authenticationmobilesecurity

Using RCS as a Second Factor: Implementation and Security Considerations

UUnknown

2026-02-14

11 min read

Practical, engineer-focused guide to implementing RCS as a secure second factor for signing workflows, with threat-model comparisons to SMS and authenticators.

Cut friction, not security: why engineers should care about RCS 2FA for signing

Signing workflows — legal documents, high-value transactions, administrative approvals — need a second factor that balances low friction with high assurance. Teams tired of SMS-fueled account takeovers and product teams reluctant to force users into authenticator apps are looking at RCS (Rich Communication Services) as a modern alternative. RCS promises richer UX (buttons, receipts, structured payloads) and, as of late 2025–early 2026, increasing support for end-to-end encryption (E2EE) across vendors. But friction without proper controls becomes a new attack surface. This article breaks down how to implement RCS-based second-factor verification for signing workflows, including practical architecture, code patterns, and a threat-model comparison with SMS and authenticator apps.

The state of RCS in 2026 — why now?

The mobile messaging landscape changed materially in 2024–2026: the GSMA's Universal Profile 3.0 and vendor moves have pushed RCS toward stronger security guarantees. Apple signaled support for RCS E2EE in iOS 26.3 beta, and several carriers began enabling RCS E2EE pilots in late 2025. Android clients (e.g., Google Messages) already implemented MLS-based E2EE in many regions. That momentum means RCS is no longer just a UX upgrade — it can be a viable security channel if you design for its current constraints and future capabilities.

High-level decision guide

Use RCS as primary 2FA for mid-risk signing flows that prioritize conversion and user experience, when E2EE is verified and device-binding controls exist.
Use RCS as fallback where regulatory or non-repudiation requirements demand hardware-backed cryptographic factors (WebAuthn / FIDO2) as primary.
Do not rely on RCS alone for high-assurance legal signatures unless you pair it with cryptographic attestation and server-side controls.

Threat modeling: RCS vs SMS vs authenticator apps

The right 2FA choice depends on the adversary model. Below I list key adversary capabilities and how each channel resists them.

Adversary capabilities (short list)

Passive network eavesdrop (intercepting messages in transit)
Active network attacks (SS7/Diameter routing abuse, IMS/short code compromises)
Carrier collusion or access (a carrier or infrastructure provider intercepts or injects)
SIM swap and number takeover (social-engineered porting)
Device compromise (malware, rooted/jailbroken devices)
Phishing / UI redirection (convincing user to approve fraudulent transaction)
Server-side compromise (API keys/webhooks hijacked)

SMS

Strengths: Universal, works without smartphone app, low engineering overhead.
Weaknesses: Vulnerable to SS7/Diameter attacks, SIM swap, lack of message integrity, no standard E2EE. Easy to intercept or port. Poor anti-phishing properties (codes are static OTPs or links).

Authenticator apps (TOTP / push)

Strengths: Strong against network interception and SIM swap. TOTP has no network delivery. Push approvals can use app-local cryptographic signing and device attestation (platform APIs).
Weaknesses: UX friction (users must install/use app). Vulnerable to device compromise and sophisticated phishing that proxies session (prompt bombing/approval fatigue). Requires secure backup/recovery strategy; see guidance on certificate recovery and fallback planning in Design a Certificate Recovery Plan.

RCS (in 2026)

Strengths: Rich UX (suggested replies, approval buttons, rich cards), receipt/read-state signals, and growing E2EE support (MLS). Can deliver contextual transaction payloads and make approval flows frictionless. Less vulnerable to SS7-only attacks where E2EE is enforced and carrier doesn't terminate encryption.
Weaknesses: Carrier and client fragmentation means E2EE is not universal; some operators still terminate messages in the network. RCS lacks a universal, standardized message signature model that verifies authenticity server-side today. SIM swap and account takeover remain threats if enrollment binds only to a phone number without device attestation.

"RCS can bridge the gap between SMS's reach and authenticator apps' security — but only if you design around its remaining weak points: carrier heterogeneity, enrollment, and server-side verification." — Senior Security Architect

Design principles for secure RCS 2FA in signing workflows

Transaction binding: Always cryptographically bind the signing payload to the 2FA approval. Include a short hash of the transaction in the message so approvals cannot be replayed for another transaction.
Channel attestation: Verify E2EE capability and carrier behavior at enrollment time. Record capability metadata (carrier, client, E2EE supported, provider) and refuse RCS-only approval for high-risk cases unless attestation present.
Device binding: Combine phone-number binding with device attestation (where feasible). Use platform attestation APIs when you have a companion app; otherwise use SIP/CPaaS signals and session heuristics.
Sign and verify webhooks: Use cryptographically signed callbacks from your CPaaS/RCS provider and enforce strict TLS + certificate validation on inbound webhooks. See the integration blueprint for patterns on validating webhooks and keeping integration hygiene.
Fallback and escalation: Define deterministic fallbacks (SMS, push to authenticator, email + phone-call) and escalation rules for failures or anomalies (new device, unusual geolocation, multiple attempts).
Minimum viable cryptography: Use short nonces/OTPs with 6–8 digits for UX but pair them with transaction hashes and tight expiry (60–120 seconds) and one-time-use enforcement.
Audit and non-repudiation: Record message IDs, delivery receipts, client capabilities, IPs of callbacks, and a transaction hash. For high-assurance legal signing, use an additional cryptographic layer (e.g., server logs time-stamped and anchored in a tamper-evident store / HSM).

