Security & Cryptography
Protocol Specification v1.0 • Ed25519 • X25519 • ChaCha20-Poly1305 • BLAKE3
1. Threat Model & Design Goals
OffMesh is architected under the zero-trust assumption that the radio communication medium (Bluetooth LE, Wi-Fi Direct, and ad-hoc networks) and intermediate relay devices may be passive observers, malicious nodes, or compromised radios.
End-to-End Secrecy
Only the intended recipient possessing the corresponding private key can decrypt message payloads.
Forward Secrecy
Compromise of a long-term device key does not expose past encrypted communications.
Relay Metadata Obfuscation
Intermediate mesh relays only handle encrypted envelopes with blinded hop headers.
Cryptographic Tamper-Resistance
All envelopes are authenticated via BLAKE3 Merkle roots and digital signatures.
2. Cryptographic Primitives
| Layer | Algorithm | Function & Implementation |
|---|---|---|
| Identity Keys | Ed25519 | Asymmetric digital signatures for peer authentication and delivery confirmation receipts. |
| Key Agreement | X25519 | Diffie-Hellman ephemeral key exchange establishing session keys per conversation. |
| Symmetric Encryption | ChaCha20-Poly1305 | Authenticated Encryption with Associated Data (AEAD) delivering 256-bit security with high performance on ARM mobile chipsets. |
| Merkle Tree Sync | BLAKE3 | Ultra-fast cryptographic hashing for 5-minute time bucket reconciliation without leaking individual message IDs. |
| Local Key Storage | Android Keystore / Secure Enclave | Hardware-backed key encryption preventing memory dumps and physical extraction. |
3. Operating System Permission Audit (Least-Privilege)
OffMesh does not request location access, contact book access, phone state, microphone, or external storage:
- Bluetooth Low Energy: Uses
android.permission.BLUETOOTH_SCANwithandroid:usesPermissionFlags="neverForLocation"so Google Play certifies that no location data is derived or shared. - Wi-Fi Direct: Uses
android.permission.NEARBY_WIFI_DEVICESfor device-to-device transport without location access. - Background Execution: Standard foreground service with an un-intrusive notification allows the mesh engine to relay delay-tolerant packets when you opt-in.
4. Responsible Vulnerability Disclosure
We welcome independent cryptographic scrutiny. If you discover a vulnerability or cryptographic weakness, please report it via our responsible disclosure channel:
OffMesh Security Advisory
Email: security@offmesh.org
PGP Key Fingerprint: E4F2 981A 3B2C 7D10 99AF 8C4E 1120 54BB 8890 A0C1