400 lines
16 KiB
Markdown
400 lines
16 KiB
Markdown
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# Fumi - Smol Mail Client (Rust)
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A Rust implementation of the Smol Mail client protocol, compatible with version 1.2 (including RNS transport). Fumi is the client counterpart to [bunshin](../bunshin), the Smol Mail server implementation.
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## Overview
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Smol Mail is a minimalist, end-to-end encrypted mail protocol. This client implementation supports:
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- **Full protocol v1.1**: All operations over TCP with Noise_NX_25519_ChaChaPoly_SHA256
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- **Full protocol v1.2**: Reticulum Network Stack (RNS) transport for mesh networking
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- **Shared codebase**: Single binary supports both transports (via feature flags)
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- **Interoperability**: Works with reference Python implementations and bunshin server
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## Protocol Compatibility
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| Feature | v1.1 (TCP) | v1.2 (RNS) |
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|---------|------------|------------|
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| AUTH | ✅ | ✅ |
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| RESOLVE | ✅ | ✅ |
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| SEND | ✅ | ✅ |
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| FETCH | ✅ | ✅ |
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| DELETE | ✅ | ✅ |
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| REGISTER | ✅ | ✅ |
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| Accept Tokens | ✅ | ✅ |
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| Key Rotation | ✅ | ✅ |
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| Message Encryption | ✅ | ✅ |
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## Address Formats
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### TCP Transport (v1.1)
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```
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smol://alice@example.com[:1961]
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smol://alice@example.com/abcdefghijklmnopqrstuvwxyz234567 # self-certifying
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```
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### RNS Transport (v1.2)
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```
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smol+rns://alice@8f2c1d0a7b6e5f4c3d2a1b0e9f8c7d6a
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smol+rns://alice@8f2c1d0a7b6e5f4c3d2a1b0e9f8c7d6a/abcdefghijklmnopqrstuvwxyz234567
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```
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The RNS address uses the server's 32-character hex destination hash as the host.
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---
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## Architecture
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```
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┌─────────────────────────────────────────────────────────────┐
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│ fumi client │
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├─────────────────────────────────────────────────────────────┤
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│ │
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│ ┌─────────────────────────────────────────────────────────┐ │
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│ │ CLI (main.rs) │ │
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│ │ keygen, import, trust, register, send, │ │
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│ │ fetch, delete, resolve, show │ │
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│ └──────────────────────────────┬──────────────────────────┘ │
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│ │ │
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│ ┌──────────────────────────────▼──────────────────────────┐ │
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│ │ Client Core (client.rs) │ │
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│ │ - Session management with servers │ │
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│ │ - Message composition and parsing │ │
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│ │ - Envelope sealing/unsealing │ │
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│ └──────────────────────────────┬──────────────────────────┘ │
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│ │ │
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│ ┌──────────────────────────────▼──────────────────────────┐ │
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│ │ Account (account.rs) │ │
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│ │ - Master secret management │ │
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│ │ - Identity derivation (rotation chains) │ │
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│ │ - Token generation │ │
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│ └──────────────────────────────┬──────────────────────────┘ │
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│ │ │
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│ ┌─────────────────┐ ┌─────────────────────────────┐ │ │
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│ │ TCP Transport │ │ RNS Transport │ │ │
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│ │ (tcp_transport) │ │ (rns_transport + FFI) │ │ │
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│ └────────┬────────┘ └──────────┬───────────────────┘ │ │
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│ │ │ │
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│ ▼ ▼ │
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│ ┌────────────────────────────────────────────────────────┐ │
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│ │ Transport Trait (transport.rs) │ │
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│ │ - connect(address) : Establish session │ │
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│ │ - request(op, body) : Send frame, get response │ │
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│ │ - bind_values() : Get auth binding values │ │
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│ │ - close() : Clean up connection │ │
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│ └────────────────────────────────────────────────────────┘ │
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│ │
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│ ┌─────────────────────────────────────────────────────────┐ │
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│ │ Store (store.rs) │ │
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│ │ - SQLite local mailbox │ │
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│ │ - Accounts, identities, tokens, messages │ │
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│ └─────────────────────────────────────────────────────────┘ │
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└─────────────────────────────────────────────────────────────┘
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```
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---
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## Implementation Plan
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### Phase 1: Core Client (TCP Only)
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#### Dependencies
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```toml
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[package]
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name = "fumi"
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version = "0.1.0"
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edition = "2021"
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description = "Smol Mail client"
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[dependencies]
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snow = "0.9" # Noise protocol
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ed25519-dalek = "2" # Ed25519 signatures
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x25519-dalek = "2" # X25519 key agreement
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sha2 = "0.10" # SHA-256
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rand_core = "0.6" # Randomness
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hmac = "0.12" # HMAC-SHA256
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clap = "4" # CLI parsing
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log = "0.4" # Logging
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env_logger = "0.11" # Logger setup
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anyhow = "1" # Error handling
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rusqlite = "0.32" # SQLite local store
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hex = "0.4" # Hex encoding
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data-encoding = "2" # Base32 encoding
