Secure Bridge: Encrypting and Auditing Messages When Bridging Matrix ↔ RCS

Worried your Matrix ↔ RCS bridge will break end-to-end (E2E) guarantees — or silently leak metadata? You should be.

Bridging Matrix to the rapidly maturing, partially E2E-capable RCS ecosystem (driven by GSMA updates and vendor efforts through 2025–2026) is already attractive: it promises seamless messaging between federated privacy-first rooms and billions of phone numbers. But without careful design, bridges become the weakest link: they can break end-to-end (E2E) guarantees, leak sensitive metadata, or create audit blind spots that operators and compliance teams cannot explain.

Executive summary — what you must know (inverted pyramid)

• E2E across protocols is hard: preserving genuine end-to-end guarantees when translating between Matrix and RCS requires either client-side bridging, protocol-level shared cryptography (e.g. MLS-compatible flows), or strong attestation holds on the bridge.
• Threat model clarity is non-negotiable: you must enumerate actors (bridge operator, carrier, network attacker, compromised client) and put mitigations in place for each.
• Auditable audit trails: design append-only, signed logs (Merkle roots + periodic anchoring) that prove message flow without exposing plaintext or unnecessary metadata.
• Practical paths: the safest deployments in 2026 are client-side user agents that run bridging on-device, MLS-aware cross-protocol experiments, or server bridges running inside an HSM/TEE with strict attestation and minimal key access.

Context & 2026 trends

By early 2026, RCS has evolved quickly: the GSMA's Universal Profile 3.x introduced more robust feature parity, and major vendors (notably Google and Apple) have been moving toward optional E2EE RCS using MLS-like mechanisms. Apple’s iOS 26.x betas and vendor carrier bundles showed progress on E2EE RCS signaling in late 2024–2025; by 2026, a growing subset of carriers and device stacks can negotiate E2EE-capable RCS sessions. That trend drives renewed interest in bridging Matrix to RCS without destroying user privacy.

For self-hosters and operators, the key question is not whether RCS supports E2EE somewhere in the stack — it's how you retain that property when a message flows from a Matrix room to a telephone-based chat participant and back.

Threat model — enumerate to secure

Before any implementation decisions, map your threat model. Here are the principal actors and capabilities to include:

• Bridge operator (internal admin): can access bridge host, potentially intercept keys and plaintext.
• Bridge developer / third-party maintainer: might have supply-chain or CI access that can introduce backdoors.
• Carrier or RCS provider: may possess metadata and, on legacy stacks, plaintext unless E2EE is negotiated.
• Network attacker (MITM): tries to intercept or tamper with transport TLS.
• Compromised client device: user device may leak keys or content.
• Legal / lawful intercept requests: operators may be compelled to provide logs or keys.

Adversary goals to mitigate

• Recover plaintext for particular conversations.
• Map Matrix IDs to phone numbers (linkability and deanonymization).
• Forge messages or replay them across domains.
• Cause denial-of-integrity or confidentiality breaks silently.

Design patterns to preserve E2E guarantees

Not all bridges are equal. Choose a pattern based on trust assumptions, user experience, and regulatory requirements.

1) Client-side (user-agent) bridging — strongest practical privacy

Description: Bridge code runs on users' devices (or in user-owned VMs). The device holds both RCS and Matrix keys and forwards messages between the two stacks without exposing private keys to a remote server.

Pros:

• Preserves true end-to-end semantics — bridge never sees plaintext if implemented correctly.
• Minimal metadata leakage from server-side components.

Cons:

• Requires client software for phones/desktop. Not feasible for pure server-side bot scenarios.
• UX and stability concerns across OS updates and RCS vendor behavior.

Use when: You control endpoints (enterprise fleet) or target power-users willing to run the agent.

2) MLS-synchronized bridging (protocol-level)

Description: Leverage the growing adoption of MLS or MLS-like crypto in RCS and create parallel group state on Matrix clients, with the bridge acting only as a neutral packet forwarder for MLS messages.

Key idea: If both endpoints speak the same group crypto protocol, the bridge can route ciphertext blobs without decrypting them. This requires both RCS and Matrix clients to agree on a compatible message codec or wrapper.

Pros:

• Bridge never decrypts content; integrity and forward secrecy preserved.

Cons:

• Requires clients and OS vendors to support compatible MLS flows; still experimental in 2026.
• Group membership semantics differ between phone number models and federated Matrix rooms; mapping is complex.

