Multi-Layer E2EE Architecture
Arc Protocol: Signal Protocol (libsignal) as the foundation, with PQXDH (ML-KEM-1024) post-quantum cryptography, XEdDSA signatures, and client-side Sealed Sender.
PQXDH Key Exchange
Hybrid post-quantum key exchange combining X25519 (Curve25519) and ML-KEM-1024 (NIST FIPS 203). Resistant to both classical and quantum attacks.
Double Ratchet
Per-message forward secrecy using DH ratchet + symmetric ratchet. Compromise of one key cannot decrypt past or future messages.
AES-256-GCM Encryption
Authenticated encryption with 256-bit keys. Each message encrypted with a unique key derived from the Double Ratchet.
XEdDSA Signatures (libsignal)
XEdDSA signatures via libsignal. Identity Key (X25519) doubles as signing key. Device list integrity verification included.
Client-Side Sealed Sender
Ephemeral X25519 key per message + HKDF-SHA256 derived AES key. Even the server cannot see who sent a message.
AEGIS β Arc Enhanced Guard & Integrity System
AEGIS (Arc Enhanced Guard & Integrity System) is Arc's message authentication layer built on XEdDSA digital signatures via libsignal. Every single message you send is cryptographically signed, providing three critical guarantees:
Anti-Impersonation
Even if an attacker compromises the server, they cannot forge messages in your name. Your Identity Key (X25519) is used for XEdDSA signing and never leaves your device.
Tamper Detection
Any modification to a message β even a single character β instantly invalidates the signature. Recipients can verify that every message arrived exactly as sent.
Non-Repudiation
Cryptographic proof that you authored a message. XEdDSA uses your X25519 Identity Key for EdDSA-compatible signatures β no separate Ed25519 key needed.
XEdDSA (libsignal) Β· X25519 Identity Key Β· 64-byte signatures Β· Per-message signing
Unlike standard messaging apps, AEGIS goes beyond encryption. While E2EE protects message content, AEGIS protects message identity β ensuring that every message truly comes from who it claims to come from. Powered by libsignal's XEdDSA implementation.
Zero Key Conversion
Arc separates X25519 (key exchange) and Ed25519 (signatures) at the type level. Unlike Signal/WhatsApp, there is no Ed25519βX25519 key conversion, minimizing the attack surface.
Signal/WhatsApp vs Arc V2
| Feature | Signal / WhatsApp | Arc V2 |
|---|---|---|
| Key Design | EdβX conversion | Complete separation |
| Message Signatures | None (DH only) | AEGIS XEdDSA |
| Sealed Sender | Server-side | Client-side |
| Offline E2EE | Not supported | BLE Mesh (Bridgefy SDK) |
| Multi-Device | Primary/Linked | Independent key sets + Key Sync |
| Key Rotation | Fixed interval | Multi-stage automatic |
| Device List | No signature | Ed25519 signed |
| Device Key Transfer | Via Primary | QR + X25519 ECDH + AES-256-GCM |
| Background E2EE | Foreground only | β All App States (FG/BG/Terminated) |
ML-KEM-1024 Parameters
NIST FIPS 203 standardized post-quantum Key Encapsulation Mechanism.
| Algorithm | ML-KEM-1024 (via libsignal-client) |
| NIST Security Level | Level 3 (AES-192 equivalent) |
| Classical Security | 2^128 bit (X25519) |
| Quantum Security | NIST Level 3 |
| Foundation | Module Lattice (FIPS 203) |
| Public Key Size | 1,184 bytes |
| Secret Key Size | 2,400 bytes |
| Ciphertext Size | 1,088 bytes |
Compliance & Standards
πΊπΈNIST CSF 2.0
NIST framework. 6 functions for systematic cyber risk management.
πΊπΈNIST SP 800-53
Federal security controls. FedRAMP foundation.
πΊπΈFIPS 140-3
Federal cryptographic module validation. Covers ML-KEM-1024.
πΊπΈNIST SP 800-175B
Cryptographic algorithm selection guide. Based on FIPS 203.
πISO/IEC 27001
International ISMS certification. B2B/B2G trust foundation.
πͺπΊGDPR
EU General Data Protection Regulation. World's highest protection standard.
π¬π§UK Cyber Essentials+
UK government-certified cybersecurity certification.
π¬π§NCSC Cloud Security
UK NCSC 14 Principles. Cloud security assessment standard.
Developed in compliance with all prerequisites of US NIST, EU GDPR, UK NCSC, and international ISO.
Arc Protocol: Security Whitepaper
A comprehensive technical document describing Arc's cryptographic architecture, threat model, and security guarantees. Covers PQXDH key exchange (ML-KEM-1024), Double Ratchet forward secrecy, XEdDSA per-message signatures, Sealed Sender, and BLE Mesh offline communication.
15 Sections
Complete protocol documentation
Signal Comparison
Honest feature-by-feature analysis
Threat Model
STRIDE analysis with residual risks
System Architecture Documentation
Complete technical specification of Arc V2 β covering the 5-layer architecture, E2EE pipeline, PQXDH implementation, key lifecycle, group encryption, multi-device architecture, BLE Mesh networking, and on-device AI.
15+ Technical Sections
Layer architecture, PQC implementation, encryption pipeline, data storage, and more
Bilingual (EN/JP)
Full documentation available in English and Japanese with one-click toggle
Version 3.1 β March 2026
Includes Elixir PreKey Server, Sealed Sender, Sender Keys, and PQXDH ML-KEM-1024
This document is classified as confidential and available exclusively to NDA-bound partners, investors, and authorized technical reviewers. Access requires prior approval by Atlas Associates.