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End-to-end encryption is necessary to keep message contents private from a messaging service, but it does not by itself verify who users are talking to, hide all metadata, protect a compromised device, or make lost message history recoverable. A privacy-first design has to make deliberate choices about identity, delivery, devices, metadata, compromise, and backups—and explain those choices to users.
Signal’s public protocol specifications are useful design references, not a security guarantee for a new app. Protocol properties, the quality of an implementation, and product policies are separate things; the sources do not establish one universally best protocol for every product.
Start with the threat model and data inventory
Before choosing a cryptographic library, define what the app must protect, from whom, and where the protection ends. Consider network observers, a malicious or compelled service operator, account takeover, compromised devices, lost devices, malicious insiders, and abusive users. These are distinct risks and may call for different controls.
Map what each component can see. A service that routes messages may need account and device records, recipient information, and temporary queues. In its server model, Signal’s Sesame specification describes user and device records, sender identifiers, and temporarily stored messages. The specification is Revision 2, dated April 14, 2017, so treat it as a model for security considerations rather than current implementation documentation.
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Inventory data by type rather than grouping everything under “message data”:
- Content: message bodies, attachments, and other user-created material.
- Routing and account data: identifiers needed to find recipients and deliver messages.
- Device and group data: device records, group membership, and membership changes.
- Operational data: timestamps, IP or network data, diagnostics, and abuse-prevention records.
- Recovery data: backup archives, recovery keys, and records associated with restoring history.
For each category, record who can access it, why it is needed, how long it is kept, and whether deletion removes all copies. Do not call the service “anonymous” or “metadata-free” unless the design supports that specific claim.
Compose protocol roles instead of inventing cryptography
A secure messaging system combines distinct cryptographic and application functions. Signal’s public specifications separate initial key agreement, message-key updates, asynchronous session management, and post-quantum key agreement; the app must compose those roles correctly.
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| Component | Role described in the specification | Design implication |
|---|---|---|
| X3DH or PQXDH | Establishes shared secrets for starting asynchronous sessions and uses public keys to authenticate parties. | Plan how identity keys are authenticated; key agreement alone does not tell a user which person a key belongs to. |
| Double Ratchet | Derives changing message keys and mixes in Diffie–Hellman values to provide protection for earlier and later messages after certain compromises. | Protect and correctly maintain ratchet state; understand the limits when a device or its keys are compromised. |
| Sesame | Addresses session management across asynchronous messages and multiple devices. | Design for queues, device changes, and messages that arrive late, out of order, more than once, or not at all. |
| ML-KEM Braid | Describes a sparse continuous key-agreement protocol using standardized ML-KEM to provide post-quantum shared secrets to higher-level protocols. | Evaluate its communication pattern and deployment constraints; its stated post-compromise behavior depends on participants’ messages and replies. |
These specifications describe protocol designs, not the security of your product. Do not implement a home-grown protocol. Select a maintained implementation only after checking its threat model, supported platforms, state-handling requirements, update path, licensing, and operational support. The cited material does not establish those selection details for any particular library.
Make identity verification and account recovery separate
Encryption can establish that a participant has the corresponding private key. It cannot, by itself, prove that the key belongs to the person a user intends to contact. Signal’s X3DH specification puts the consequence plainly: “If authentication is not performed, the parties receive no cryptographic guarantee as to who they are communicating with.”
Offer an authenticated way for users to check identity keys, such as comparing safety numbers or fingerprints, or scanning a QR code in person. Explain what the check confirms and what it does not. A service’s unauthenticated assertion that a key belongs to a name is not a substitute for an authenticated channel.
Make identity-key changes visible. Sesame notes that a changed key can follow a reinstall or added device, but can also signal impersonation; one approach it describes is pausing communication until the user acknowledges the change. The product should make that choice understandable rather than silently treating the new key as the old identity.
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Keep login recovery distinct from cryptographic identity. A reset to an account credential should not quietly bind a new identity key in a way that lets an attacker appear to be an established contact. Define policies for lost devices, SIM or email takeover, device additions and removals, and key rotation before these events occur.
Design delivery and multi-device state for failure
Asynchronous messaging means the service and clients hold state. Sesame’s model includes user and device records, per-device mailboxes, and temporary message storage. It accounts for messages being delayed, lost, reordered, duplicated, corrupted, deleted, or forged. The client therefore needs authenticated parsing and deliberate handling of replays and duplicates, as well as recovery paths when local session state is deleted or rolled back.
Document the service’s trust boundary and decide:
- Whether identity keys belong to users or individual devices.
- How a newly added device is authenticated and how a removed device is revoked.
- What happens to queued messages when a device or group membership changes.
- How clients recover from reinstall, stale sessions, restore, simultaneous device changes, and unreliable clocks.
Test these transitions as security-critical flows, not merely usability edge cases. A delivery system that behaves correctly only when both participants are online and their devices never change is not a complete asynchronous design.
