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* docs: correct inaccurate privacy and metadata claims Several documented guarantees did not match the implementation. These matter more than ordinary doc drift: someone deciding whether to carry this phone to a protest reads these sentences as the threat model. - Peer IDs were described as "short ephemeral IDs derived per session" that "rotate periodically" and "prevent tracking". They are the first 8 bytes of the Noise static key fingerprint, stable across sessions and reboots, and replaced only by a panic wipe. Corrected in the whitepaper (§3, §8), IdentityModels, and BitchatProtocol, whose header notes claimed "no persistent identifiers in protocol headers" while every header carries exactly one. - "No plaintext message content is ever written to disk" was false for accepted media, which is stored unsealed under the platform's data-protection class. Narrowed to what actually holds. - Padding was described as applying to all packets but fragments. Only noiseEncrypted and noiseHandshake frames are padded; the pad bytes equal the pad length rather than being random; and because that length must fit one byte, a frame needing over 255 bytes of padding is emitted unpadded. Documented in the whitepaper (§4.1) and MessagePadding. - The gossip archive window is 6 hours in production, not the 15 minutes claimed in PRIVACY_POLICY.md and the privacy assessment. The 15-minute figure is the struct default that BLEService overrides. - The privacy assessment credited iOS BLE address randomization without noting that stable app-layer identifiers defeat it. The whitepaper's future-work list now names the changes these corrections imply: rotating on-air identity, padding for non-Noise types, and making the announce neighbor list optional. No behavior change; comments and documentation only. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> * docs: correct the same claims in the README The README repeats two of the claims corrected elsewhere in this PR, and it is the document people actually read before deciding to trust the app. - "no persistent identifiers" is the inverse of what the mesh does; it now points at the whitepaper's identity and metadata sections. - "end-to-end encryption with forward secrecy" holds for live Noise sessions but not for sealed store-and-forward mail, which the whitepaper already flags as its main cryptographic trade-off. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> --------- Co-authored-by: jack <jackjackbits@users.noreply.github.com> Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
102 lines
8.9 KiB
Markdown
102 lines
8.9 KiB
Markdown
# bitchat Privacy Assessment
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Last reviewed: July 2026
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## Scope
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- BLE discovery, mesh routing, gossip sync, private delivery, and courier behavior
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- Nostr private fallback, bridge courier drops, mesh bridging, geohash channels, and notices
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- CoreLocation and reverse geocoding
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- Local persistence, panic wipe, logging, and App Store privacy manifests
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The user-facing contract is `PRIVACY_POLICY.md`. This document records implementation-level behavior and residual risks that should be re-audited when storage or transport semantics change.
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## Current Posture
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- The project operates no account system, analytics pipeline, advertising SDK, or project-owned messaging backend.
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- Mesh transport is peer-to-peer. Optional internet features use third-party Nostr relays and can expose public content, coarse geohashes, timing, relay, and network metadata.
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- Private payloads are end-to-end encrypted, but public mesh, board, bridge, and geohash content is intentionally visible to its participants.
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- Local storage is bounded where practical and included in panic wipe, but it is not wholly ephemeral. The app persists the stores listed below.
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- The app and share extension each bundle a privacy manifest declaring their actual required-reason API use.
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## BLE Discovery and Metadata
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Signed announces can expose:
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- Nickname, persistent Noise public key, and Ed25519 signing public key
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- Capability flags
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- A bounded set of short direct-neighbor identifiers
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- A coarse rendezvous geohash when the bridge capability is enabled
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The app does not advertise the device's assigned name. iOS manages BLE address randomization; bitchat does not attempt to create a stable MAC address.
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That randomization does not deliver the unlinkability it might suggest, because the application layer publishes stable identifiers above it. The 8-byte peer ID in every packet header is the first 8 bytes of the Noise static key fingerprint, so it does not rotate; announces carry the static keys themselves; and the fixed service UUID makes any bitchat device detectable as such by a passive scanner. A receiver in radio range can therefore recognise a specific device across sessions and locations, and detect that the app is in use at all. RSSI, timing, traffic volume, and radio fingerprints remain observable as well.
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Ingress validates announce structure, sender binding, signatures, payload sizes, and freshness. Current-link Noise authentication is required before destructive courier handoff or strict directed delivery. Floods, queues, fragments, ingress work, and per-peer state are bounded.
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## Private Messaging and Courier Delivery
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- Direct mesh sessions use Noise XX with X25519, ChaCha20-Poly1305, and SHA-256.
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- Undelivered outgoing private messages remain in a bounded, ChaChaPoly-sealed outbox for at most 24 hours. Its key is stored in the keychain.
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- Physical couriers store opaque Noise-sealed envelopes, not plaintext. Deposits have trust-tier quotas, per-depositor caps, a global cap, and at most a 24-hour lifetime.
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- Spray-and-wait copies are bounded and progress cannot be replenished by replaying a deposit.
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- Delivery status advances only after real transport admission or explicit relay acceptance; late failure cannot downgrade a delivered/read message.
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- Panic wipe deletes the outbox, courier mail, keys, dedup state, and active transport state.
