally useful audit log combines message transport events with signing artifacts. At minimum, store:

• Message-level: messageId, channel, to, from, providerId, payload snapshot, timestamps (queued/sent/delivered/read/failed), delivery receipts, click events, provider raw webhook.
• Document-level: documentId, docHash (SHA-256), version, hosted URL, retention policy.
• Signature event: signerId, authMethod (OTP/passkey/ID verification), actionToken, signatureBlob (base64), signatureHash, signingTimestamp, IP, device info, user agent, geolocation (if collected legally).
• Cryptographic proof: signed timestamp tokens from your KMS or timestamping authority (RFC 3161), and optionally a chain-of-custody hash that includes the message payload and final document hash.

Store this data immutably where possible (append-only logs, WORM storage) and ensure retention aligns with your compliance regime. Include human-readable summaries for auditors.

Security and anti-fraud best practices

Token and link hardening

• Use single-use JWT/action tokens signed by your backend with short TTLs and nonce tracking.
• Bind tokens to recipient phone numbers and device fingerprints where feasible.
• Require re-authentication for high-risk documents (ID verification, biometric, or passkeys).

Device and channel risk signals

Leverage carrier and provider fraud signals in real time: SIM swap alerts, number porting events, and telemetry such as device model and user agent. If the capability lookup shows mismatched carrier data or if the number recently ported, raise the auth requirements.

E2EE considerations

Where E2EE is available (MLS-based or vendor-specific), transport confidentiality is stronger, but it does not replace signer authentication or server-side audit recording. Always capture server-side evidence (receipt timestamps, provider raw events, and the final signed document hash). If your provider marks E2EE available, note it in the audit record.

Fallback strategies and cost trade-offs

RCS cannot reach every user. Implement progressive fallback paths to preserve conversion while keeping the audit chain identical.

Fallback hierarchy (recommended)

1. RCS (rich card + deep link)
2. SMS (short link + OTP verification)
3. App push notifications (if user has app and registered token)
4. Email (last resort for desktop users)

Implementation details:

• Unified link tokens: Use the same action token regardless of channel; the token resolves to the same document and logs channel usage.
• Consistent audit fields: Ensure channel and transport metadata are always recorded so auditors can see how the user was reached.
• Retry/backoff: If an RCS send returns a transient error, retry with exponential backoff, then fallback to SMS after N attempts.
• Cost management: RCS messaging rates vary; measure cost per signed document and route by priority (RCS for high-value docs, SMS for low-value or large-volume notices).

Webhook handler example and mapping to audit log

Below is a minimal pseudo-code handler that verifies the webhook signature and appends events to an append-only audit store.

POST /webhooks/messages
// Verify provider signature header X-Provider-Signature (HMAC-SHA256)
function handleWebhook(req, res) {
  const raw = req.rawBody;
  const signature = req.headers['x-provider-signature'];
  if (!verifyHMAC(raw, signature, PROVIDER_SECRET)) {
    return res.status(401).send('invalid signature');
  }

  const event = JSON.parse(raw);
  const record = {
    eventId: event.id || uuid(),
    messageId: event.messageId,
    channel: event.channel,
    type: event.type,
    providerPayload: event,
    receivedAt: nowISO(),
  };

  auditStore.append(record); // append-only

  // Map to application state
  updateMessageStatus(event.messageId, normalizeStatus(event.type));

  res.status(200).send('ok');
}

Mobile SDK integration tips

When you have an app, combine RCS deep links with SDK-based signing to reduce friction and create stronger device binding.

• Universal links / App links: Ensure the deep link includes the action token and fallback web URL. Validate token server-side via your SDK before allowing signing.
• In-app identity: If the user is signed in, bind the signing event to their account and store device identifiers and push tokens in the audit record.
• Offline signing: For offline-capable apps, cache the signed artifact locally and upload on next network availability with server-signed timestamping.
• Platform specifics: Android RCS clients (Messages) will render rich cards; on iOS, RCS support improved in 2025–2026, but client behavior can still differ — run device matrix tests.

