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smolmail/smolmaild.py

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#!/usr/bin/env -S uv run --quiet --script
# /// script
# requires-python = ">=3.11"
# dependencies = ["noiseprotocol>=0.3.1", "cryptography>=42"]
# ///
# Copyright 2026 randogoth
# SPDX-License-Identifier: Apache-2.0
"""Smol Mail reference server.
Implements SPEC.md version 1.1: a store-and-forward mailbox reachable over TCP
with a Noise_NX handshake. The server never sees plaintext, sender identities,
or any private key belonging to a user; it stores sealed envelopes addressed to
a recipient public key and hands them back to whoever can sign for that key.
uv run smolmaild.py keygen --key server.key
uv run smolmaild.py serve --key server.key --db mail.db
"""
from __future__ import annotations
import argparse
import base64
import hashlib
import hmac
import logging
import os
import socket
import socketserver
import sqlite3
import struct
import sys
import threading
import time
from cryptography.exceptions import InvalidSignature
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
from noise.connection import Keypair, NoiseConnection
log = logging.getLogger("smolmaild")
# ---------------------------------------------------------------------------
# Protocol constants (SPEC.md §4, §6, §11, §12)
# ---------------------------------------------------------------------------
NOISE_PROTOCOL = b"Noise_NX_25519_ChaChaPoly_SHA256"
PROLOGUE = b"smolmail/1"
LABEL_AUTH = b"smolmail/1 auth"
LABEL_ID = b"smolmail/1 id"
LABEL_MAC = b"smolmail/1 mac"
LABEL_ROTATE = b"smolmail/1 rotate"
LABEL_REGISTER = b"smolmail/1 register"
OP_AUTH = 0x00
OP_RESOLVE = 0x01
OP_SEND = 0x02
OP_FETCH = 0x03
OP_DELETE = 0x04
OP_REGISTER = 0x05
OK = 0
MALFORMED = 1
BAD_VERSION = 2
UNKNOWN_USER = 3
AUTH_REQUIRED = 4
AUTH_FAILED = 5
QUOTA_EXCEEDED = 6
TOO_LARGE = 7
RATE_LIMITED = 8
NOT_PERMITTED = 9
INTERNAL_ERROR = 10
ENVELOPE_MAGIC = b"SMOL"
ENVELOPE_VERSION = 1
ENVELOPE_HEADER = 69 # magic 4 + version 1 + to 32 + epk 32
ENVELOPE_MIN = ENVELOPE_HEADER + 16 # + Poly1305 tag
ID_LEN = 32
KEY_LEN = 32
SIG_LEN = 64
TOKEN_LEN = 32 # §5.8 accept token, and the MAC derived from it
CERT_LEN = 200 # old_pub 32 + new_pub 32 + time 8 + two signatures
MAX_CHAIN = 16
SEPARATORS = "._-"
TIER_MAIN = 0
TIER_REQUESTS = 1
FLAG_REQUESTS = 0x01 # §6.1 record flags
DEFAULT_PORT = 1961
MAX_FRAME = 1 << 20 # §4 application frame ceiling
NOISE_MAX = 65535 # §4 Noise message ceiling
NOISE_PAYLOAD = NOISE_MAX - 16 # minus the AEAD tag
FETCH_BUDGET = 512 * 1024 # §6.1, must stay under MAX_FRAME
RECORD_OVERHEAD = ID_LEN + 8 + 1 + 4 # id + received_at + flags + env_len
IDLE_TIMEOUT = 120.0
PURGE_INTERVAL = 60.0
class ProtocolError(Exception):
"""A peer sent something unparseable. Always answered with MALFORMED."""
# ---------------------------------------------------------------------------
# Encoding helpers
# ---------------------------------------------------------------------------
def b32(raw: bytes) -> str:
"""RFC 4648 base32, lowercase and unpadded, as §3 requires."""
return base64.b32encode(raw).decode("ascii").rstrip("=").lower()
class Reader:
"""Fail-closed reader over a frame body.
