smol mail

A minimalist, cypherpunk mail protocol. Fun, simple, decentralised, end-to-end encrypted.

Online correspondence reduced to an address, a key and a message.

Five operations, four primitives, zero extensibility mechanisms. One 32-byte master key as the only identity. No certificates, no CAs, no expiry: one round trip and the server's key pin are the entire trust model. A fresh key seals every message: untraceable, unforgeable, and safe by construction.

Anti-spam without censorship: quotas and tokens do the filtering, and servers never see a word. Clients talk directly to the recipient's server. Registration is bound to one server by proof-of-possession. Double-signed certificate chains move a username to a new key, and old keys keep receiving until every contact catches up.

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. That is what makes mail addressed to a superseded key readable years later, with nothing archived.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.

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.

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 defines the wire format, the operations and the trust model.

Inspiration

The smolmail project takes inspiration from Gemini Protocol, Misfin, and LXMF.

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).