Xylem Messenger

Problem

Messaging apps usually ask you to trust the server. The goal was a messenger where the server only ever holds public keys and ciphertext, built end to end as an undergraduate thesis.

What I built

Xylem Messenger is an E2EE messenger in two parts. The server stores public identity and signed pre-keys (X3DH-style), authenticates users with Ed25519 signatures over one-time nonces, and syncs messages over WebSockets, falling back to silent push when a client is offline. The Android client does the X3DH handshake and all encryption on-device, keeps keys in the Android KeyStore and chat history in an SQLCipher database.

Decisions & trade-offs

• Signed one-time nonces (Ed25519) instead of passwords, so the server never stores a secret that could leak.
• One MongoDB Change Stream for the whole app, dispatched to per-user queues, instead of a stream per connection.
• The server deletes a message only after the client confirms it saved it, so a dropped connection loses nothing.
• Silent push to wake offline clients instead of keeping long-lived connections open.

Highlights

• Signature-based auth with expiring one-time nonces (Ed25519) on both REST and WebSocket, no passwords
• One MongoDB Change Stream for the whole app, dispatched to per-user asyncio queues, with a backlog fetch so nothing is lost on reconnect
• Server deletes a message only after the client confirms it persisted it (COMPLETED acknowledgment)
• Silent high-priority FCM pushes wake offline clients to sync
• Android client: X3DH handshake on-device, private keys in Android KeyStore, history encrypted at rest with SQLCipher, signed pre-key rotation every 28 days
• Tests on both sides (pytest for routes/database/websocket; Android test sources); server ships a Dockerfile and a mock browser client

Stack

• FastAPI
• Python 3.13
• MongoDB (Motor)
• WebSockets
• Ed25519
• Firebase Cloud Messaging
• Docker
• Kotlin
• Jetpack Compose
• Hilt
• Room + SQLCipher
• Google Tink