Every parse path raises rather than reading past the end, so a truncated
frame can never be mistaken for a short but valid one.
"""
def __init__(self, buf: bytes) -> None:
self.buf = buf
self.pos = 0
def take(self, n: int) -> bytes:
if n < 0 or self.pos + n > len(self.buf):
raise ProtocolError(f"short read: want {n}, have {len(self.buf) - self.pos}")
out = self.buf[self.pos : self.pos + n]
self.pos += n
return out
def u8(self) -> int:
return self.take(1)[0]
def u16(self) -> int:
return struct.unpack(">H", self.take(2))[0]
def u32(self) -> int:
return struct.unpack(">I", self.take(4))[0]
def i64(self) -> int:
return struct.unpack(">q", self.take(8))[0]
def rest(self) -> bytes:
return self.take(len(self.buf) - self.pos)
def done(self) -> None:
if self.pos != len(self.buf):
raise ProtocolError(f"{len(self.buf) - self.pos} trailing bytes")
def valid_username(name: str) -> bool:
"""§3: 1-63 bytes of [a-z0-9._-], alphanumeric ends, no adjacent separators."""
if not 1 <= len(name) <= 63:
return False
if not all("0" <= c <= "9" or "a" <= c <= "z" or c in SEPARATORS for c in name):
return False
if name[0] in SEPARATORS or name[-1] in SEPARATORS:
return False
return not any(a in SEPARATORS and b in SEPARATORS for a, b in zip(name, name[1:]))
def message_id(envelope: bytes) -> bytes:
"""§5.4. Derived from the envelope, so a sender cannot choose it."""
return hashlib.sha256(LABEL_ID + envelope).digest()
def verify(pubkey: bytes, signature: bytes, message: bytes) -> bool:
try:
Ed25519PublicKey.from_public_bytes(pubkey).verify(signature, message)
return True
except (InvalidSignature, ValueError):
return False
# ---------------------------------------------------------------------------
# Storage
# ---------------------------------------------------------------------------
SCHEMA = """
CREATE TABLE IF NOT EXISTS users (
username TEXT PRIMARY KEY,
identity BLOB NOT NULL
);
-- Every key ever bound to a username, so a superseded key stays addressable
-- across a rotation (§7).
CREATE TABLE IF NOT EXISTS keys (
identity BLOB PRIMARY KEY,
username TEXT NOT NULL
);
CREATE TABLE IF NOT EXISTS rotations (
username TEXT NOT NULL,
seq INTEGER NOT NULL,
cert BLOB NOT NULL,
PRIMARY KEY (username, seq)
);
-- Accept tokens (§5.8): opaque secrets the mailbox owner uploads with AUTH.
-- The server can match them against a message's MAC and nothing else; it never
-- learns which correspondent a token belongs to.
CREATE TABLE IF NOT EXISTS accepted (
username TEXT NOT NULL,
token BLOB NOT NULL,
PRIMARY KEY (username, token)
);
CREATE TABLE IF NOT EXISTS messages (
id BLOB PRIMARY KEY,
recipient BLOB NOT NULL,
received_at INTEGER NOT NULL,
tier INTEGER NOT NULL,
envelope BLOB NOT NULL
);
-- (received_at, id) is FETCH's sort and cursor order (§6.1).
CREATE INDEX IF NOT EXISTS messages_by_recipient
ON messages (recipient, received_at, id);
-- Identifiers of every envelope accepted within the retention window, so an
-- envelope captured and resent after DELETE does not reappear (§10).
CREATE TABLE IF NOT EXISTS seen (
id BLOB PRIMARY KEY,
at INTEGER NOT NULL
);
"""
class Store:
"""SQLite behind a connection per thread.
ThreadingTCPServer hands each connection its own thread and sqlite3
connections are not shareable across threads, so they are kept in
thread-local state rather than guarded by a lock.