Practical examples: two concrete flows

1) Quick-sign consumer contract (low-friction, medium-risk)

1. Capability lookup -> RCS supported
2. Send RCS rich card with single CTA deep link and docHash
3. User taps -> opens web signing page -> authenticate via 6-digit OTP sent over RCS or SMS
4. User signs -> server records signatureBlob and docHash -> create signed PDF and sign timestamp using KMS
5. Webhook delivery/read events + signature event logged to audit store

2) High-value transaction (high security)

1. Capability lookup reveals RCS but number recently ported -> escalate
2. Send RCS card plus push an in-app notification if app present
3. Require OOB ID verification (ID scan via SDK) or passkey sign-in
4. Sign via in-app signing component -> sign with local key and upload signature to server, server attests with timestamping authority
5. All transport events and device signals stored in WORM-compliant repository

Testing, monitoring, and rollout

Implement a phased rollout and measure KPIs:

• RCS deliverability rate
• Click-through and signing conversion rate (RCS vs SMS)
• Time-to-sign (median)
• Fraud incidence by channel

Use synthetic tests that simulate capability lookups and full message lifecycle (queued → delivered → read → clicked). Monitor webhook latencies and implement alerting for gaps between delivered and signed events beyond an SLA threshold.

Compliance and legal considerations

RCS can be part of a compliant e-signature solution but does not by itself create a legally binding signature. Ensure:

• Strong signer identity verification aligned with your jurisdiction (eIDAS for EU, ESIGN/UETA for US).
• Immutable, timestamped evidence: delivery/read receipts, document hash, signer authentication method, and signed document artifact.
• Retention and data residency policies are met; use regional storage and apply encryption-at-rest.
• Auditability: make both human-readable and machine-readable evidence available for disputes.

2026 trends and future-proofing

As of 2026, expect the following trends to affect your integrations:

• Wider E2EE adoption: MLS-based E2EE is becoming common; annotate audit logs with E2EE flags and keep transport-level receipts.
• Interoperability improvements: Carriers and OS vendors are closing gaps — design integrations that gracefully handle client differences.
• Hybrid identity: Increased use of passkeys and mobile-native identity (SIM-fed signals combined with device-bound keys) will tighten signer binding.
• Regulatory focus: Regulators will expect stronger audit evidence for remote notarization and high-value signatures; include timestamping authorities in your stack.
• Multichannel orchestration: RCS will be part of a multi-channel, AI-assisted orchestration layer that picks channel based on risk, cost, and user preference.

Checklist: Deploy RCS-enabled signature flows

1. Integrate provider capability lookup and normalize responses.
2. Implement rich RCS card templates with single CTA and short-lived tokens.
3. Configure webhook subscriptions for message events and verify signatures.
4. Design an append-only audit log storing provider raw payloads and signature artifacts.
5. Implement secure token issuance, binding tokens to phone and device.
6. Create a fallback strategy (SMS → push → email) using the same token and logging semantics.
7. Test at scale: deliverability, receipts, end-to-end signing latency, and fraud scenarios.
8. Document your retention and compliance posture for auditors.

Practical takeaway: the technical lift to integrate RCS is front-loaded (capability checks, webhook mapping, token security), but the payoff is a measurable lift in conversion, stronger audit trails, and reduced friction for remote signing.

Ready-to-use code snippets and libraries

Start with these primitives in your stack:

• HTTP client that supports retries and exponential backoff
• Webhook signature verifier (HMAC-SHA256 or JWT)
• Short-lived JWT/action token generator with nonce store
• Append-only audit store (e.g., write-once S3 with signed manifests or a blockchain-backed ledger)

// Pseudo: generate action token (Node.js)
const jwt = require('jsonwebtoken');
function createActionToken({docId, to}){
  return jwt.sign({docId, to, jti: uuid(), iat: Date.now()}, process.env.ACTION_SECRET, {expiresIn: '10m'});
}

Conclusion and next steps

RCS is a production-ready channel for e-signature flows in 2026 — when integrated with capability checks, secure token patterns, delivery/read receipts, and strong audit logging it reduces friction and strengthens legal evidence. Build the capability lookup first, then ramp RCS for high-conversion, low-risk documents while keeping a robust SMS fallback. For high-value signatures, layer additional identity checks and cryptographic timestamping.

Want a starter project? Download our reference implementation (server + webhook handler + audit store) or spin up a trial on our API to test RCS capability lookups, rich card sends, and a unified audit log. Deploy a proof-of-concept to compare RCS vs SMS conversion and measure time-to-sign improvements in your workflows.

Call to action

Try a guided POC: sign up for a 14-day sandbox at declare.cloud to run capability lookups, send RCS-rich signing requests with SMS fallback, and capture a compliance-ready audit trail. Get sample code, webhook handlers, and templates that follow the exact patterns in this guide — production-ready and tuned for signature workflows.

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