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docs(auth): enforce nine-byte contract for client_id with MUST-level language #45

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docs(auth): enforce nine-byte contract for client_id with MUST-level language #45

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15 changes: 11 additions & 4 deletions auth.proto
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Original file line number Diff line number Diff line change
Expand Up @@ -9,9 +9,14 @@ package proto;
// against its current (and previous) pre-shared key and uses the
// recovered identity to set up the tunnel session.
message Auth {
// Stable client identifier, exactly nine raw UTF-8 bytes; signed
// by the HMAC together with timestamp. Not a UUID — the Hub
// generates it once per installation.
// Stable client identifier. MUST be exactly nine bytes. The HMAC
// (field 3) covers (client_id || timestamp); any other length
// passes the wire and the HMAC verifies cleanly against the same
// non-nine-byte input on both sides, silently breaking the
// identity contract. Both sender and receiver MUST validate the
// length before computing or checking the HMAC. Not a UUID — the
// client generates it once per installation using a CSPRNG and
// persists it across launches.
bytes client_id = 1;

// Minutes since the project epoch (2024-01-01 UTC), used as a
Expand All @@ -24,7 +29,9 @@ message Auth {
// possession of the PSK without revealing it. Receivers MUST
// assert len(hmac) == 12 and reject the Auth before performing any
// comparison; a shorter or absent value weakens authentication
// non-deterministically across implementations.
// non-deterministically across implementations. Implementations
// MUST also verify client_id is exactly 9 bytes before computing
// or checking this tag.
bytes hmac = 3;

// Free-form device label reported by the client (e.g. "SM-G998B",
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