Code is a better thing to argue with than prose, so the spec now has a
working, tested base under it. Every number and context string is a
concrete proposal you can reject by changing one function and watching a
test vector move.
What is implemented:
- PeerIDRotation: hour epochs with a ±1 matching window, the rotation
secret from the Noise static *private* key, per-epoch peer IDs, pairwise
recognition keys and tags from an X25519 shared secret, the fixed-width
tag block with CSPRNG padding and constant-time matching, and the
canonical bytes for the identity binding.
- AnnounceV2Packet (announceV2 = 0x05): TLV wire format carrying an epoch,
a 64-byte tag block, capabilities and an optional bridge cell — and
nothing else. No nickname, no public keys, no neighbour list. Rejects a
wrong-width tag block on both encode and decode, since a short block
would disclose how many mutual favourites someone has, and rejects
non-canonical capability encodings the way AuthenticatedPeerStatePacket
does. Unknown TLVs are skipped for forward compatibility.
- 37 tests, three of which are hex vectors cross-checked against an
independent implementation written from the spec alone (Python
hmac/hashlib, HKDF extract-then-expand, empty salt) and matching byte
for byte. That is the property Android needs: the document is sufficient
to reproduce the numbers without reading this code.
What is deliberately NOT implemented: nothing emits a v2 announce, and
BLEService parses the type and explicitly ignores it. Consuming presence
needs both the replacement identity binding and a decision on how
unverified presence appears in the peer list, and accepting it now would
put unauthenticated entries in front of people.
Adding the message type forced three policy decisions, all reviewable:
- Not gossip-synced. Syncing presence would defeat the point — a device
never in radio range could collect tag blocks, turning a local beacon
into a network-wide one.
- Not padded. At ~75 bytes the smallest bucket would triple the airtime of
the most frequent packet in the protocol; the format is already
near-constant width, and fixing the capability and geohash field widths
would be cheaper than padding.
- Parsed but ignored on receive, as above.
Notably the v2 announce is *smaller* than v1 (~75 vs ~229 bytes): dropping
two 32-byte keys, the neighbour list and the signature more than pays for
64 bytes of tags, so unlinkability here costs less airtime rather than
more.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