"""
def __init__(self, path: str) -> None:
self.path = path
self.local = threading.local()
with sqlite3.connect(path) as db:
db.execute("PRAGMA journal_mode=WAL")
db.executescript(SCHEMA)
@property
def db(self) -> sqlite3.Connection:
conn = getattr(self.local, "conn", None)
if conn is None:
conn = sqlite3.connect(self.path, timeout=10.0)
conn.execute("PRAGMA busy_timeout=10000")
self.local.conn = conn
return conn
def identity_of(self, username: str) -> bytes | None:
row = self.db.execute(
"SELECT identity FROM users WHERE username = ?", (username,)
).fetchone()
return row[0] if row else None
def chain(self, username: str) -> list[bytes]:
return [
row[0]
for row in self.db.execute(
"SELECT cert FROM rotations WHERE username = ? ORDER BY seq", (username,)
)
]
def keys_of(self, username: str) -> list[bytes]:
return [
row[0]
for row in self.db.execute(
"SELECT identity FROM keys WHERE username = ?", (username,)
)
]
def username_for_key(self, identity: bytes) -> str | None:
row = self.db.execute(
"SELECT username FROM keys WHERE identity = ?", (identity,)
).fetchone()
return row[0] if row else None
def register(self, username: str, identity: bytes) -> bool:
try:
with self.db:
self.db.execute(
"INSERT INTO users (username, identity) VALUES (?, ?)",
(username, identity),
)
self.db.execute(
"INSERT INTO keys (identity, username) VALUES (?, ?)",
(identity, username),
)
return True
except sqlite3.IntegrityError:
# Username taken, or this key is already bound to another username.
return False
def rotate(self, username: str, new_key: bytes, cert: bytes, seq: int) -> bool:
try:
with self.db:
self.db.execute(
"UPDATE users SET identity = ? WHERE username = ?",
(new_key, username),
)
self.db.execute(
"INSERT INTO keys (identity, username) VALUES (?, ?)",
(new_key, username),
)
self.db.execute(
"INSERT INTO rotations (username, seq, cert) VALUES (?, ?, ?)",
(username, seq, cert),
)
return True
except sqlite3.IntegrityError:
return False
def accepted_tokens(self, username: str) -> list[bytes]:
return [
row[0]
for row in self.db.execute(
"SELECT token FROM accepted WHERE username = ?", (username,)
)
]
def set_accepted(self, username: str, tokens: list[bytes]) -> int:
"""§4: an AUTH with sync = 1 replaces the whole set, which is how a
token is both added and removed."""
with self.db:
self.db.execute("DELETE FROM accepted WHERE username = ?", (username,))
self.db.executemany(
"INSERT OR IGNORE INTO accepted (username, token) VALUES (?, ?)",
[(username, token) for token in tokens],
)
return self.count_accepted(username)
def count_accepted(self, username: str) -> int:
return self.db.execute(
"SELECT COUNT(*) FROM accepted WHERE username = ?", (username,)
).fetchone()[0]
def tombstoned(self, mid: bytes) -> bool:
return (
self.db.execute("SELECT 1 FROM seen WHERE id = ?", (mid,)).fetchone()
is not None
)
def mailbox_bytes(self, keys: list[bytes], tier: int) -> int:
marks = ",".join("?" * len(keys))
row = self.db.execute(
f"SELECT COALESCE(SUM(LENGTH(envelope)), 0) FROM messages "
f"WHERE tier = ? AND recipient IN ({marks})",
[tier] + keys,
).fetchone()
return row[0]
def store_message(self, mid: bytes, recipient: bytes, tier: int, envelope: bytes) -> None:
with self.db:
now = int(time.time())
self.db.execute(
"INSERT OR IGNORE INTO messages "
"(id, recipient, received_at, tier, envelope) VALUES (?, ?, ?, ?, ?)",
(mid, recipient, now, tier, envelope),
)
self.db.execute(
"INSERT OR IGNORE INTO seen (id, at) VALUES (?, ?)", (mid, now)
)
def pending(
self, keys: list[bytes], after_time: int, after_id: bytes, budget: int
) -> list[tuple[bytes, int, int, bytes]]:
marks = ",".join("?" * len(keys))
rows = self.db.execute(
f"SELECT id, received_at, tier, envelope FROM messages "
f"WHERE recipient IN ({marks}) "
f"AND (received_at > ? OR (received_at = ? AND id > ?)) "
f"ORDER BY received_at, id",
keys + [after_time, after_time, after_id],
)
out: list[tuple[bytes, int, int, bytes]] = []
used = 0
for mid, received_at, tier, envelope in rows:
size = RECORD_OVERHEAD + len(envelope)
