Comparing Top Message Clients: Pros, Cons, and Use Cases

How to Build a Secure Message Client — Best PracticesBuilding a secure message client requires careful design across architecture, cryptography, user experience, and operational practices. This article guides you through core principles, practical decisions, and implementation details needed to create a modern, secure messaging application for mobile, desktop, or web.

Threat model first: know what you’re protecting

Before designing, define a clear threat model. Decide which actors and risks you’ll protect against:

Adversaries: server operators, network attackers, device thieves, other users, malicious developers, nation-states.
Assets: message content, metadata (who, when, IPs), attachments, contact lists, user credentials, cryptographic keys.
Goals: confidentiality, integrity, authentication, forward secrecy, deniability, availability, metadata minimization.

Design choices should map to the threat model (e.g., defending against server compromise requires end-to-end encryption).

Architectural patterns

End-to-end encryption (E2EE): encrypt messages so only sender/recipient can read them. Servers should only handle ciphertext.
Signal-style server model: centralized servers for metadata routing and push notifications, with cryptographic protections at the client.
Federated model: multiple interoperable servers (e.g., Matrix), which can reduce central points of failure but complicate trust and metadata exposure.
Peer-to-peer: minimizes servers but can be complex for NAT traversal, syncing, and scaling.

Choose the model that balances usability, scalability, and the privacy guarantees required.

Cryptography: protocols and key management

Use well-vetted protocols (Signal Protocol, Double Ratchet, X3DH) rather than designing your own.
Key types:
- Long-term identity keys (Ed25519/Ed448) for authentication.
- Signed pre-keys and one-time pre-keys (X25519) for initial DH key exchange.
- Ephemeral keys for forward secrecy (X25519 ephemeral DH).
- Symmetric keys for message encryption (AES-GCM, ChaCha20-Poly1305).
Forward secrecy: ensure compromise of long-term keys doesn’t decrypt past messages (Double Ratchet).
Post-compromise recovery: provide mechanisms to re-establish secure sessions after compromise.
Authenticated encryption: use AEAD constructions (ChaCha20-Poly1305 or AES-GCM).
Perfect forward secrecy vs. future secrecy: consider “future secrecy” (post-compromise secrecy) with key rotation and server-assisted rekeying.
Key storage: store private keys in secure enclaves (Secure Enclave, Android Keystore) where available; use OS-provided APIs for hardware-backed keys.
Protect against downgrade and replay attacks: include protocol versioning and unique nonces/sequence numbers.

Metadata protection

Metadata often reveals more than message content. Mitigation strategies:

Minimal metadata retention: store only what is required, purge logs regularly.
Private contact discovery: use techniques like cryptographic contact discovery, Bloom filters, or trusted contact indexing to avoid uploading plaintext address books.
Use ephemeral connection identifiers and rotate them to avoid long-term correlation.
Routing obfuscation: integrate mix networks, message batching, or delayed delivery options where appropriate.
Consider using onion routing or proxies to hide IP addresses for sensitive users.
Implement decentralized or federated architectures to distribute metadata exposure.

Authentication and account security

Use authenticated identity verification: verify other users’ public keys via safety numbers, QR codes, or out-of-band channels.
Strong password policies for account access; prefer passphrases and length over complexity.
Multi-factor authentication (MFA): combine device-bound keys (hardware security keys, platform authenticators) with passwords when server-side accounts exist.
Credential storage: never store plaintext passwords; use salted hashing (Argon2id/BCrypt/Scrypt) for server-side credentials where relevant.
Device management: allow users to view, revoke, and name devices; require re-verification when adding a new device.

Secure message storage and transport

Encrypt messages at rest on the device using keys derived from user credentials and device keys; consider per-conversation keys.
Use authenticated APIs (HTTPS/TLS 1.3) with certificate pinning or DANE where feasible.
Limit server-side plaintext: servers should store only encrypted blobs and minimal metadata.
Secure attachments: encrypt attachments with per-file keys; upload only ciphertext to storage servers.
Implement message expiration/self-destruct timers with secure deletion when possible (note: secure deletion on SSDs/flash is difficult).

Group messaging

Group chats introduce complexity for E2EE:

Use group key agreements (Sender keys or MLS — Messaging Layer Security) to scale efficiently while maintaining security properties.
For small groups, pairwise sessions can be used; for larger groups, use a group ratchet (MLS) to manage membership changes, forward secrecy, and post-compromise recovery.
Authenticated membership changes: require signatures from authorized members for invites and removals.
Handle offline members: support out-of-order delivery and rekeying so offline devices can join securely later.

Usability and secure defaults

Security only helps if users actually use it. Prioritize usability:

Make E2EE the default with clear but minimal user prompts about verification.
Simplify key verification: provide QR codes, short numeric safety codes, or contact-based verification flows.
Provide clear UI for device list, session status, and warnings for unverified devices.
Make secure recovery reasonable: offer encrypted backups protected by user-chosen passphrases (with strong KDFs like Argon2id), but warn users about tradeoffs.
Avoid security dialogs too often — only show when user action or risk is present.

Privacy-preserving features

Read receipts and typing indicators should be opt-in to limit metadata leakage.
Offer disappearing messages and message retention controls.
Implement per-conversation privacy settings and per-contact block/ignore controls.
Minimize analytics and telemetry; if collected, aggregate and anonymize on-device where possible.

Logging, monitoring, and incident response

Log only what’s necessary, avoid storing message content or identifiable metadata.
Use secure, access-controlled audit logs for admin actions.
Establish incident response plans for key compromise, server breaches, or zero-day vulnerabilities.
Provide transparent breach notifications and, when possible, allow users to rotate keys and re-establish secure sessions.

Open source and transparency

Open-source cryptographic and protocol implementations to enable third-party audits.
Publish security design documents, threat models, and bug-bounty programs.
Regular third-party audits and reproducible builds increase trust.

Compliance, legal, and policy considerations

Be aware of export controls, local data retention laws, and lawful access requirements.
Design to minimize the amount of data that could be subject to legal requests; use transparency reporting.
Consider safe defaults to resist overbroad legal demands (e.g., not storing plaintext backups).

Performance and scaling

Use efficient cryptographic primitives (Curve25519, ChaCha20) to reduce battery and CPU use.
Cache session state securely to speed up reconnection and message sending.
For large-scale deployments, design stateless servers for routing and stateful services for storage with strict access controls.

Testing and secure development lifecycle

Threat modeling during design, static analysis, fuzz testing, and regular code reviews.
Use memory-safe languages where possible (Rust, Go) for critical components to reduce memory-safety bugs.
Continuous integration with security tests, dependency scanning, and automated cryptographic checks.
Offer bug bounties and coordinated disclosure processes.

Example technology stack (suggested)

Client: Kotlin (Android), Swift (iOS), Rust backend libs, React/Electron for desktop/web with WASM for crypto components.
Crypto: libsodium, libsodium-wrappers, or well-maintained implementations of the Signal Protocol or MLS.
Server: TLS 1.3, PostgreSQL/Encrypted blob stores, horizontally scalable message routers.
Key storage: platform keystores, optional hardware-backed HSMs for server operations.

Summary checklist

Define threat model and assets.
Use established E2EE protocols (Signal/MLS).
Implement forward secrecy and post-compromise recovery.
Minimize metadata and implement private discovery.
Secure key storage and device management.
Make secure defaults and prioritize usability.
Open source critical components and run audits.
Prepare incident response, testing, and monitoring.

Building a secure message client is a systems problem that spans cryptography, UX, infrastructure, and policy. Following established protocols, minimizing metadata, and making security usable are the pillars that produce a trustworthy messaging app.