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```
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#### Modules
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1. **crypto.rs** - Cryptographic primitives
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- Base32 encoding/decoding
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- HKDF-SHA256
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- HMAC-SHA256
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- Identity derivation
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- Accept token generation
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- Message ID computation
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2. **address.rs** - Address parsing
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- Parse TCP addresses: `smol://user@host[:port][/identity]`
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- Parse RNS addresses: `smol+rns://user@desthash[/identity]`
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- Validate usernames
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- Generate self-certifying URIs
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3. **account.rs** - Account management
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- Master secret storage
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- Identity key derivation with rotation
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- Current identity tracking
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- Token generation for correspondents
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4. **transport.rs** - Transport trait
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- Define `Transport` trait with connect/request/close
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- Define `TransportBindValues` for auth bindings
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- Define operation and status constants
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5. **tcp_transport.rs** - TCP transport
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- Noise_NX handshake
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- Frame reading/writing
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- Server key pinning (optional)
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6. **client.rs** - Core client logic
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- Session management
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- REGISTER operation
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- AUTH operation
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- RESOLVE operation
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- SEND operation (envelope creation)
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- FETCH operation
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- DELETE operation
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- Message sealing/unsealing
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7. **store.rs** - Local SQLite store
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- Account storage
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- Trusted server keys
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- Accept tokens
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- Outbound message queue
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- Inbound message storage
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8. **main.rs** - CLI
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- keygen: Generate new identity
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- import: Import existing identity
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- accounts: List accounts
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- trust: Trust server key (TCP)
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- register: Register username
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- send: Send message
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- fetch: Fetch messages
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- delete: Delete messages
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- resolve: Resolve username
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- show: Display message info
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---
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### Phase 2: RNS Transport
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Same approach as bunshin's RNS integration:
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1. **FFI with microReticulum** (recommended for production)
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- Use C++ microReticulum via FFI bindings
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- Official implementation, full compliance
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- Complex but most reliable
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2. **Python IPC** (quick validation)
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- Call `smolmail_rns.py` as subprocess
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- Share data via files or pipes
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- Fast to implement
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3. **Native Rust** (future)
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- Wait for mature Rust RNS crate
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- Or implement minimal subset
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---
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## Key Differences from Server (bunshin)
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| Aspect | bunshin (server) | fumi (client) |
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|--------|------------------|---------------|
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| Noise role | Responder (has static key) | Initiator (no static key) |
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| Session | Accepts connections | Initiates connections |
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| Trust | Server key pinned by client | Client pins server key |
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| Auth | Verifies client signatures | Signs requests |
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| Rate limiting | Enforces limits | Respects limits |
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---
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## CLI Commands
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```bash
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# Identity management
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fumi keygen # Generate new identity
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fumi keygen --name alice # Generate with name
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fumi import --name alice -m ABCD... # Import master secret
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fumi accounts # List accounts
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# Server trust (TCP only)
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fumi trust smol://alice@example.com ABCD... # Trust server key
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# Account registration
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fumi register smol://alice@example.com --username alice
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fumi register smol://alice@example.com --username alice --invite TOK...
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# Messaging
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fumi send smol://bob@example.com --subject "Hello" --body "World"
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fumi send smol://bob@example.com --file message.txt
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fumi fetch smol://alice@example.com
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fumi delete smol://alice@example.com --ids ABCD...
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fumi resolve smol://alice@example.com --username bob
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fumi show ABCD... # Show message details
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```
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---
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## Project Structure
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```
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fumi/
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├── Cargo.toml # Project configuration
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├── Cargo.lock
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├── src/
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│ ├── main.rs # CLI entry point
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│ ├── lib.rs # Library exports
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│ ├── client.rs # Core client logic
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│ ├── account.rs # Account/identity management
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│ ├── address.rs # Address parsing
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│ ├── store.rs # Local SQLite store
