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docs: add PROTOCOL.md and retire the stale audit writeup

Wire format lives in docs/PROTOCOL.md so the handshake, invite, and
frames can be read without the Rust. Threat model points at it. The
in-house audit notes are a finding-status table, not a third-party
audit. SDK README no longer claims the legacy concat invite signature
is what we encode.

2026-09-11 13:15:51 -04:00

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OnionWire threat model (v1)

OnionWire is a two-party chat over Tor onion services. There is no chat server. Wire format: PROTOCOL.md. In-house review notes: SECURITY_AUDIT.md — not a public third-party audit.

Tor relays are not a chat server

Each install hosts its own v3 onion and dials friends’ onions through in-process Arti. Relays move cells; they never see plaintext, never store messages, and never hold a roster. Compromising a relay is not compromising OnionWire’s message store. The store is the sqlite file on the endpoint.

Location updates are not a missing-person finder

A signed loc frame (onion, ts, sig) rewrites a friend’s locator only if that friend is already in the roster, the signature verifies against the pinned pubkey, and ts is newer. There is no directory, DHT, or introduction server. If you rotated while they were offline, they cannot find you until they F3-paste your new invite. OnionWire will not hunt for a missing person.

Rotating the onion does not revoke a friend

F4 changes the locator only. The identity key is unchanged. Anyone who already has your pubkey can still prove they are talking to you, and a later invite paste with the same k updates their row. To become a new person, /wipe-all (new identity key). There is no in-band unfriend/revoke in v1.

Message bodies are encrypted at rest; keys are not

Chat bodies in sqlite are ChaCha20-Poly1305 (nonce || ciphertext in the messages.plaintext column) with AAD owmsg1 || friend_id_le64 || dir || 0x00 || row_id_le64. Swapping ciphertext between rows fails closed. A random 32-byte data key is wrapped with Argon2id (same params as identity backup) from a non-empty passphrase. Salt + wrapped key live in store_meta. Unlock is fail-closed: wrong or empty passphrase does not open chat. Empty-AAD v0.2 blobs are rewrapped once on unlock; list_messages never falls back to empty AAD.

Identity secret key, friend public keys, and locators remain plaintext in the same db. The message key is not wrapped with identity_sk (that key is already on disk). A seized laptop still yields who you talk to and your identity unless you add OS/FDE. sqlcipher is out of v1. /wipe deletes chat and payments history (overwrite message bodies, VACUUM, WAL checkpoint); roster and identity stay. It is not a forensic erase — SSD wear-leveling can keep copies. /wipe-all deletes the data dir (new identity); same disk caveat.

Fail closed

Peer onion down → send fails. Fingerprint mismatch vs the pinned key → hard fail, no send. Arti HS experimental: if it cannot publish, OnionWire stops; it does not fall back to C-tor.

Profile is friend-visible, not a directory

A signed prf frame is shown to people who already have a session with you. Apply only for existing friends. There is still no name lookup, DHT, or public profile server. Anyone who already has a Noise session can see the profile you send them; that is not confidentiality against that friend.

Monero sidecar is not a wallet

OnionWire never holds spend keys. Optional ONIONWIRE_WALLET_RPC talks HTTP Digest to a user-hosted monero-wallet-rpc on loopback (--rpc-login required; open RPC is refused). A Noise friend can sign any rcp; the signature proves who sent the claim, not that a payment happened. verified=1 only after a conjunctive RPC match: one get_transfers row with the same non-empty txid, amount, and address. Incoming rcp stays verified=0 if RPC is down, errors, or no exact row. Subaddress reuse is the user’s wallet policy.

Backup file is the identity

/backup writes the identity and Noise static secrets. Treat the file like onionwire.db. Restore overwrites self keys and does not rewrite the friends table.

Rate limit is availability, not anonymity

Incoming rendezvous accepts are token-bucket limited (30/60s, burst 10). Excess is dropped without handshake. That is a DoS/availability control. It does not hide that you run an onion, and it is not a traffic-analysis defense.

Network (UDP / clearnet)

The application does not open UDP sockets and does not resolve friends via system DNS. Peer dials are Arti connect((onion, 80)) only. The one intentional clearnet socket is optional monero-wallet-rpc on loopback TCP (numeric loopback preferred; localhost still does getaddrinfo).

Do not add STUN/ICE/WebRTC, a DHT, SOCKS/C-tor fallback, or “direct TCP when the onion is slow.” Those are the Tox-over-Tor leak class.

Out of v1

Prosody, XMPP, s2s, MAM, carbons, outbox, multi-device, DHT / name server, sqlcipher, A/V.

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