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A privacy-first messaging app needs more than end-to-end encryption. Encryption can protect message contents from the service, but it does not by itself verify who a user is talking to, hide all metadata, secure a compromised device, or make message history recoverable. Build the product around a defined threat model, authenticated identities, carefully managed device and delivery state, and an explicit recovery policy.
Start with the threats and data the app must handle
Before choosing a protocol or library, decide what the app is intended to protect, from whom, and under what conditions. A useful threat model distinguishes a network observer from a malicious or compelled service operator; account takeover from endpoint compromise; and lost devices from abusive users. These threats require different controls. End-to-end encryption, for example, can limit what a service operator learns from message bodies but cannot keep an attacker with access to an unlocked endpoint from reading those messages.
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Inventory the data each part of the system sees and retains. Keep message contents distinct from account identifiers, device identifiers, group membership, sender and recipient routing data, timestamps, IP or network information, diagnostics, and backup records. For each category, document why it is needed, where it is processed, who can access it, how long it is retained, and what deletion means in practice. Do not describe the service as anonymous or metadata-free unless the architecture and evidence support that specific claim.
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- FIDO2 & Passkey Ready: Business-ready and FIDO2 L1 certified. This key is supported by major management suites and is ideal for both individual and enterprise deployment. Works seamlessly with Gmail, Facebook, GitHub, Dropbox, Coinbase, and more.
- Universal Connectivity (USB-A ): Features a built-in USB-A connector—simply unfold the key and plug it into your compatible PC or laptop for seamless authentication on the go.
- Dedicated Manager App: Use the Thetis Manager App for the initial hardware PIN setup. Setting the PIN on the device first ensures a smooth registration process. Once the PIN is configured, you can begin registering the key across your favorite FIDO2-compatible online services.
- Ultra-Durable & Portable: Featuring a rotating metal cover, this key is water, crush, and tamper-resistant. It fits easily on a keychain and requires no batteries or network connectivity.
- Check FIDO2 compatibility before purchase - Known limitations: ID Austria is not supported (requires FIDO2 Level 2). Windows Hello login only works with Windows Enterprise editions that support Entra ID, and NFC is NOT supported.
Compose cryptographic functions instead of inventing a protocol
Secure messaging is a set of connected functions, not one encryption switch. Signal’s public documentation separates session establishment, message-key evolution, asynchronous session management, and post-quantum key agreement. An app must compose the chosen functions correctly and maintain their state safely across delays, device changes, updates, and recovery.
| Protocol role | What it addresses | Design implication |
|---|---|---|
| X3DH or PQXDH | Establishing shared secrets for asynchronous sessions and associating them with public keys. X3DH describes forward secrecy and deniability properties, while warning that unauthenticated identity keys do not establish whom a party is communicating with. | Session setup and identity authentication are related but distinct. Users need a way to authenticate the identity key through a suitable channel. |
| Double Ratchet | Deriving changing message keys and incorporating Diffie-Hellman values as communication proceeds. | Key evolution can limit exposure of recorded messages after certain later compromises; it does not make a compromised endpoint safe for future communication. |
| Sesame | Managing asynchronous session state across messages and multiple devices. | Clients and service design must account for delay, loss, reordering, duplication, device changes, and local state recovery. Sesame is Revision 2, dated April 14, 2017, so treat it as a model and security reference, not current implementation documentation. |
| ML-KEM Braid | A sparse continuous key-agreement protocol using NIST-standardized ML-KEM to provide post-quantum shared secrets for higher-level protocols. | Its specified post-compromise behavior depends on message patterns and replies. Revision 1 is dated February 21, 2025, and the document shows a last update of September 26, 2025; evaluate deployment maturity and client/network constraints rather than assuming a universal upgrade path. |
These components have different jobs; adopting one does not automatically provide the properties of the others. Do not implement a home-grown cryptographic protocol. Select a maintained implementation with a documented threat model, support for the target platforms, secure handling of persistent state, and a clear update process. Verify licensing and operational support for the specific implementation before committing to it.
Make identity-key verification understandable and consequential
A service can deliver a public key, but an unauthenticated service assertion alone does not prove that the key belongs to the intended contact. The X3DH specification states: “If authentication is not performed, the parties receive no cryptographic guarantee as to who they are communicating with.” That distinction is central: encryption to the wrong key can still be strong encryption, but it protects communication with the wrong identity.
Give users a practical way to compare or otherwise authenticate identity keys outside an unauthenticated assertion by the service. Safety-number or fingerprint comparison and in-person QR scanning are examples, not the only possible authenticated channels. Explain what the user is verifying and what a successful match means.
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- Ultra-Compact FIDO2 Security Key - Plug-and-stay or carry on a keychain. This USB-A hardware security key offers portable, always-on protection for desktop and mobile use. (Item Size: 0.75 X 0.74 IN x 0.25 IN)
- USB-A Hardware Key for All Devices - Works with USB-A ports on PC, Mac, Android, and other laptop/notebook device. Enables secure, cross-platform login with FIDO2.0 passkey support.