# Always return at least one record, even if it alone exceeds the
# budget, so an oversized envelope cannot wedge a mailbox shut.
if out and used + size > budget:
break
out.append((mid, received_at, tier, envelope))
used += size
return out
def delete(self, keys: list[bytes], ids: list[bytes]) -> int:
kmarks = ",".join("?" * len(keys))
imarks = ",".join("?" * len(ids))
with self.db:
cur = self.db.execute(
f"DELETE FROM messages WHERE id IN ({imarks}) "
f"AND recipient IN ({kmarks})",
ids + keys,
)
return cur.rowcount
def purge(self, main_cutoff: int, requests_cutoff: int, seen_cutoff: int) -> int:
with self.db:
cur = self.db.execute(
"DELETE FROM messages WHERE (tier = ? AND received_at < ?) "
"OR (tier = ? AND received_at < ?)",
(TIER_MAIN, main_cutoff, TIER_REQUESTS, requests_cutoff),
)
gone = cur.rowcount
self.db.execute("DELETE FROM seen WHERE at < ?", (seen_cutoff,))
return gone
# ---------------------------------------------------------------------------
# Rate limiting
# ---------------------------------------------------------------------------
class RateLimiter:
"""Fixed-window counter, keyed by IP address or by accept token (§10)."""
def __init__(self, limit: int, window: float = 60.0) -> None:
self.limit = limit
self.window = window
self.lock = threading.Lock()
self.hits: dict[str, tuple[float, int]] = {}
def allow(self, key: str) -> bool:
if self.limit <= 0:
return True
now = time.monotonic()
with self.lock:
start, count = self.hits.get(key, (now, 0))
if now - start >= self.window:
start, count = now, 0
if count >= self.limit:
return False
self.hits[key] = (start, count + 1)
if len(self.hits) > 4096:
self._evict(now)
return True
def _evict(self, now: float) -> None:
stale = [key for key, (start, _) in self.hits.items() if now - start >= self.window]
for key in stale:
del self.hits[key]
# ---------------------------------------------------------------------------
# Framed transport (§4)
# ---------------------------------------------------------------------------
def recv_exact(sock: socket.socket, n: int) -> bytes:
chunks = []
remaining = n
while remaining:
chunk = sock.recv(min(remaining, 65536))
if not chunk:
raise EOFError("peer closed")
chunks.append(chunk)
remaining -= len(chunk)
return b"".join(chunks)
class Channel:
"""Noise messages under a u16 length prefix, application frames on top.
The two layers are independent: an application frame is split across as
many Noise messages as it needs, and reassembled from them.
