Smol Mail

A minimalist, decentralized, end-to-end encrypted mail protocol.

Email should be simple again. Inspired by Spartan, Misfin, and LXMF. Smol Mail reduces electronic correspondence to its essentials: an address, a key and a message. One keypair per user, five operations, one binary message format. No sprawling infrastructure, no proprietary accounts, no advertising, no tracking. Anyone can run a server, anyone can write a client, and only the intended recipient can read a message.

A server learns which mailbox a message is for, how big it is and when it arrived. Not the sender, not the subject, not the content.

• Minimal — five operations, four primitives, no extensibility mechanisms.
• Private — messages are sealed and signed; senders are absent from the wire format.
• Decentralized — independent servers, no federation, no directories.
• Portable — identity is a keypair, not an account.

Identity

The only secret is a 32-byte master. Back that up and nothing else: the Ed25519 signing key, the encryption key derived from it, and the key that issues accept tokens all come out of it. One thing to hold, move between servers, print on paper or scan from a screen.

Signatures prove authorship. A throwaway key per message means a stolen identity key cannot decrypt anything already sent.

Rotating a key advances an index rather than inventing an unrelated key, so the master alone can re-derive every key you have ever used — which is what makes mail addressed to a superseded key readable years later, with nothing archived. restore rebuilds a client from the master and a username.

Addressing

alice@example.org                                     short form, resolved via the server
smol://alice@example.org/mfrggzdfmztwq2lknnwg23tpo…   self-certifying, carries its own key

The short form is typeable. The long form carries the key itself, so an address shared by QR code, contact file or link needs no trust in any server at all. Either way, changing servers never changes an identity.

Keys learned from a server are pinned on first use. Later changes need a rotation certificate signed by both the old and the new key, or explicit confirmation.

Recipients learn only the sender's key, never a name: a first contact arrives as a bare fingerprint until its address is known. A client may carry its full smol:// address in a signed Reply-To field so first contacts can be answered; receivers bind it only when its key matches the message's signer (SPEC.md §5.7).

A mailbox nobody else can fill

A server cannot see senders, so it cannot tell wanted mail from a flood, and without help anyone who knows an address could fill a mailbox to its quota.

An accept token fixes that without telling the server anything. It is a 32-byte secret you derive per correspondent, hand to them inside a sealed reply, and upload to your own server. Their messages carry a MAC over it; your server matches the MAC and puts those messages in your mailbox proper. Everything else — a first contact, a stranger, a flood — goes to a small requests tier with a short retention, which your client shows separately. Accepting someone is one command, or just replying to them; withdrawing the token puts them back in requests.

Your server learns from this how many correspondents you have accepted and which token a message matched, so a token is a stable pseudonym. It still never learns who is behind one (SPEC.md §5.8, §9).

Cryptography

Ed25519 · X25519 · ChaCha20-Poly1305 · SHA-256. Four established primitives, no novel cryptography, nothing else anywhere in the protocol. Transport is TCP with a Noise handshake — no certificates, no CA, no expiry.

What is not protected

Traffic analysis, delivery timing, mailbox size, and whether a user has an account on a given server. Run a server as a Tor onion service for metadata resistance; the transport needs no changes for it.

There is no forward secrecy on the recipient side: a seized recipient key decrypts ciphertext recorded while it was valid. Messages are signed, so they carry non-repudiation rather than deniability.

Version 1.1 excludes attachments, group messaging, federation, anonymous routing, multi-device synchronisation and key revocation.

Reference server

smolmaild.py is a complete server in one file. It declares its own dependencies inline, so there is nothing to install:

uv run smolmaild.py keygen
uv run smolmaild.py serve

keygen writes the server's static key to server.key and prints its public key in base32. Publish that public key through a trusted channel — clients pin it, and a mismatch aborts the handshake.

serve listens on 127.0.0.1:1961 by default and stores mail in mail.db. Use --host 0.0.0.0 to accept remote connections, and --invite-token to close registration. --help lists the size, quota, retention and rate limits, including the separate --requests-quota for mail that arrives without an accept token.

Reference client

smolmail.py is a complete client in one file, with the same inline dependencies:

uv run smolmail.py keygen
uv run smolmail.py trust example.org <server-key>
uv run smolmail.py register alice@example.org
uv run smolmail.py resolve bob@example.org
echo "hello" | uv run smolmail.py send bob@example.org --subject Hi
uv run smolmail.py fetch
uv run smolmail.py list
uv run smolmail.py read <id>

Sent mail carries the sender's full smol:// address in a signed Reply-To field (SPEC.md §5.7), so a first-time recipient can answer without an out-of-band channel; pass --anonymous to omit it.

accept admits a contact to your mailbox proper and pushes the token to your server at once; block withdraws it; list --requests shows what arrived without one. import adds a contact from a smol:// address without trusting any server, contacts lists known keys and how each was learned, and rotate advances the identity with a certificate signed by both keys, which your contacts accept automatically.

fetch deletes what it has verified and stored. fetch --keep leaves mail on the server and remembers where it stopped, and --reset pages through it again.

Mail is stored sealed and opened on demand, so the local database holds no plaintext.

Specification

SPEC.md — wire format, operations and trust model.

License

The protocol definition — SPEC.md — is licensed under the Creative Commons Attribution-ShareAlike 4.0 International license (CC BY-SA 4.0).

The reference code — smolmail.py, smolmaild.py, smolmail_rns.py and smolmaild_rns.py — is licensed under the Apache License 2.0 (Apache-2.0).