Implementation guide — practical steps (engineer-oriented)

Below is a step-by-step blueprint for adding RCS as a second factor for signing approval. This assumes you have an application server, a signing service, and an RCS-capable messaging provider (CPaaS or direct carrier RBM API).

Prerequisites

A business account with an RCS-capable CPaaS or direct RBM access (Google Business Messages, operator RBM gateways, or vendors like Twilio/Vonage where supported).
Server-side webhook endpoints with HTTPS and strict TLS policy. Validate provider-signed webhooks (JWTs or HMAC signatures).
Instrumentation for per-number capability discovery (E2EE supported flag, client type, last-seen, carrier info).
Optional: a light companion mobile app to provide device attestation and better cryptographic proving for approvals.

Enrollment flow (secure binding)

User adds a phone number to their profile. Record IP, UA, geo, and time. Send an RCS capability probe (via CPaaS API) to detect if the device/client supports RCS and whether E2EE metadata is advertised.
Deliver a two-step verification: initial RCS probe with a temporary code, plus an out-of-band verification (email or push) to confirm the user controls both channels if high risk.
- Record the provider response: message ID, receipt timestamps, and capability flags (E2EE true/false/unknown).
If the user has a companion app, use platform attestation (Play Integrity / DeviceCheck) to bind the device to the number and store an attestation token server-side.
Mark the number as "RCS-enrolled" with metadata including e2ee_status, carrier, client, and last verification timestamp.

Approval flow for signing (transaction confirmation)

Two robust patterns: code-based and approval (push) button. Approval buttons give best UX; code-based is simpler if provider doesn't support rich replies.

Core steps (both patterns)

Server generates a signing transaction ID (txn_id) and canonical transaction payload (canonicalize the document hash, signer identity, timestamp, and nonce).
Compute a short transaction hash: txn_hash = SHA256(txn_payload) → base64url → take 8 chars for human-reading (collision window managed by server logs).
Create a time-limited challenge: challenge = HMAC(server_key, txn_id || txn_hash || user_id || ts). Store as one-time token with TTL (e.g., 120 seconds).
Send RCS message with structured JSON or rich card, including:
- clear human text describing the transaction
- the short txn_hash
- an approval button that invokes a deep link to your app or submits a suggested reply (e.g., APPROVE:challenge_token)
Provider delivers message; on user action provider calls your webhook with message metadata and the user response.
Server verifies webhook authenticity, validates the challenge token, confirms txn_hash matches the stored payload, ensures single-use, then records approval and releases the signing key to complete the signature process.

Example message payload (pseudo-JSON for CPaaS)

{
  "to": "+15551234567",
  "type": "rich_card",
  "title": "Approve contract #1234",
  "body": "You are asked to sign 'Sales Contract' for $25,000. Txn ID: abc123. Code: X7Y9-A2B1",
  "suggested_replies": ["Approve", "Decline"],
  "buttons": [
    {"label": "Approve", "action": "deep_link", "url": "myapp://approve?token=..."},
    {"label": "Decline", "action": "postback", "payload": "decline:..."}
  ]
}

Important: do not rely solely on the textual OTP if you support approval buttons and deep links. Textual OTPs are fallback for non-RCS clients.

Server-side verification checklist

Validate webhook signature (JWT/HMAC). Reject unsigned or malformed callbacks.
Ensure the response maps to an outstanding transaction by txn_id or challenge token.
Verify the message originated from the expected number and provider by comparing provider message_id and recorded delivery receipts.
Check rate limits and anomaly signals (multiple attempts, geographic mismatch, rapid rebindings).
Record final proof: store message_id, delivery receipt, approval timestamp, client capability flags, and the server-side confirmation token hash.