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│ ├── crypto.rs # Cryptographic primitives
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│ ├── transport.rs # Transport trait
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│ ├── tcp_transport.rs # TCP transport implementation
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│ └── error.rs # Error types
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├── tests/
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│ └── integration/ # Integration tests
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├── flake.nix # Nix flake (optional)
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└── README.md # This file
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```
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---
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## Implementation Priority
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### Phase 1: TCP Client (High Priority)
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| # | Module | Description | Dependencies |
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| 1 | crypto.rs | Base32, HKDF, HMAC, identity derivation | sha2, hmac, data-encoding |
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| 2 | address.rs | Address parsing and validation | regex, anyhow |
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| 3 | account.rs | Account and identity management | ed25519-dalek, rand_core |
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| 4 | transport.rs | Transport trait and types | None |
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| 5 | tcp_transport.rs | TCP/Noise transport | snow, std::net |
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| 6 | client.rs | Core operations (REGISTER, AUTH, etc.) | All above |
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| 7 | store.rs | SQLite local storage | rusqlite |
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| 8 | main.rs | CLI | clap, all above |
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| 9 | Tests | Unit and integration tests | All above |
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### Phase 2: RNS Support (Medium Priority)
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| # | Module | Description | Dependencies |
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|---|--------|-------------|--------------|
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| 1 | rns/ffi.rs | FFI bindings to microReticulum | libffi |
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| 2 | rns/transport.rs | RNS transport implementation | ffi.rs |
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| 3 | rns/mod.rs | RNS module exports | transport.rs |
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| 4 | Update transport.rs | Add RNS feature flag | None |
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| 5 | Update client.rs | Handle RNS-specific logic | rns module |
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| 6 | Update main.rs | Add RNS CLI commands | rns module |
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| 7 | Tests | RNS integration tests | All above |
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---
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## Testing Strategy
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### Unit Tests
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- Address parsing (TCP and RNS)
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- Identity generation and rotation
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- Message sealing and unsealing
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- Envelope format validation
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- Token generation and verification
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### Integration Tests
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- Register with bunshin server (TCP)
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- Send and receive messages (TCP)
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- Key rotation interoperability
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- Accept token usage
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- Error response handling
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### Interoperability Tests
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- Against `smolmail.py` reference client
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- Against `smolmaild.py` reference server
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- Against `smolmail_rns.py` reference client
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- Against `smolmaild_rns.py` reference server
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---
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## Compatibility Matrix
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| Client \ Server | bunshin (TCP) | bunshin (RNS) | smolmaild.py | smolmaild_rns.py |
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|------------------|---------------|---------------|---------------|------------------|
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| fumi (TCP) | ✅ Full | ❌ No | ✅ Full | ❌ No |
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| fumi (RNS) | ❌ No | ✅ Full | ❌ No | ✅ Full |
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Both transports share the same account and store, so a user can use both transports with the same identity.
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---
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## Key Implementation Notes
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### Message Sealing (SPEC.md S5)
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1. Generate ephemeral X25519 keypair
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2. Convert recipient Ed25519 public key to X25519 (per SPEC.md S2)
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3. Perform X25519 key agreement
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4. Derive two 32-byte keys using HKDF-SHA256:
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- `k1 = HKDF(shared, "", "smolmail/1 msg", 64)[0:32]`
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- `k2 = HKDF(shared, "", "smolmail/1 msg", 64)[32:64]`
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5. Build payload with frontmatter (unencrypted): version, sender, timestamp
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6. Build body with: version, timestamp, body_length, body (padded to 1024-byte boundary)
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7. Encrypt body with ChaCha20-Poly1305 using k1
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8. Build envelope: magic ("SMOL"), version, recipient, ephemeral_pub, payload
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### Transport Differences
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#### TCP (v1.1)
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- Noise_NX handshake (client is initiator, server has static key)
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- Frames: u32 length || u8 op || body (split across Noise messages)
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- Binding values: Noise handshake hash and server static public key
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- Server pinning: Client verifies server's Noise static key
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#### RNS (v1.2)
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- Reticulum link establishment
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- Frames: u8 op || body (RNS delimits messages)
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- Binding values: SHA-256("smolmail/1 bind" || dest_hash || link_id) and SHA-256("smolmail/1 bind" || dest_hash)
|
||
|
|
- Server pinning: Destination hash IS the server's identity (no separate pinning)
|
||
|
|
|
||
|
|
---
|
||
|
|
|
||
|
|
## Next Steps
|
||
|
|
|
||
|
|
1. **Implement Phase 1 (TCP Client)**
|
||
|
|
- Start with crypto, address, and account modules
|
||
|
|
- Then transport and tcp_transport
|
||
|
|
- Then client core operations
|
||
|
|
- Finally store and CLI
|
||
|
|
- Test against bunshin server
|
||
|
|
|
||
|
|
2. **Implement Phase 2 (RNS Support)**
|
||
|
|
- Research microReticulum C API
|
||
|
|
- Create FFI bindings
|
||
|
|
- Implement RNS transport
|
||
|
|
- Add tests with reference implementations
|
||
|
|
|
||
|
|
3. **Polish and Package**
|
||
|
|
- Documentation
|
||
|
|
- Nix flake integration (optional)
|
||
|
|
- Performance optimization
|
||
|
|
- Error handling improvements
|
||
|
|
|
||
|
|
---
|
||
|
|
|
||
|
|
## References
|
||
|
|
|
||
|
|
- [Smol Mail SPEC.md](https://code.randogoth.com/randogoth/smolmail/src/branch/main/SPEC.md)
|
||
|
|
- [Smol Mail RNS.md (v1.2)](https://code.randogoth.com/randogoth/smolmail/src/branch/main/RNS.md)
|
||
|
|
- [smolmail.py - Reference TCP client](https://code.randogoth.com/randogoth/smolmail/src/branch/main/smolmail.py)
|
||
|
|
- [smolmail_rns.py - Reference RNS client](https://code.randogoth.com/randogoth/smolmail/src/branch/main/smolmail_rns.py)
|
||
|
|
- [bunshin - Rust server implementation](../bunshin)
|
||
|
|
- [microReticulum - C++ RNS implementation](https://github.com/attermann/microReticulum)
|