"""
def __init__(self, sock: socket.socket, noise: NoiseConnection) -> None:
self.sock = sock
self.noise = noise
self.buf = bytearray()
def _read_noise(self) -> bytes:
(length,) = struct.unpack(">H", recv_exact(self.sock, 2))
return self.noise.decrypt(recv_exact(self.sock, length))
def _write_noise(self, payload: bytes) -> None:
packet = self.noise.encrypt(payload)
self.sock.sendall(struct.pack(">H", len(packet)) + packet)
def read_frame(self) -> tuple[int, bytes]:
while len(self.buf) < 5:
self.buf += self._read_noise()
(length,) = struct.unpack(">I", self.buf[:4])
if length < 1 or length > MAX_FRAME:
raise ProtocolError(f"frame length {length} out of range")
while len(self.buf) < 4 + length:
self.buf += self._read_noise()
frame = bytes(self.buf[4 : 4 + length])
del self.buf[: 4 + length]
return frame[0], frame[1:]
def write_frame(self, op: int, body: bytes) -> None:
frame = struct.pack(">I", 1 + len(body)) + bytes([op]) + body
for off in range(0, len(frame), NOISE_PAYLOAD):
self._write_noise(frame[off : off + NOISE_PAYLOAD])
def handshake(sock: socket.socket, static_key: bytes) -> NoiseConnection:
"""Noise_NX responder. The initiator stays anonymous; only we hold a static."""
noise = NoiseConnection.from_name(NOISE_PROTOCOL)
noise.set_as_responder()
noise.set_prologue(PROLOGUE)
noise.set_keypair_from_private_bytes(Keypair.STATIC, static_key)
noise.start_handshake()
(length,) = struct.unpack(">H", recv_exact(sock, 2))
noise.read_message(recv_exact(sock, length))
reply = noise.write_message()
sock.sendall(struct.pack(">H", len(reply)) + reply)
if not noise.handshake_finished:
raise ProtocolError("handshake did not complete")
return noise
# ---------------------------------------------------------------------------
# Operations (§6)
# ---------------------------------------------------------------------------
class Session:
"""Per-connection state: the authenticated username, if any."""
def __init__(self, server: "MailServer", peer_ip: str, handshake_hash: bytes) -> None:
self.server = server
self.peer_ip = peer_ip
self.handshake_hash = handshake_hash
self.username: str | None = None
@property
def store(self) -> Store:
return self.server.store
def dispatch(self, op: int, body: bytes) -> tuple[int, bytes]:
handlers = {
OP_AUTH: self.op_auth,
OP_RESOLVE: self.op_resolve,
OP_SEND: self.op_send,
OP_FETCH: self.op_fetch,
OP_DELETE: self.op_delete,
OP_REGISTER: self.op_register,
}
handler = handlers.get(op)
if handler is None:
return MALFORMED, b""
if op in (OP_FETCH, OP_DELETE) and self.username is None:
return AUTH_REQUIRED, b""
return handler(Reader(body))
def op_auth(self, r: Reader) -> tuple[int, bytes]:
username = r.take(r.u8()).decode("utf-8", "strict")
identity = r.take(KEY_LEN)
signature = r.take(SIG_LEN)
sync = r.u8()
tokens = [r.take(TOKEN_LEN) for _ in range(r.u16())]
r.done()
bound = self.store.identity_of(username)
# A wrong username and a wrong signature are both AUTH_FAILED: telling
# them apart would turn this into an account-existence oracle.
if bound is None or bound != identity:
return AUTH_FAILED, b""
if not verify(identity, signature, LABEL_AUTH + self.handshake_hash):
return AUTH_FAILED, b""
if sync not in (0, 1) or (sync == 0 and tokens):
return MALFORMED, b""
# Refused before authenticating, so the client has to trim and retry
# rather than silently running with a truncated set (§4).
if len(tokens) > self.server.max_accepted:
return TOO_LARGE, b""
self.username = username
held = self.store.set_accepted(username, tokens) if sync else \
self.store.count_accepted(username)
return OK, struct.pack(">H", held)
def op_resolve(self, r: Reader) -> tuple[int, bytes]:
username = r.take(r.u8()).decode("utf-8", "strict")
r.done()
identity = self.store.identity_of(username)
if identity is None:
return UNKNOWN_USER, b""
chain = self.store.chain(username)
return OK, identity + bytes([len(chain)]) + b"".join(chain)
def match_token(self, username: str, mid: bytes, mac: bytes) -> bytes | None:
"""§5.8. Trial-match the MAC against the mailbox's tokens.
