文 fumi

A Rust client for Smol Mail: a minimalist, end-to-end encrypted mail protocol over a Noise-secured TCP connection. fumi derives an identity from one 32-byte master secret, seals and opens mail, and talks to a mailbox; it is the counterpart to bunshin, the server.

Status

Both carriers are implemented: the TCP carrier (SPEC.md 1.1) and the Reticulum carrier of RNS.md (1.2), the latter behind the rns cargo feature (nix build .#rns). fumi interoperates in both directions with the reference stack — smolmail.py/smolmaild.py over TCP and smolmail_rns.py/smolmaild_rns.py over Reticulum — and with bunshin over both carriers, including the cross-transport case upstream section 15 promises: send over TCP, fetch over the mesh, one mailbox.

Addressing

alice@example.org[:1961]                                        short form, resolved via the server
smol://alice@example.org/mfrggzdfzt...                          self-certifying, 52-char base32 identity
smol+rns://alice@8f2c1d0a7b6e5f4c3d2a1b0e9f8c7d6a               short form over Reticulum (rns feature)
smol+rns://alice@8f2c1d0a7b6e5f4c3d2a1b0e9f8c7d6a/mfrggzdfzt... self-certifying over Reticulum

Usage

One master per store, selected by the global --key and --db flags; a second identity is a second pair of files.

fumi [--key identity.key] [--db fumi.db] [--timeout 30] <command>

# identity
fumi keygen [--force]                      create a master secret
fumi whoami                                address, public key, fingerprint, rotation index
fumi restore <address>                     recover local state from the master alone
fumi rotate                                advance the rotation index, sign and push the certificate

# servers and contacts
fumi trust <host> <key-b32> [--force]      pin a server's Noise static key
fumi register <address> [--invite TOKEN]   bind this identity to a username
fumi resolve <address>                     look up a contact's key, walk the chain, pin it
fumi import <smol-uri>                     add a contact from a self-certifying address
fumi contacts                              known keys and how each was learned

# accept tokens
fumi accept <address>                     issue a token, admit to the main tier, sync the set
fumi block <address>                       withdraw it, sync the set

# mail
fumi send <address> [--subject S] [--body TEXT | --file F | -] [--header K:V] [--reply-to ID] [--anonymous] [--no-pad]
fumi fetch [--keep] [--reset]             retrieve, verify, store, acknowledge
fumi delete <id>...                       remove from the server explicitly
fumi list [--sent] [--requests]
fumi read <id> [--sent]

# RNS carrier (built with --features rns / nix build .#rns); addresses select
# it by scheme, so these flags apply only to smol+rns:// dials:
fumi --rns-udp <host:port> --rns-udp-forward <host:port> [--rns-storage DIR] <command>

On the mesh the destination hash is the pin (RNS.md 13.4): there is no trust, no static key, and a client creates no Reticulum identity (13.8). The --rns-udp-forward target is effectively required for a client, which speaks first; it should point at an rnsd or at the server's UDP interface. Requests carry an explicit 30-second timeout (13.5), links are torn down when a session ends (13.10), and a resend is safe because ids are derived.

Mail is stored sealed and opened on demand; there is no plaintext at rest. A first contact is learned by trust on first use and marked unverified when the server's key was not pinned; a key change is accepted only when a signed rotation chain leads from the key held to the one offered, and is surfaced rather than applied silently.

Build

nix build              # or: nix develop -c cargo build
nix develop -c cargo test --workspace

The crate is a workspace: core/ is the fumi-core library — everything but the command line — and cli/ is the fumi binary. GUI hosts depend on fumi-core alone; it pulls no argument parsing, no logging globals, and no keyfile conventions. bundled-sqlite (default) vendors libsqlite3; embedders that link their own SQLite, as Android's NDK does, build with default-features = false.

bunshin and the reference clients make a complete test rig: register against a local server, exchange mail in both directions, then rotate and fetch the mail that the superseded key still receives.

Embedding

fumi-core is the protocol core of a GUI client, not just this CLI's engine. The contract an embedder can rely on:

• Secrets stay bytes in the API, files stay in the CLI. Account::new(master, index) takes the master as bytes and Store::open(path) takes only mail and state; identity.key next to fumi.db is a CLI convention. A GUI keeps the master wherever its platform wants it (Android Keystore, OS keychain) and hands it over per operation.
• One shareable store handle. Store is Send + Sync: every operation locks an inner mutex, and the database runs in WAL mode, so a host holds one Store behind an executor and reads it while a fetch writes. Each envelope is committed as it is verified — a concurrent reader listing the inbox sees fetch progress without any callback.
• Decisions, not prose. error::Error is an enum: NotPinned, PinMismatch, KeyChanged { address, known, offered }, NotRegistered, AuthFailed, RateLimited, QuotaExceeded, SchemaVersion, ... — the distinctions a UI routes on. Display still produces the message the CLI prints.
• Long operations can stop. fetch_with takes a FetchOptions with a cancellation token checked between pages; the cursor is saved per page, so a cancelled fetch resumes rather than repeats.
• Composable flows. restore remains the one-call convenience, but its pieces are public: connect + resolve, account::find_rotation_index, then the Store setters — a wizard can hold the master and the address before it has connectivity, and show each step. Trust outcomes arrive as values: TrustChange from trust_key and send, Session::unpinned_static for the sec 4 warning.
• A versioned store. The schema carries a user_version; a store written by a newer build is refused (Error::SchemaVersion) rather than misread, and within a version the schema is stable. The host, not the library, decides when to upgrade the binary.
• Binding from other languages. The API is Send-friendly and free of CLI globals, so plain Rust bindings (flutter_rust_bridge, uniffi) work against it. core/vectors.json holds the reference vectors the unit tests pin — derivations, seals, a full envelope — so a port or FFI binding can verify itself end to end without the reference stack.

The intended call graph: Store::open once, Account::new(master, rotations) per session, then client::* operations taking &Store and &Account. The store outlives accounts; nothing in the library caches between calls.

Modules

File	Contents
core/src/crypto.rs	base32, HKDF, HMAC, SHA-256, the Ed25519→X25519 map with SPEC.md §2's checks
core/src/address.rs	parsing for all four address forms, username validation, fingerprints
core/src/account.rs	master, seed_n derivation, accept-key and tokens, rotation certificates, find_rotation_index
core/src/message.rs	envelope seal/open (§5.1–§5.3), message id (§5.4), frontmatter (§5.5)
core/src/transport.rs	Transport trait, bind values, operation and status constants
core/src/tcp.rs	Noise_NX initiator, len u32 || op u8 || body framing, static-key pinning
core/src/rns/	the Reticulum carrier behind the rns feature: FFI over core/shim/, path discovery, one link per session
core/src/client.rs	the six operations, AUTH, chain walking, token sync, fetch pipeline with cancellation
core/src/store.rs	SQLite: state, contacts, accepted, tokens, seen ids, inbox, sent; Send + Sync, WAL, versioned schema
core/src/error.rs	the Error enum embedders match on, the status-code mapping
core/vectors.json	the committed reference vectors, the tests' source of truth
cli/src/main.rs	the command line: keyfile and --db conventions, hints, output

Unit tests pin every derivation against vectors.json, whose values were generated from smolmail.py, including a full envelope reproduced byte for byte.

References

• SPEC.md — protocol 1.1
• RNS.md — the 1.2 Reticulum addendum
• bunshin — the server