Why Big Group Chats Break Traditional Encryption
End‑to‑end encryption is easy to picture when two people are chatting. Each person has keys. Messages are encrypted on one device and decrypted on the other. But once you move to large group chats with people joining and leaving all the time, the old recipes start to crack.
The popular pairwise approach, where the app makes a secure session with each member and then sends the message N times, does not scale. It’s slow, burns battery and bandwidth, and is hard to keep consistent across many devices. Membership changes make it worse. If someone leaves, every session might need a fresh key. If someone is compromised and then recovers their account, you need to roll the keys forward for everyone to regain safety. It’s a tangle.
This is where MLS—Messaging Layer Security—comes in. MLS is an IETF standard designed specifically for private group messaging at internet scale. It keeps the end‑to‑end security people expect, but it does so with a design that’s friendly to large, dynamic groups.
MLS in Plain Language
MLS gives your app a way to create and maintain a shared secret for a group that grows and shrinks over time. It uses a tree to structure group keys. When something changes, you don’t rebuild every session. You update part of the tree. This makes the cost of changes grow like logarithm of the group size, not linearly. In simple terms: if a group has 1,024 members, an update touches around 10 places, not 1,024.
MLS also tracks time in epochs. An epoch is a version of the group. Each epoch has new keys. Each message is tied to an epoch, so you always know which keys to use and whether you’re up to date. That structure gives you forward secrecy (compromising current keys doesn’t reveal past messages) and post‑compromise security (if a device is recovered, future messages can be safe again).
Crucially, MLS does not make you invent a new backend. It expects a normal delivery server that stores messages, fans them out, and checks basic authentication. The server never gets the plaintext. It just moves encrypted packets.
Core Pieces: Trees, Epochs, and Commits
The Tree (TreeKEM)
Imagine the group arranged as leaves on a tree. Each node has a secret. Each member holds just enough secrets to decrypt the path from their leaf to the root. The root secret becomes the group key. When someone updates their key material, they replace secrets along their path to the root. They send encrypted path secrets so others can update their view of the tree.
This is called TreeKEM (Tree‑based Key Encapsulation Mechanism). Its magic is that each update involves sending a small set of encrypted values, not rebuilding every session. That’s where the O(log N) cost comes from.
Epochs and the Handshake
All structural changes—adds, removes, key updates—happen through a handshake process. You propose a change, then a commit message applies it and advances the epoch. Every member processes the commit, updates their local tree, and derives new keys. The app uses those keys to encrypt regular chat messages. Each message includes the epoch number so receivers know how to decrypt.
Authentication and Identities
Encryption is not enough. You must know who you’re talking to. MLS relies on a signature key for each device. That key is bound to an identity by an external system—your app’s login, a company’s directory, or a public credential. Every handshake message is signed. That means members can verify that changes come from legitimate devices, not from the server pretending to be someone else.
Welcome Messages and New Members
When you add a new member, you still need to bootstrap them with secrets. MLS uses a welcome message to deliver what they need to join the current epoch, without exposing secrets to anyone else. The welcome is itself encrypted to the new member’s published key package.
What MLS Does Well (and What It Doesn’t)
Strengths
- Efficient at scale: O(log N) updates and compact messages, even for huge groups.
- Asynchronous friendly: Works with people offline. The server can hold handshake messages until devices reconnect.
- Post‑compromise recovery: Members can update their leaf and push the group to a fresh epoch after a suspected compromise.
- Built for multi‑device: Each device is its own member. You can add or remove devices cleanly without tearing down the whole group.
Limitations
- Metadata leaks: MLS encrypts content, not metadata. The server may learn who’s in a group and when messages flow.
- Server honesty still matters: The server can drop or delay messages. It can’t read them, but it can cause confusion if not handled.
- Identity is external: MLS doesn’t solve identity by itself. You need a strong, user‑facing way to bind people to signing keys.
- Federation is separate: MLS defines the crypto, not inter‑service rules. Cross‑app messaging needs extra agreements.
Designing an MLS‑Backed Chat System
You can add MLS to an existing chat app without rebuilding everything. Think in layers.
1) Authentication Service
This service verifies users, binds them to signature keys, and issues credentials. It could be your existing login with an additional signature key registration step. It must let users:
- Register new devices and publish key packages.
- Rotate signing keys if a device is lost.
- View and confirm the identity and devices of other members.
Do not hide this from users. Give them a clear, human‑readable identity check, such as a short safety phrase or QR code to verify devices when needed. Trust UX is a feature, not a footnote.