Bounded by --max-accepted, so an unmatchable MAC costs a known amount
of HMAC rather than an unbounded scan.
"""
if len(mac) != TOKEN_LEN:
return None
for token in self.store.accepted_tokens(username):
want = hmac.new(token, LABEL_MAC + mid, hashlib.sha256).digest()
if hmac.compare_digest(want, mac):
return token
return None
def op_send(self, r: Reader) -> tuple[int, bytes]:
mac = r.take(r.u8())
envelope = r.rest()
if not self.server.send_limiter.allow(self.peer_ip):
return RATE_LIMITED, b""
if len(envelope) > self.server.max_envelope:
return TOO_LARGE, b""
if len(envelope) < ENVELOPE_MIN or envelope[:4] != ENVELOPE_MAGIC:
return MALFORMED, b""
if envelope[4] != ENVELOPE_VERSION:
return BAD_VERSION, b""
recipient = envelope[5:37]
username = self.store.username_for_key(recipient)
if username is None:
return UNKNOWN_USER, b""
mid = message_id(envelope)
# A resend is answered with the same id and stores nothing, whether the
# original is still here or was deleted (§10).
if self.store.tombstoned(mid):
return OK, mid
# An unmatched MAC is not an error: SEND must not reveal whether a
# token is still accepted (§5.8).
token = self.match_token(username, mid, mac)
if token is None:
tier, quota = TIER_REQUESTS, self.server.requests_quota
else:
tier, quota = TIER_MAIN, self.server.quota
if not self.server.token_limiter.allow(token.hex()):
return RATE_LIMITED, b""
keys = self.store.keys_of(username)
if self.store.mailbox_bytes(keys, tier) + len(envelope) > quota:
return QUOTA_EXCEEDED, b""
# The ciphertext is never inspected; the server cannot read it.
self.store.store_message(mid, recipient, tier, envelope)
return OK, mid
def op_fetch(self, r: Reader) -> tuple[int, bytes]:
after_time = r.i64()
after_id = r.take(ID_LEN)
r.done()
assert self.username is not None
keys = self.store.keys_of(self.username)
records = self.store.pending(keys, after_time, after_id, FETCH_BUDGET)
out = [struct.pack(">H", len(records))]
for mid, received_at, tier, envelope in records:
flags = FLAG_REQUESTS if tier == TIER_REQUESTS else 0
out.append(
mid + struct.pack(">qBI", received_at, flags, len(envelope)) + envelope
)
return OK, b"".join(out)
def op_delete(self, r: Reader) -> tuple[int, bytes]:
count = r.u16()
ids = [r.take(ID_LEN) for _ in range(count)]
r.done()
assert self.username is not None
if not ids:
return OK, struct.pack(">H", 0)
# Scoped to the caller's own keys, so ids cannot be used to probe or
# delete another mailbox.
keys = self.store.keys_of(self.username)
removed = self.store.delete(keys, ids)
return OK, struct.pack(">H", removed)
def op_register(self, r: Reader) -> tuple[int, bytes]:
username = r.take(r.u8()).decode("utf-8", "strict")
identity = r.take(KEY_LEN)
signature = r.take(SIG_LEN)
token = r.take(r.u8())
cert = r.take(r.u8())
r.done()
if not valid_username(username):
return MALFORMED, b""
try:
Ed25519PublicKey.from_public_bytes(identity)
except ValueError:
return MALFORMED, b""
