Atlas Associates

Arc / SECURITY

多层 E2EE 架构

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ILLUSTRATED 1:1 FLOW · 10-SECOND PREVIEW

Encrypted here.Opened there.

A message between people. Private keys stay on the devices; servers carry ciphertext and delivery data.

  1. 01
    Encrypt on device

    The message is encrypted before it leaves.

  2. 02
    Transport ciphertext

    Firestore carries ciphertext and delivery data.

  3. 03
    Open on device

    The recipient’s device decrypts the message.

FunctionsExpiry & notifications
FCMPush notification

Concept animation of one delivery path, not a recording of the app. Other paths, server-visible metadata and exceptions are documented below.

ARCHITECTURE

Arc system architecture

Explore the system diagram

Private keys are stored in the device Keychain / Keystore and never sent to servers. Messages to other people are encrypted on the device before they are sent. Phoenix hands out public PreKeys, Firestore carries ciphertext, receipts and delivery data, and Cloud Functions handle expiry deletion and notifications. Mesh bypasses servers.

One role at a time.

Open full-size diagram

Devices (iOS / Android / macOS)

Arc Protocol

1:1 · libsignal v0.96.2 · Rust FFI

  • PQXDH
  • Kyber-1024
  • Double Ratchet
Private keys
Keychain / Keystore
Decrypted text
SQLite / Hive

Message text and private keys never go to servers (except AI chats, link previews)

Explore the message flows

The dots move in the numbered order. On each of these paths, message text is encrypted on the device before it is sent.

Follow one path at a time.

Open full-size diagram
1:1 message. Post-quantum. First time: fetch B's keys. Encrypt on the device. Send ciphertext only. Notify (no content). Decrypt on B's device. Read / decrypt receipts. Only A's and B's devices can read the text

Post-quantum

1:1 message

  1. First time: fetch B's keys
  2. Encrypt on the device
  3. Send ciphertext only
  4. Notify (no content)
  5. Decrypt on B's device
  6. Read / decrypt receipts

Only A's and B's devices can read the text

On smaller screens, scroll within the diagram. Open the original using the link above.

1:1 is post-quantum (PQXDH). Group messages are not (shared key + X25519 ECIES). On Mesh, nearby Arc phones may forward the ciphertext without being able to read it. Routed relay is in development.

Arc Protocol — 1:1 encryption pipeline

Phase 1

PQXDH

Key agreement with X25519 + Kyber-1024 (post-quantum)

Phase 2

Double Ratchet

A new key for every message

Phase 3

AES-256-GCM

Encrypts message bodies and media

Technology stack

Flutter 3.44
Dart
Riverpod 2.6
Rust
Arc Protocol

App

Flutter 3.44 / Riverpod 2.6 / Rust FFI (libsignal v0.96.2)

Elixir
Phoenix
Cloud Run

Key distribution API

Elixir / Phoenix on Google Cloud Run (europe-west1)

Authentication
Firestore
Storage
Functions
Messaging
App Check
Crashlytics

Delivery and auth

Firebase (Auth / Firestore / Storage / Functions / FCM / App Check / Crashlytics)

Android
Kotlin
Swift

Mesh

BLE implementation (Dart + Android Kotlin + iOS Swift, in-house, beta)

Technical specifications

The four encryption layers

Arc core specification

Arc's core specification is ML-KEM-1024/PQXDH, E2EE for 1:1 and group chats, IGF, and offline Mesh. This Security page documents the scope and technical details.

01

PQXDH 密钥交换

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02

Double Ratchet

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03

AES-256-GCM 加密

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04

XEdDSA 签名（libsignal）

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ML-KEM-1024 参数

POST-QUANTUM

NIST FIPS 203 标准化的后量子密钥封装机制。

算法ML-KEM-1024 (via libsignal-client)
NIST 安全级别Security category 5
经典安全2^128 bit (X25519)
量子安全NIST security category 5
基础Module Lattice (FIPS 203)
公钥大小1,568 bytes
私钥大小3,168 bytes
密文大小1,568 bytes

合规与标准

NIST CSF 2.0

NIST框架。6大功能系统化管理网络风险。

NIST SP 800-53

联邦安全控制。FedRAMP基础。

FIPS 140-3

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NIST SP 800-175B

加密算法选择指南。基于FIPS 203。

ISO/IEC 27001

国际ISMS认证。B2B/B2G信任基础。

GDPR

欧盟通用数据保护条例。全球最高保护标准。

UK Cyber Essentials+

英国政府认证网络安全认证。

NCSC Cloud Security

英国NCSC 14原则。云安全评估标准。

参考美国NIST、欧盟GDPR、英国NCSC和ISO公开发布的指南进行设计。这并非认证或第三方审计。