Per-user keys versus per-device keys
| Approach | Consequence to plan for |
|---|---|
| Per-user identity key | Decide how adding or removing a device affects verification and how a user’s identity is represented across devices. |
| Per-device identity keys | Provide a way to verify devices and revoke a particular device; account recovery must not make an unverified device appear trusted. |
Be precise about metadata and group privacy
End-to-end encryption primarily protects message contents from the service and network observers. It does not mean every piece of metadata is hidden. Authenticated encryption on device-to-service connections protects traffic from network eavesdroppers, but does not eliminate what the service can observe. Sesame explicitly distinguishes secure server communications from service-visible information such as sender and device identifiers in its model.
Minimize the routing and abuse-prevention data the service needs, limit retention, and state what remains visible. Assess content, account identifiers, sender and recipient identifiers, device identifiers, group membership, timing, IP or network information, diagnostics, and backup records separately. A privacy claim should match the specific exposure the design actually reduces.
Groups add membership, update, and delivery decisions. Compare candidate designs against product requirements rather than naming a universal winner:
- How securely membership changes are handled.
- Forward secrecy and post-compromise security.
- What group or delivery metadata infrastructure can see.
- Group size, latency, bandwidth, and offline behavior.
- Implementation maturity and the client complexity required.
The available specifications are not enough to choose a single best group protocol for a hypothetical app.
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Ratchet-based key evolution can protect previously recorded encrypted messages after certain later key compromises. That is not protection against every form of compromise: Signal’s Double Ratchet specification warns that access to secret keys or device integrity can devastate future communications. If an attacker controls an endpoint, encryption performed by that endpoint cannot keep the endpoint’s plaintext secret from the attacker.
Deletion also has limits. Removing a message from an app’s visible history does not necessarily remove plaintext copies, attachments, keys, queued copies, or backups. The Double Ratchet specification notes that low-level device access may recover deleted keys or plaintext. Define what deletion means for each storage location, and avoid promising secure erasure unless the implementation and device assumptions support it.
Choose backup behavior as a security and usability policy
A backup is another copy of message history and another key-management problem. Decide whether history is recoverable, what a backup contains, whether it is optional, how its key is generated and stored, and what users can do if that key is lost. A design that makes recovery easy for the service may also give the service a route to the archive; a design that keeps the recovery key from the service places more responsibility on the user.
| Choice | Recovery benefit | Security and usability trade-off |
|---|---|---|
| No cloud backup | A cloud archive is not available for restoration. | History may be lost when devices are lost or replaced; no cloud backup copy is created. |
| Optional end-to-end encrypted backup | Users can restore covered history if they retain the recovery key and the backup remains available. | Creates an encrypted archive and makes recovery depend on safe key custody; losing the key can make restoration impossible. |
As a product-specific example—not a default every app should copy—Signal’s current support page describes optional Secure Backups protected by a 64-character recovery key that Signal says it never receives. Its page says the archive includes text and the last 45 days of media, with a paid option for up to 100 GB of media at $1.99 per month at the time those details were checked. Signal says it cannot read, decrypt, or restore the archive without the key. Availability and price can change; these are Signal product details, not independent security measurements or a recommendation for another app.
Evaluate post-quantum key agreement against the product’s needs
Signal’s ML-KEM Braid specification describes a sparse continuous key-agreement protocol using standardized ML-KEM to provide post-quantum shared secrets to higher-level protocols. It is revision 1 dated February 21, 2025, with a last update shown as September 26, 2025. That is a design reference, not evidence that adopting it automatically makes an app post-quantum secure.
Compare classical and post-quantum approaches by the security objective, message size and round behavior, deployment maturity, and client and network constraints. The specification’s stated post-compromise behavior depends on whether and how participants reply: a long-offline or one-way pattern can change which messages remain vulnerable. Evaluate that behavior against how users actually communicate rather than treating “post-quantum” as a blanket guarantee.
Quick Recap
Turn the architecture into a product security plan
- Write the threat model and data inventory. Identify attackers, assets, service-visible fields, retention periods, and deletion semantics.
- Select protocol roles and maintained implementations. Check threat models, platform support, state handling, updates, licensing, and support before committing.
- Specify identity and recovery flows. Define verification, key-change warnings, account recovery, device addition, device removal, and key rotation.
- Specify delivery and state transitions. Define queues, per-device behavior, duplicate and replay handling, stale-state recovery, and offline operation.
- Choose metadata and group policies. Minimize retained data and compare group designs against the app’s membership, delivery, and performance requirements.
- Set compromise, deletion, and backup expectations. State what survives endpoint compromise, what deletion removes, and who can restore history under which key conditions.
- Test failure paths before claiming security properties. Exercise lost devices, reinstall, state rollback, key changes, account takeover, delayed delivery, and recovery. A design based on public references is not thereby audited.
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