# §6.1 proof of possession, bound to this server's static key so the
# attestation cannot be replayed elsewhere.
if not verify(
identity,
signature,
LABEL_REGISTER + self.server.static_pub + username.encode() + identity,
):
return AUTH_FAILED, b""
expected = self.server.invite_token
if expected is not None and token != expected:
return NOT_PERMITTED, b""
if not cert:
if not self.store.register(username, identity):
return NOT_PERMITTED, b""
return OK, b""
if len(cert) != CERT_LEN:
return MALFORMED, b""
old_pub, new_pub, when = cert[:32], cert[32:64], cert[64:72]
sig_old, sig_new = cert[72:136], cert[136:200]
if new_pub != identity:
return MALFORMED, b""
bound = self.store.identity_of(username)
if bound is None:
return UNKNOWN_USER, b""
if bound != old_pub:
# Only the currently bound key may hand the username on (§7).
return NOT_PERMITTED, b""
# Both keys sign: the old one alone could otherwise rotate to a key
# nobody controls (§7).
signed = LABEL_ROTATE + username.encode() + old_pub + new_pub + when
if not verify(old_pub, sig_old, signed) or not verify(new_pub, sig_new, signed):
return AUTH_FAILED, b""
chain = self.store.chain(username)
if len(chain) >= MAX_CHAIN:
return NOT_PERMITTED, b""
if not self.store.rotate(username, new_pub, cert, len(chain)):
return NOT_PERMITTED, b""
return OK, b""
# ---------------------------------------------------------------------------
# Server
# ---------------------------------------------------------------------------
class Handler(socketserver.BaseRequestHandler):
server: "MailServer"
def handle(self) -> None:
sock: socket.socket = self.request
peer_ip = self.client_address[0]
if not self.server.conn_limiter.allow(peer_ip):
log.warning("rate limited %s", peer_ip)
return
sock.settimeout(IDLE_TIMEOUT)
try:
noise = handshake(sock, self.server.static_key)
except Exception as exc:
log.info("handshake failed from %s: %s", peer_ip, exc)
return
session = Session(self.server, peer_ip, noise.get_handshake_hash())
channel = Channel(sock, noise)
while True:
try:
op, body = channel.read_frame()
except (EOFError, OSError):
return
except ProtocolError as exc:
log.info("bad frame from %s: %s", peer_ip, exc)
self._try_send(channel, OP_AUTH, MALFORMED)
return
try:
status, payload = session.dispatch(op, body)
except (ProtocolError, UnicodeDecodeError) as exc:
log.info("bad body from %s: %s", peer_ip, exc)
self._try_send(channel, op, MALFORMED)
return
except Exception:
log.exception("handler error from %s", peer_ip)
self._try_send(channel, op, INTERNAL_ERROR)
return
try:
channel.write_frame(op, bytes([status]) + payload)
except OSError:
return
def _try_send(self, channel: Channel, op: int, status: int) -> None:
try:
channel.write_frame(op, bytes([status]))
except OSError:
pass
def static_public(static_key: bytes) -> bytes:
return (
X25519PrivateKey.from_private_bytes(static_key)
.public_key()
.public_bytes(serialization.Encoding.Raw, serialization.PublicFormat.Raw)
)
class MailServer(socketserver.ThreadingTCPServer):
allow_reuse_address = True
daemon_threads = True
def __init__(self, address, args: argparse.Namespace, static_key: bytes) -> None:
super().__init__(address, Handler)
self.static_key = static_key
self.static_pub = static_public(static_key)
self.store = Store(args.db)
self.max_envelope = args.max_envelope
self.quota = args.quota
self.requests_quota = args.requests_quota
self.retention = args.retention_days * 86400
self.requests_retention = args.requests_retention_days * 86400
self.max_accepted = args.max_accepted
self.invite_token = args.invite_token.encode() if args.invite_token else None
self.conn_limiter = RateLimiter(args.rate_connections)
self.send_limiter = RateLimiter(args.rate_sends)
self.token_limiter = RateLimiter(args.rate_token)
threading.Thread(target=self._purge_loop, daemon=True).start()
def _purge_loop(self) -> None:
store = Store(self.store.path)
while True:
time.sleep(PURGE_INTERVAL)
try:
now = int(time.time())