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Residual risk: private-message metadata such as timing, radio adjacency, ciphertext size, rotating recipient tags, and relay connections remains observable. A compromised recipient device can disclose plaintext.
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## Public Gossip, Boards, and Media
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- Recent signed public mesh messages are archived in Application Support for up to 6 hours so gossip sync survives a relaunch and can cross mesh partitions.
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- Signed public board posts and tombstones persist until author-selected expiry, at most seven days. Stores are bounded by global and per-author quotas.
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- Group metadata (name, roster, creator, epoch) persists as protected JSON; group keys live in the keychain until leave/removal/wipe.
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- Voice notes and images are stored in Application Support. Incoming media has a 100 MB oldest-first quota; outgoing media does not have an equivalent automatic lifetime and remains until cleanup, panic wipe, or app removal. Panic wipe invalidates detached preparation work, cancels active transfers, closes live capture files, and removes the managed media tree before returning.
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Public archives contain content already intended for public mesh/board distribution, but a seized unlocked device can reveal it. Group metadata and media can reveal relationships or content even when the in-memory chat timeline has gone away.
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## Nostr and Mesh Bridge
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- BitChat's proprietary private-envelope fallback protects plaintext with secp256k1 key agreement, HKDF-SHA256, and XChaCha20-Poly1305. It is not NIP-17, NIP-44, or NIP-59 compatible and does not provide forward secrecy against later compromise of the recipient's static Nostr private key. Relays still see the recipient public-key tag, event timing and size, and network metadata.
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- Bridge courier drops use a throwaway publisher key, an opaque Noise-sealed envelope, and a day-rotating recipient tag. Only a party already holding the recipient's Noise static key can compute candidate tags.
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- Relay publication is considered successful only after an explicit NIP-01 `OK true` from at least one target relay. Rejected, disconnected, timed-out, or merely socket-written events stay retryable.
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- When mesh bridge is enabled, public mesh messages not marked “nearby only” are signed under a per-cell Nostr identity and published to a neighborhood rendezvous geohash. Presence and public bridge traffic therefore expose a coarse area to relays and participants.
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- A bridge gateway can carry signed bridge/location events and opaque courier drops for nearby mesh-only peers. It cannot validly publish a neighbor's radio-only message because the author must first sign the bridge event.
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Residual risk: Nostr relay retention and logging are outside project control. Public events may be copied indefinitely. Timing, coarse location, and participation can be correlated even when content is encrypted or per-cell identities are used.
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## Location
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- When-in-use CoreLocation access computes geohash choices and bridge cells. Permission revocation stops live sampling and releases subscriptions.
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- Exact coordinates are not persisted by bitchat or placed into mesh/Nostr payloads.
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- Selected/bookmarked geohashes, teleport flags, and display names persist in local preferences; a fine geohash can identify a small area.
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- Friendly place names use `CLGeocoder.reverseGeocodeLocation`. Apple may process the supplied location under its own privacy terms, so this operation is not accurately described as wholly on-device.
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- Automatic presence is limited to lower-precision geohashes; precise posts occur through user-selected channels, notes, notices, or the bridge behavior presented in the UI.
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## Logging and Telemetry
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- `SecureLogger` uses OSLog privacy markers and filters likely secrets. Release builds suppress debug verbosity.
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- No project analytics or telemetry endpoint exists.
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- Apple system logs, Nostr relays, network providers, and nearby radios can still observe operational metadata outside the project's logging layer.
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## Privacy Manifests
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`bitchat/PrivacyInfo.xcprivacy` declares:
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- UserDefaults: `CA92.1` for app-only preferences and `1C8F.1` for the shared app group
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- File timestamps/metadata: `C617.1` for app-container files and `3B52.1` for user-granted files
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- System boot time: `35F9.1` for elapsed-time deadlines and timers
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`bitchatShareExtension/PrivacyInfo.xcprivacy` declares app-group UserDefaults reason `1C8F.1`. Both manifests declare no tracking domains and no data collection by the app developer. They must remain bundled in their respective executable bundles.
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## Panic Wipe Coverage
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The panic action clears identity/session state, preferences, location state, groups, prekeys, outbox mail, courier mail, bridge dedup state, gossip archive, board data, managed media, and active subscriptions/transports. Managed media deletion completes synchronously, after active media work has been invalidated. Keychain secrets use device-only accessibility, and an install marker detects and clears app keys that survive uninstall before a later reinstall can use them. New persistent stores must add an explicit wipe hook and a regression test.
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## Release Review Checklist
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- Reconcile every new Application Support, UserDefaults, keychain, cache, or relay write with this assessment and `PRIVACY_POLICY.md`.
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- Re-scan required-reason APIs and validate both bundled `PrivacyInfo.xcprivacy` files before archive submission.
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- Verify panic wipe reaches any newly added persistent store.
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- Treat geohash precision, bridge-cell changes, new relay tags, and announce fields as privacy-surface changes.
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- Re-run real-device Bluetooth, background/locked-device recovery, location revocation, and audio-route checks; simulators cannot validate the physical side of those behaviors.
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