2) Delivery Service
The delivery service stores handshake and application messages and fans them out to devices. It enforces basic rules: who can speak for which group, rate limits, and replay protections. It sees ciphertext only. It must:
- Handle out‑of‑order and delayed messages.
- Keep a limited window of handshake history so devices can catch up.
- Reject obviously invalid commits (wrong epoch, bad signature) without learning contents.
3) Storage and Backup
Backups must be end‑to‑end encrypted. Your server should never have the keys to decrypt stored transcripts. Consider per‑device encrypted backups, tied to the device’s signature identity. Group state (the tree, current epoch, and your leaf secrets) must be protected with platform secure storage.
4) Device Lifecycle
Every device is a member. When a new device is added, add it to each group it should join and send a welcome. When a device is removed, issue a remove proposal and commit to rotate group keys. If a device is lost, err on the side of immediate removal. Do not delay updates for “convenience.” Privacy depends on timely key rotation.
A Step‑By‑Step MLS Flow
Creating a Group
- Alice picks members and sends an add proposal for each.
- Alice issues a commit. This advances the epoch and creates welcome messages.
- New members receive welcomes, install the group state, and ack.
Sending a Message
- The sender derives an application message key from the current epoch’s secret.
- The app encrypts the payload, attaches the current epoch number, and signs the message.
- The delivery service fans it out. Each receiver uses their state to derive the same key and decrypt.
Adding a Member
- Any member proposes an add with the new member’s key package.
- Someone (often the proposer) sends a commit that updates the tree and produces a welcome.
- All members update to the new epoch. The new member processes the welcome and joins.
Removing a Member
- Any member proposes a remove for a target.
- A commit applies it and rotates secrets along the tree path to prevent the removed member from deriving future keys.
- Everyone moves to the new epoch. The removed member cannot decrypt messages from that point on.
Performance Expectations Without Hype
Let’s use round numbers. Suppose you have a group with 1,024 members. In a tree of that size, the height is about 10. A key update touches at most 10 nodes and delivers a handful of encrypted path secrets. Compare that to pairwise encryption, where you would encrypt the same message 1,024 times, or rekey 1,024 sessions when membership changes. MLS turns a quadratic headache into a compact, predictable operation.
On the wire, handshake messages are larger than ordinary messages, but they happen only when the group changes. Application messages are small and constant size, regardless of group size. This is why MLS is a practical choice for big public channels, private communities, or enterprise teams with many devices per user.
Security Pitfalls and How to Avoid Them
Identity Drift
MLS enforces that a device with a specific signature key sent the message, but it doesn’t verify that this device belongs to the person you think it does. Your app must show the binding between a human identity and the device’s signature key. Offer a trust ceremony with a clear comparison process when users first connect.
Server Misbehavior
A server can drop commits and cause members to diverge. Build in checks:
- Show a “not up to date” indicator if a device is missing recent epochs.
- Require a minimum quorum acknowledgement for high‑risk actions like removing someone.
- Log and surface signature and epoch mismatches to users with plain language.
Device Compromise and Recovery
When a device is suspected compromised, remove it immediately. Then rotate the group keys (a plain update commit) and nudge users to re‑verify if appropriate. MLS gives you post‑compromise security only if you take action quickly.
Backup Hazards
A convenient but risky pattern is to back up decrypted transcripts to cloud storage “for search.” Don’t do this. If you need server‑side search, use encrypted indexes or keep search on device. End‑to‑end means end‑to‑end.
Shipping MLS Without Breaking UX
Keep Things Boring for Users
Most people just want to send messages. MLS does not need to add friction. Keep day‑to‑day flows the same as any modern chat:
- Sending messages should be instant. Handshake traffic runs in the background.
- When members change, show a short banner—no pop‑ups—unless a risky event happens.
- If a device is out of date, display a discreet warning and a one‑tap fix.
Make Safety Visible, Not Noisy
Expose a simple, human‑readable “group safety” panel. Show which devices are in the group, when the last key update happened, and whether everyone is in the latest epoch. Use clear language. Avoid dumping technical terms like “TreeKEM” in the UI.
Handle Multi‑Device Cleanly
If users sign in on a new laptop, add it to all relevant groups automatically with welcomes. If they sign out, remove it cleanly. For shared devices or BYOD setups, make the device list very easy to review and prune. Good hygiene reduces surprise risks.
Adoption Paths: Consumer, Community, and Enterprise
Consumer Apps
MLS is a way to offer truly private group chats without the scale tax of pairwise encryption. It’s a quiet upgrade. It also helps with long‑lived groups like families or hobby groups where people join and leave over years.
