WebRTC E2EE with Insertable Streams

Security is now a buying criterion, not a checkbox. Teams want end-to-end encryption (E2EE) for browser calls, but run into the same blockers: “Will it break the SFU?”, “How do we manage keys?”, “What about recordings and moderation?”. Here’s a practical, product-centred guide to E2EE that answers those questions and shows how to ship it.

The problem: SFUs see media by design

Selective Forwarding Units need to inspect and route RTP. With ordinary SRTP, the SFU can’t understand media, but it can decrypt and re-encrypt if it terminates DTLS. That’s great for features (recording, layout, server-side effects) but conflicts with true E2EE, where only endpoints hold content keys.

The fix: Encrypt above SRTP with Insertable Streams

Modern browsers let you transform frames before they’re handed to the WebRTC engine. Using Insertable Streams you apply an extra encryption layer on the sender and remove it on the receiver—so the SFU only forwards opaque packets:

• Concept & tutorial: Insertable Streams overview
• API surface: RTCRtpScriptTransform (MDN)
• Working demo: E2EE sample with insertable streams

What this solves

• SFU-friendly E2EE: media stays unintelligible to the server.
• Mixed reliability: works with simulcast/SVC; SFU features like bandwidth adaptation still operate.
• Granular control: encrypt video only, or both audio and video; rotate keys per participant.

Key management (the non-negotiable)

The hard part isn’t the cipher; it’s distributing and rotating keys. Practical approaches:

• Per-room key with join-time exchange via your signalling channel, then epoch-based rotation (e.g., every N seconds/frames).
• Per-sender keys for better blast containment; receivers track the active epoch for each SSRC.
• Out-of-band verification (display/confirm emoji or SAS code in UI) to defeat MITM on signalling.
• For high-assurance environments, study double encryption ideas from PERC (SRTP Double, RFC 8723) even if you don’t implement PERC itself.

Trade-offs you must design for

• Server-side recording/transcription: can’t read E2EE payloads. Offer client-side recording, or a compliance key (org-controlled) with clear UI.
• Moderation & blur: server-side ML effects won’t see the media. Prefer client-side background blur/NS.
• Metrics & troubleshooting: still fine via WebRTC stats, but never log decrypted frames.
• Performance: JavaScript transforms add a few ms. Use WASM or worker-based transforms for heavier pipelines.

Implementation checklist (copy this to your tracker)

1. Design the trust model (who can decrypt? do you need a compliance key?).
2. Choose a crypto format (frame-level AEAD, counter/nonce strategy; evaluate SFrame if available).
3. Build key exchange/rotation in signalling; add epoch numbers to frame metadata.
4. Wire in Insertable Streams with RTCRtpScriptTransform; unit-test for renegotiations, mute/unmute, and simulcast changes.
5. Ship UI for verification & fallbacks (poor devices can disable E2EE with an explicit banner).
6. Document feature caveats (recording, server-side effects) and offer alternatives.
7. Run abuse and failure drills (key leak rotation, participant removal, late joiners).

Security hygiene (don’t skip)

• HTTPS/WSS everywhere and HSTS.
• Certificate pinning for apps, if applicable.
• Zero-log policy for media transforms; sensitive telemetry stays aggregate.
• Sanity-check against OWASP WebRTC Security Cheat Sheet.

---

For more articles like this, visit SoftpageCMS.