- FIDO Certified Security Key - Meets FIDO and FIDO2 standards. Works with Google, Microsoft, GitHub, Dropbox, and more. Please check service compatibility before purchase.
- Passwordless Login with Passkey - Supports passkey login via WebAuthn and CTAP2. Enjoy password-free sign-ins where supported. Not all websites or services currently support passkeys.
- Advanced Multi-Factor Authentication - Offers 200 FIDO2 passkey slots and 50 OATH-TOTP slots. Strong, flexible 2FA/MFA support across various apps and authentication platforms.
Do not silently treat a changed identity key as the same established identity. Sesame notes that a key change may follow a reinstall or added device, but may also indicate impersonation; one approach it describes is pausing communication until the user acknowledges the change. Design a visible, comprehensible change flow and decide whether users can pause, verify, or explicitly proceed.
Keep account authentication separate from cryptographic identity. A login reset should not quietly rebind an identity key so that someone who took over an account appears to be the established contact. Define what happens after a lost device, an email or SIM takeover, a new device addition, device removal, and key rotation. In particular, specify which identity keys are verified by users and which keys identify individual devices.
Design asynchronous delivery for unreliable networks and changing devices
Messages may be sent while recipients are offline, so the server and clients have state even when message contents are end-to-end encrypted. Sesame’s model includes user and device records, per-device mailboxes, and temporary message storage. It also considers messages that are delayed, lost, reordered, duplicated, corrupted, deleted, or forged. Its Revision 2 is dated April 14, 2017; use it to understand the model and its security considerations, not as current implementation documentation.
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Rank #3
- USB-C or tap via NFC for easy authentication on any compatible device. No drivers needed; optional Kensington software available for advanced management features.
- Works across Windows, macOS, iOS, Android, ChromeOS, and supports Passkeys and Apple ID.
- Slim, keychain-ready form for easy carry and on-the-go authentication
- IP68-rated for dependable performance
- FIDO CTAP 2.1 for enhanced security features (e.g. resident credentials, Passkey support) and backwards compatibility with CTAP 2. FIDO2 L2 certified security for phishing resistant protection against identity theft and unauthorized access.
Exercise recovery paths as deliberately as the normal send path. Include an offline recipient, simultaneous device changes, stale sessions, reinstall, restore from backup, and clock manipulation. Sesame notes that reliable clocks matter for some mitigations, so document which protections depend on time and how clock irregularities affect them.
Choose a device model and define revocation behavior
Deciding between per-user and per-device keys changes both verification and operations. With per-device keys, each device can have a distinct cryptographic identity, which makes it possible to remove one device without treating all devices as the same key. It also means a new device must be authenticated and recipients may need to recognize device-level key changes. With a shared user-level identity, the product still needs a secure way to authorize devices and revoke a lost or compromised one; a single visible identity should not conceal an untrusted device addition.
| Design choice | Benefit to consider | Risk or operational question |
|---|---|---|
| Per-device identity keys | Can make device-specific verification and revocation more explicit. | How are new device keys authenticated, and what do contacts see when devices are added or removed? |
| Per-user identity key | Can present a stable identity across a user’s devices. | How is device membership authorized without letting an attacker add a device under the established identity? |
For either model, decide how queued messages are handled when a device is added, removed, or loses session state. Revocation should affect future delivery according to a documented policy; it cannot make plaintext already copied to a device disappear. Treat reinstall and restore as security-sensitive transitions, not just account setup screens.
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End-to-end encryption primarily protects message content from the service and network observers; it does not automatically conceal every fact needed to route, synchronize, or moderate communication. In the Sesame server model, the service has user and device records and sender identifiers associated with queued messages. That is enough to show why “the service cannot see messages” is narrower than “the service sees nothing.”
Rank #4
- FIDO2 & Passkey Ready: Business-ready and FIDO2 L1 certified. This key is supported by major management suites and is ideal for both individual and enterprise deployment. Works seamlessly with Gmail, Facebook, GitHub, Dropbox, Coinbase, and more.
- Dedicated Manager App: Use the Thetis Manager App for the initial hardware PIN setup. Setting the PIN on the device first ensures a smooth registration process. Once the PIN is configured, you can begin registering the key across your favorite FIDO2-compatible online services.
- Universal Connectivity (USB-C, USB-A, & NFC): Designed for PCs, Macs, iPhones, and Android. For mobile use, simply unfold the key, align it with your phone’s NFC antenna, and hold for a few seconds to authenticate.
- Enhanced MFA (FIDO2 & TOTP/HOTP): Strengthen your security with flexible options. Use the Manager App to access TOTP/HOTP features for accounts that do not yet support FIDO2.