Use when: You operate in environments where participants have MLS-capable RCS stacks and you can coordinate protocol mapping.

3) Server-side bridge with attested key handling (HSM / TEE)

Description: The bridge runs on a server but protects long-term private keys inside an HSM or TEE; the host cannot extract keys but can forward messages and produce attestations that keys were used correctly.

Pros:

• Better UX for general users than client-side bridging; can run at scale.
• Hardware-backed protection reduces risk of key exfiltration by operators.

Cons:

• HSM/TEE complexity and cost; availability varies between cloud and on-prem.
• Attestation is only as trustworthy as the TEE model and verifier implementation.

Use when: You need server-side convenience but must limit key exposure. Combine with strong logging and attestation verification (local, auditable attestations and human review).

Preserving metadata privacy across the bridge

Even if the bridge never sees plaintext, it can leak strong metadata signals: phone numbers, join/leave events, timing correlations, and device identifiers. Minimization techniques include:

• Pseudonymous mapping: store a per-bridge ephemeral mapping from Matrix ID to tel: URI, rotated frequently and not backed up in cleartext.
• Batching and timing obfuscation: delay or batch messages for low-risk conversations to reduce correlation risk.
• Selective forwarding: avoid forwarding full address books or presence unless explicitly consented.
• Tokenized addressing: use short-lived tokens for bridge routing so long-term mapping isn't stored.

Auditing without hurting privacy — architecture

Operators usually want evidence of what happened (for debugging, compliance, or forensics). Design an audit system that is tamper-evident and privacy-aware.

Core building blocks

• Signed append-only logs: each bridge operation (message in/out, key operation) is recorded with metadata and signed by the bridge’s attested signing key.
• Merkle trees and anchoring: periodically compute Merkle roots and publish them to an external, immutable anchor (e.g., a Matrix room dedicated to published roots — see approaches to rapid content publishing and anchoring).
• Redaction-aware records: store only hashed payloads where plaintext is not necessary, or encrypt logged plaintext with a per-case escrow key (see lawful access policy below).
• Access controls & audit gates: human-in-the-loop approval for decrypting any stored plaintext; strong logging of any decryption requests.

Example: a privacy-preserving audit record

Record schema (JSON) example—store minimal fields and a signature:

{
  "ts": "2026-01-15T12:34:56Z",
  "event": "message_forward",
  "direction": "matrix->rcs",
  "room": "!id:example.com",
  "to_token_hash": "sha256:...",
  "payload_hash": "sha256:...",
  "bridge_sign": "base64(signature)"
}

Notes:

• Never store phone numbers in cleartext — replace them with salted, rotation-aware hashes (to_token_hash).
• If plaintext must be retained (e.g., for compliance), encrypt it with an escrow key stored in an HSM and record only the ciphertext and the attestation that encryption occurred.
• Sign each record with a key protected by your HSM/TEE so records are tamper-evident.

Practical deployment checklist (actionable steps)

1. Clarify objectives and consent: Which users opt into RCS bridging? Document a privacy policy stating what metadata is logged and why. Consider linking consent design to established consent flow patterns.
2. Prefer client-side bridging: when possible, ship a user agent or recommend a client-side agent for users who require real E2E guarantees.
3. Harden server bridges: run bridges under minimal-privilege containers, limit network access, and use HSMs/TEEs for signing keys.
4. mTLS between components: enforce mutual TLS between your Matrix homeserver, bridge, and RCS gateway. Use your own CA or ACME + small PKI for issuer control.
5. Rotate and audit keys: perform scheduled key rotation and publish rotation events to your audit logs and Merkle anchor (see publishing approaches).
6. Implement signed append-only logs: sign records, generate Merkle roots hourly, and publish roots to a trust-minimized anchor (Matrix room + public timestamp).
7. Limit metadata persistence: store only salted hashes for identifiers and rotate salts quarterly.
8. Test recovery and forensic procedures: run tabletop exercises where you must justify audit entries or respond to a legal request.

Configuration examples — concrete steps

1) Appservice registration (Matrix Synapse example)

{
  "id": "rcs.bridge.com",
  "hs_token": "HS_TOKEN",
  "as_token": "AS_TOKEN",
  "sender_localpart": "rcsbot",
  "namespaces": {
    "users": [{"exclusive": true, "regex": "@rcs_.*:example.com"}],
    "rooms": [],
    "aliases": []
  },
  "rate_limited": false
}

Steps:

• Place this registration file on your homeserver and configure Synapse to accept appservices.
• Limit the as_token to a single bridge host and secure it with firewall rules.
• Use a short-lived TLS cert for the bridge endpoint and require client certs from your homeserver (mTLS).