# Tombstones outlive both tiers, so a replay cannot slip in
# between a message expiring and its id being forgotten.
gone = store.purge(
now - self.retention,
now - self.requests_retention,
now - max(self.retention, self.requests_retention),
)
if gone:
log.info("expired %d message(s)", gone)
except Exception:
log.exception("purge failed")
# ---------------------------------------------------------------------------
# CLI
# ---------------------------------------------------------------------------
def cmd_keygen(args: argparse.Namespace) -> int:
if os.path.exists(args.key) and not args.force:
print(f"{args.key} exists; refusing to overwrite (use --force)", file=sys.stderr)
return 1
private = X25519PrivateKey.generate()
raw = private.private_bytes(
serialization.Encoding.Raw,
serialization.PrivateFormat.Raw,
serialization.NoEncryption(),
)
# Written 0600 before any bytes land, so the key is never briefly readable.
fd = os.open(args.key, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
with os.fdopen(fd, "wb") as fh:
fh.write(raw)
public = private.public_key().public_bytes(
serialization.Encoding.Raw, serialization.PublicFormat.Raw
)
print(f"private key: {args.key}")
print(f"public key: {b32(public)}")
print()
print("Publish the public key through a trusted channel; clients pin it (SPEC.md §4).")
return 0
def cmd_serve(args: argparse.Namespace) -> int:
try:
with open(args.key, "rb") as fh:
static_key = fh.read()
except OSError as exc:
print(f"cannot read server key: {exc}", file=sys.stderr)
return 1
if len(static_key) != KEY_LEN:
print(f"server key must be {KEY_LEN} raw bytes, got {len(static_key)}", file=sys.stderr)
return 1
server = MailServer((args.host, args.port), args, static_key)
log.info("listening on %s:%d", args.host, args.port)
log.info("server public key: %s", b32(server.static_pub))
try:
server.serve_forever()
except KeyboardInterrupt:
log.info("shutting down")
finally:
server.shutdown()
server.server_close()
return 0
def main(argv: list[str] | None = None) -> int:
parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
parser.add_argument("-v", "--verbose", action="store_true")
sub = parser.add_subparsers(dest="command", required=True)
keygen = sub.add_parser("keygen", help="generate the server's static X25519 key")
keygen.add_argument("--key", default="server.key")
keygen.add_argument("--force", action="store_true")
keygen.set_defaults(func=cmd_keygen)
serve = sub.add_parser("serve", help="run the mailbox server")
serve.add_argument("--key", default="server.key")
serve.add_argument("--db", default="mail.db")
serve.add_argument("--host", default="127.0.0.1")
serve.add_argument("--port", type=int, default=DEFAULT_PORT)
serve.add_argument("--max-envelope", type=int, default=768 << 10, metavar="BYTES")
serve.add_argument("--quota", type=int, default=64 << 20, metavar="BYTES")
serve.add_argument("--requests-quota", type=int, default=2 << 20, metavar="BYTES",
help="quota for mail arriving without an accept token")
serve.add_argument("--retention-days", type=int, default=30)
serve.add_argument("--requests-retention-days", type=int, default=7)
serve.add_argument("--max-accepted", type=int, default=1024, metavar="TOKENS",
help="accept tokens a mailbox may hold (SPEC.md §5.8)")
serve.add_argument("--invite-token", default=None,
help="require this token to register; open registration if unset")
serve.add_argument("--rate-connections", type=int, default=120, metavar="PER_MIN")
serve.add_argument("--rate-sends", type=int, default=60, metavar="PER_MIN")
serve.add_argument("--rate-token", type=int, default=120, metavar="PER_MIN",
help="sends per minute per accept token")
serve.set_defaults(func=cmd_serve)
args = parser.parse_args(argv)
logging.basicConfig(
level=logging.DEBUG if args.verbose else logging.INFO,
format="%(asctime)s %(levelname)s %(message)s",
)
return args.func(args)
if __name__ == "__main__":
sys.exit(main())
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