Open Communities
Moderation often requires removing members swiftly. MLS removes a user and makes it cryptographically impossible for them to read future messages, even if the delivery server misbehaves. That reduces dependence on “trust the platform.”
Enterprise and Education
Teams and classrooms are multi‑device by default. MLS aligns well with managed identities and device lifecycle policies. You can integrate with a directory to bind signature keys to employee or student IDs, while keeping the content invisible to the server.
Federation and Interoperability
MLS is the crypto layer. To make different chat providers talk, you need agreements on transport, identity formats, and moderation rules. Standards work on this front is active. Keep an eye on messaging interoperability efforts. MLS makes it possible to maintain end‑to‑end security across services because the server never needs the keys. The hard parts become identity and policy, not cryptography.
Post‑Quantum Considerations
MLS uses modern building blocks like HPKE (Hybrid Public Key Encryption) for its key encapsulation. That gives a path to post‑quantum security by swapping or hybridizing the KEM with a post‑quantum algorithm. It won’t be a one‑click swap, but the design anticipates change. If you operate in sectors with long confidentiality lifetimes, plan for hybrid modes early: combine a classical KEM with a post‑quantum KEM and treat both as required for decryption. This protects against “harvest now, decrypt later” risks.
Testing and Monitoring
Test What Can Go Wrong
- Concurrency: Two members propose changes at the same time. Ensure only one commit advances the epoch and the other re‑tries cleanly.
- Lossy networks: Drop and reorder handshake messages in tests. Devices should still converge on the correct state.
- Replays: Try to deliver old commits. Devices must reject them and stay in the latest epoch.
- Server stalling: Simulate a server that drops commits for selective devices. Your UI should surface “out of date” clearly.
Monitor with Care
You can’t inspect message contents, by design. But you can monitor health:
- Group sizes and churn rate.
- Time to converge to a new epoch after a change.
- Percent of devices in the latest epoch.
- Handshake failure rates and error types.
Alert on persistent divergence. A prolonged “out of date” state may indicate a delivery issue or a malicious server component.
Implementation Resources You Can Use Today
You don’t need to roll your own crypto. Mature, open implementations exist. Look for libraries that:
- Implement the latest MLS specification.
- Use proven cryptographic primitives and safe defaults.
- Offer audited code and active maintenance.
Pair your library with a small delivery service that stores and forwards handshake and application messages. If you already have a messaging backend, you can add MLS envelopes on top.
A Practical Rollout Plan
Phase 1: Pilot in a Private Channel
- Enable MLS in a single, opt‑in group type.
- Instrument the client to capture state transitions (without content).
- Polish the safety panel UI and device list.
Phase 2: Multi‑Device and Account Recovery
- Build a clear, guided flow for adding and removing devices.
- Offer a secure recovery path if a user loses all devices, without server decryption.
- Automate welcome message delivery so users don’t need to think about epochs or trees.
Phase 3: Broad Release with Guardrails
- Turn on MLS for all new groups.
- Backfill old groups as members reconnect, with minimal disruption.
- Expose admin tools for moderators to remove members and rotate keys quickly.
The Bottom Line
MLS gives you a disciplined way to keep group messaging private without punishing performance. It treats dynamic membership as a first‑class problem. With trees and epochs, it avoids re‑encrypting messages for each member and lets devices converge even over flaky networks. You still need to solve identity and build a clean UX, but the cryptographic heavy lifting is standardized and ready.
If you’re building a chat app, a community platform, or enterprise collaboration tools, MLS is worth adopting. It’s the difference between “we promise your messages are private” and proving that privacy is robust, even when groups are large and messy.
Summary:
- MLS is an IETF standard for end‑to‑end encrypted group messaging that scales efficiently.
- It uses a tree of secrets (TreeKEM) and epochs to provide forward secrecy and post‑compromise security.
- Updates cost O(log N), making membership changes practical for large groups.
- MLS requires strong identity binding and clear, simple trust UX in your app.
- The server delivers ciphertext but cannot read messages, though it can still drop or delay them.
- Adopt MLS in layers: authentication, delivery, storage, and device lifecycle.
- Plan for multi‑device, asynchronous operation, and clean recovery flows.
- Test concurrency, loss, replays, and stalled servers; monitor convergence and epoch health.
- Post‑quantum migration is feasible via HPKE hybrid modes; track standards progress.
- Start with a pilot, refine UX, then roll out broadly with moderation and admin guardrails.
