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ARCHITECTURE.md

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CoreCrypto Architecture

Components

  • Keystore: Encrypted Keystore powered by SQLCipher on all platforms except WASM. WASM uses an IndexedDB-backed, encrypted store with AES256-GCM
  • MlsProvider: RustCrypto + Keystore MLS provider
  • CoreCrypto: Abstracts MLS & Proteus in a unified API (Proteus isn't supported as of 0.2.0)
  • CoreCryptoFFI: FFI bindings for C, iOS, Android and WASM

Keystore

Purpose

The keystore's purpose is to securely store the Client's keying material on-device.

As such, its requirements are:

  • Some sort of persistent data storage layer
  • Encryption at-rest of the persisted keying material
  • Compatibility with target libraries storage traits (i.e. OpenMLS & Proteus)

Implementation

On most platforms, the keystore relies on SQLCipher to persist & encrypt data On WebAssembly (TS/JS bindings for the web & electron), the keystore calls into the browser's IndexedDB to persist data and AES256-GCM to encrypt data (via RustCrypto).

Status

  • The keystore's implementation of encryption at rest on WASM isn't validated nor audited so paper cuts expected.

MlsProvider

Purpose

Interfacing with the OpenMLSCryptoProvider trait to allow OpenMLS to call into our keystore

Implementation

Just implementing a single trait and instanciating the relevant structs, namely the crypto provider from RustCrypto and our Keystore

CoreCrypto

Purpose

Implements and abstracts differences between protocols (MLS, Proteus), and also erases the API inconsistences in those APIs. CoreCrypto basically aims at being as simple as possible, erasing protocol specifics and manipulating "layman" entities.

Implementation

Based around a Central concept (see MlsCentral in lib.rs), where a single object holds ownership over what is needed at runtime.

A couple of other concepts are used:

  • Central: Represents the entry point of the instantiated state of the library once the necessary initialization is done. Takes care of finding/creating the local Client, restoring persisted conversations etc.
  • Client: Represents the local client (i.e. user device) that has the ability to produce local keying material.
  • Member: Represents a remote client (i.e. conversation member) without the ability to produce keying material.
  • Conversation: Represents a conversation group (i.e. n Sessions on Proteus, and a MlsGroup on OpenMLS) in which the local Client is taking part.

CoreCryptoFFI

Purpose

Allows other programming languages and platforms to embed and interact with CoreCrypto

Implementation

  • For iOS and Android, UniFFI is used to produce the relevant Kotlin and Swift bindings
  • For JS/TS, a WebAssembly binary is produced using wasm-bindgen and wasm-pack