2) mTLS with an internal CA (quick example)

# create CA
openssl genpkey -algorithm RSA -out ca.key -pkeyopt rsa_keygen_bits:4096
openssl req -x509 -new -nodes -key ca.key -sha256 -days 3650 -out ca.crt -subj "/CN=Example Bridge CA"

# server cert
openssl genpkey -algorithm RSA -out server.key -pkeyopt rsa_keygen_bits:2048
openssl req -new -key server.key -out server.csr -subj "/CN=bridge.example.com"
openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 825 -sha256

# configure nginx for mTLS
# in nginx server block:
# ssl_certificate /path/server.crt; ssl_certificate_key /path/server.key;
# ssl_client_certificate /path/ca.crt; ssl_verify_client on;

Enforce client cert verification so only your homeserver can talk to the bridge's admin API.

Audit logs: signing and anchoring (practical)

High-level flow for tamper-evident logs:

1. Bridge emits signed event records to an internal append-only store (e.g., WAL-backed local DB).
2. Every N minutes the service computes a Merkle tree of new entries and signs the Merkle root with the bridge’s attested signing key.
3. The bridge publishes the Merkle root to a public anchor — a Matrix room or a timestamping API — so third parties can verify a record existed at time T without needing access to plaintext logs. See practical approaches to rapid content publishing for examples.

Why this works: publishing the Merkle root provides a compact “commitment” to the set of events. If an auditor later requests specific events, the bridge can present the signed record plus Merkle proof to show it was part of the anchored set. If a bridge operator tampers with local records, the Merkle proofs won't validate against the anchor.

Lawful access and escrow — policy, not design

Be explicit about your stance. If you plan to support lawful access, separate policy from technical controls:

• Document when plaintext is escrowed and provide transparency reports.
• Use HSMs so keys cannot be trivially exported; log and require multiple admin approvals to extract anything.
• Prefer cryptographic escrow where possible (split-key escrow, multi-party authorization). See guidance on regulation and developer obligations in Europe: how startups must adapt to new AI rules.

Testing & verification

Test these properties before you go to production:

• Crypto black-box tests: verify the bridge cannot decrypt MLS/OMEMO packets when configured as a forwarder; fuzz protocol mismatches. Consider sandboxed tests and desktop-agent hardening patterns from desktop-agent sandboxing.
• Red team: attempt to coerce mapping between Matrix IDs and phone numbers via timing or API access.
• Key compromise drills: rotate keys, validate audit logs and Merkle proofs post-rotation.

Future directions and predictions (2026+)

Expect these trends through 2026 and beyond:

• MLS diffusion: broader adoption of MLS across major mobile platforms will make protocol-level bridging more feasible.
• Carrier adoption divergence: many carriers will support optional E2EE RCS, but global consistency will lag — expect hybrid deployments for years.
• Hardware attestation: market pressure for HSM/TEE-attested bridges will create managed offerings for privacy-sensitive bridges.
• Standardized audit primitives: the community will standardize privacy-first audit record formats (expect Matrix RFCs or GSMA follow-on work).

Concluding takeaways

• E2E is preserved only when you control endpoints or use genuine protocol alignment (MLS): server-side bridges that decrypt and re-encrypt cannot claim full end-to-end properties.
• Design your bridge around a clear threat model: enumerate actors, then match mitigation to capability.
• Make audit trails tamper-evident, not invasive: signed records + Merkle anchoring let you prove behaviour without storing sensitive identifiers in plain.
• Prefer client-side or attested hardware solutions: they strike the best balance between privacy and convenience in 2026.

Operators: treat bridging like a privacy boundary. Your users’ expectation of confidentiality doesn’t disappear because a message touches a telephone network.

Call to action

Ready to harden your Matrix ↔ RCS bridge? Start by running a tabletop threat model with stakeholders, adopt client-side agents for privacy-sensitive users, and implement signed append-only logs with Merkle anchoring. If you want a practical reference, check the selfhosting.cloud bridge hardening checklist and join Matrix community RFC discussions — contribute your findings so we can standardize privacy-preserving audit primitives for hybrid messaging in 2026. If you need help designing an attested bridge or an audit pipeline, reach out to experienced operators or schedule a hands-on workshop with your team.

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