- Check FIDO2 compatibility before purchase - Known limitations: ID Austria is not supported (requires FIDO2 Level 2). Windows Hello login only works with Windows Enterprise editions that support Entra ID. NFC is supported only through mobile authentication, Not MacOS/windows.
For each metadata field, decide whether it is necessary for delivery, abuse prevention, diagnostics, or product operation. Minimize collection and retention, restrict internal access, and explain residual exposure in terms users can understand. Protect client-server communications with authenticated encryption as well as message-level encryption: the former helps limit metadata exposure to network eavesdroppers, while neither eliminates information available to endpoints or the service itself.
Evaluate group messaging against product-specific requirements
Group design has to account for membership changes and how messages reach devices, not just encrypting a message for multiple recipients. Compare candidate approaches using the needs of the intended product rather than selecting a universal winner. The available protocol references do not establish one best group protocol for every app.
- Membership-change security: What happens to past and future access when a member joins, leaves, or is removed?
- Forward secrecy and post-compromise security: Which past or future messages may be exposed after a key or device compromise, and what actions restore protection?
- Delivery metadata: What can delivery infrastructure infer about membership, recipients, and activity?
- Scale and performance constraints: How do group size, message size, bandwidth, latency, and offline delivery affect the user experience?
- Implementation maturity: Is there a maintained implementation that fits the app’s platforms and state-management requirements?
These are design criteria to test against the actual group behavior of the product. Avoid inferring a group protocol’s suitability from one attractive property while leaving membership, offline delivery, or metadata questions unresolved.
Set expectations for compromise, deletion, and history recovery
Ratcheting can protect earlier recorded messages after certain later key compromises, but it is not a remedy for an attacker controlling a device. The Double Ratchet specification describes protection for earlier messages under particular compromise conditions and warns that compromise of secret keys or device integrity can devastate future communications. If an attacker controls an endpoint, they may be able to read newly received plaintext or keys regardless of the server’s inability to decrypt the message stream.
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- 3-SIDE CAPACITIVE TOUCH: Tap the end, left, or right side to authenticate, so it works in any orientation or crowded USB port.
- MULTI-COLOR LED INDICATOR: Blue means ready, blinking blue means tap now, green means success, and red means error for instant status feedback.
- IP68 WATERPROOF & BATTERY-FREE: Crush-resistant one-piece construction survives daily carry on a keychain or in a bag for years without any batteries.
- UNIVERSAL COMPATIBILITY: Works with Google, Microsoft, Apple, GitHub, AWS, and any FIDO2 / U2F / WebAuthn service, storing up to 100 passkeys.
Deletion also has limits. Removing a message from an app’s interface does not prove every plaintext copy or key has been erased. Device-level recovery, screenshots, exports, notifications, or backup copies may remain outside the deleted record. Explain what deletion removes from the service and supported clients, and avoid promising secure erasure that the app cannot establish.
Backups make history recoverable by creating another encrypted copy and another key-management problem. Decide whether backups exist, whether they are optional, what content and time range they include, how recovery keys are generated and stored, and what happens when users lose those keys. More convenient recovery can mean another place where message history is exposed if the recovery secret or backup endpoint is compromised.
Signal’s Secure Backups support page describes an optional end-to-end encrypted archive protected by a 64-character recovery key that Signal says it never receives. According to that page, the key is required to restore the archive, and Signal says it cannot read, decrypt, or restore the archive without it. The page describes text and the last 45 days of media as included, plus an option for up to 100 GB of media at $1.99 per month at the research time. Those are Signal product details, not defaults for a new app; availability and price can change. Signal’s September 8, 2025 announcement described an initial limited Android beta and said iOS and Desktop support were planned then; the later support page is the relevant source for the feature details above.
Turn architecture choices into a shipping checklist
- Define the threat model and data inventory. Identify attackers, assets, service-visible fields, retention periods, and deletion boundaries before selecting a protocol.
- Choose protocol components and implementations. Map session establishment, ratcheting, multi-device state, and any post-quantum goals to maintained implementations; review platform coverage, state handling, licensing, and update operations.
- Specify identity and account transitions. Document verification, key-change warnings, account recovery, device authorization, removal, and revocation as user-visible behaviors.
- Specify server and client delivery state. Define mailbox behavior, queue expiry, parsing and replay protections, duplicate handling, and recovery after local state loss or rollback.
- Test hostile and failure conditions. Include offline delivery, reordered and duplicated messages, device replacement, compromised endpoints, clock changes, backup restoration, and identity-key changes.
- Review every privacy claim against the data inventory. State separately what the service, network observers, and endpoints can learn; do not use broad labels such as “anonymous” or “metadata-free” without evidence.
- Make recovery a deliberate product choice. If history is recoverable, explain what is backed up, who holds the recovery secret, and what users lose if it is lost.
No design based on public specifications is secure merely because it adopts familiar protocol names. Security depends on the exact implementation and the product decisions around identity, delivery, endpoints, metadata, and recovery; those require review and testing in the system that will actually ship.
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