Operational mitigations — what to monitor and harden

SIM-swap detection: integrate carrier-provided porting/port-out alerts where available and pause approvals for recently ported numbers until re-verification. Health and identity teams should coordinate on suspicious-account workflows; see broader security playbooks like Clinic Cybersecurity & Patient Identity for operational parallels.
Anomaly scoring: combine velocity (attempts/min), geo-IP delta, device context, and age of enrollment to score each approval request. Force higher assurance (WebAuthn) above a threshold.
Webhook and API hygiene: rotate API keys, use mTLS for webhooks, and validate provider certificate pins if possible. For automation and resilient patching of integration code, consider patterns from Automating Virtual Patching.
Message receipts and delivery monitoring: correlate delivery/read receipts to detect interception proxies or duplicate deliveries via multiple providers. Portable network kits and testers help in per-region checks — see Field Review: Portable COMM Testers.
Log immutably: store signed server-side audit records in HSM-backed stores for compliance and non-repudiation. Consider periodic anchoring to an external tamper-evident ledger for legal workflows; operational guidance is summarized in the Operational Playbook: Evidence Capture and Preservation at Edge Networks.

When RCS is insufficient — escalation rules

Do not let convenience override security. If any of the following occur, escalate away from RCS approval to a higher assurance factor:

Transaction value exceeds your policy threshold.
User is onboarding a new signing device or changing the bound phone number.
Detection of suspicious activity: SIM-swap alerts, concurrent approvals from other regions, or repeated failed attempts.
Regulatory requirement for cryptographic non-repudiation (e.g., eIDAS 'qualified electronic signature' scenarios). If you support regulated signatures, audit your legal stack and controls — a useful reference is How to Audit Your Legal Tech Stack.

Comparative summary (quick reference)

SMS: Universally available, low cost, highest risk to intercept/SIM swap.
Authenticator apps: Best network resilience and phishing resistance; more friction for users; strong for high-assurance signing.
RCS: Best UX and contextual approval flows when E2EE is present and you pair with device attestation and transaction binding.

Implementation pitfalls to avoid

Assuming E2EE; always detect and record E2EE capability and gate approval flows by capability level.
Binding solely to a phone number without device or provider attestation.
Trusting webhook payload content without validating signatures and message provenance.
Using long-lived OTPs or allowing replay of approvals across transactions.
Ignoring carrier and regional fragmentation; perform per-region testing and fallbacks. Portable kits and field tests are invaluable — see the portable kits review referenced earlier.

Real-world example: low-friction signing with RCS approval

A document-signing vendor layered RCS approvals into their flow for 80% of domestic users. They used a CPaaS that provided RCS rich cards and signed webhooks. Their implementation:

At enrollment they probed RCS capability and requested app installation for device attestation when possible.
For transactions under $10k, they used RCS approval buttons with a 90-second challenge token and transaction hash.
For anything larger, the server required a second factor from a WebAuthn authenticator or video KYC.
They monitored message receipts and porting events; any suspicious signals triggered a hold and a manual review queue.

Result: conversion on signing increased by ~25% for the supported cohort, with a negligible increase in fraud after tuning anomaly thresholds.

Future predictions (2026+): what engineers should plan for

Broader E2EE adoption: expect more carriers and iOS/Android clients to enable MLS-based E2EE. Plan to make E2EE capability a gating check rather than a bonus.
Improved attestation from mobile networks: Mobile operators and CPaaS vendors will likely expose stronger attestation APIs (MNO attestations) to fight SIM fraud and port-outs.
Standardized message signatures: Watch for industry moves toward signed receipts or per-message signature primitives at the RBM layer — adopt them when available to reduce trust in provider webhooks alone.
Regulatory tightening: Financial and legal sectors are moving to mandate stronger 2FA for certain transaction classes; build configurable policy gates that escalate away from RCS when needed.

Actionable checklist — deploy RCS 2FA safely

Inventory regions and carriers you operate in; test RCS capability per region.
Choose a CPaaS provider with signed webhooks, delivery receipts, and RCS RBM support; validate E2EE metadata availability.
Implement enrollment flows that record E2EE and device attestation state.
Design transaction binding using a short txn_hash + one-time challenge token with strict TTLs.
Enforce webhook signature validation and mTLS. Rotate keys regularly.
Set anomaly thresholds and escalation paths to WebAuthn or manual review for high-risk cases.
Log message IDs, receipts, and approval proofs to an auditable store for compliance and forensics.

Final recommendations

RCS has matured into a credible channel for improving conversion in signing workflows while offering better security than SMS when implemented correctly. But it is not a drop-in replacement for cryptographic authenticators in high-assurance contexts. Treat RCS as a usable layer in a defense-in-depth approach: leverage its UX for low- to mid-risk transactions, require stronger attestations for high-risk ones, and instrument everything for detection and fast escalation.

Call to action

Ready to pilot RCS 2FA for your signing workflows? Start with a scoped proof-of-concept: run per-region capability probes, integrate a CPaaS with signed webhooks, and implement transaction binding and device attestation. If you want a security review or an implementation blueprint tailored to your environment, contact the FileVault Cloud security engineering team for a free 30-minute technical audit and rollout plan.

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