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Relabel private Nostr envelopes honestly in docs; harden legacy envelope validation (#1480)

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Split-out safe half of #1437 (the kind-1402 wire migration stays held for Android coordination). Docs (README/WHITEPAPER/PRIVACY_POLICY/privacy-assessment) now describe the actual proprietary DM construction — kind 1059 gift wrap carrying XChaCha20-Poly1305 with a 24-byte nonce, base64url v2: framing, and an HKDF that borrows the nip44-v2 info label but is not the NIP-44 key schedule — instead of claiming NIP-17/44/59.

Hardens the existing legacy inbound path: 64 KiB ciphertext cap before decode (~46x the largest producible legacy envelope), outer kind/recipient-tag/signature binding, tagless kind-13 seal binding, unsigned kind-14 inner binding, inner tags restricted to the two shapes deployed clients emit (verified against every historical iOS release and a fixture frozen from Android production), SecRandomCopyBytes failure now throws, non-UTF-8 plaintext now throws instead of returning empty. Adds frozen cross-platform fixtures with hash-pinned generators; interop-reviewed with no rejection surface for deployed clients.
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jack
2026-07-26 12:50:17 +02:00
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docs/privacy-assessment.md
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@@ -54,9 +54,9 @@ Public archives contain content already intended for public mesh/board distribut
## Nostr and Mesh Bridge
- NIP-17/NIP-44 v2 private fallback protects plaintext with secp256k1 key agreement, HKDF-SHA256, and XChaCha20-Poly1305. Relays still see event and network metadata.
- 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.
- 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.
- Relay publication is considered successful only after an explicit NIP-20 `OK true` from at least one target relay. Rejected, disconnected, timed-out, or merely socket-written events stay retryable.
- 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.
- 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.
- 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.