## Problem
The firmware computes packet content hash as:
```
SHA256(payload_type_byte + [path_len for TRACE] + payload)
```
Where `payload_type_byte = (header >> 2) & 0x0F` — just the payload type
bits (2-5).
CoreScope was using the **full header byte** in its hash computation,
which includes route type bits (0-1) and version bits (6-7). This meant
the same logical packet produced different content hashes depending on
route type — breaking dedup and packet lookup.
**Firmware reference:** `Packet.cpp::calculatePacketHash()` uses
`getPayloadType()` which returns `(header >> PH_TYPE_SHIFT) &
PH_TYPE_MASK`.
## Fix
- Extract only payload type bits: `payloadType := (headerByte >> 2) &
0x0F`
- Include `path_len` byte in hash for TRACE packets (matching firmware
behavior)
- Applied to both `cmd/server/decoder.go` and `cmd/ingestor/decoder.go`
## Tests Added
- **Route type independence:** Same payload with FLOOD vs DIRECT route
types produces identical hash
- **TRACE path_len inclusion:** TRACE packets with different `path_len`
produce different hashes
- **Firmware compatibility:** Hash output matches manual computation of
firmware algorithm
## Migration Impact
Existing packets in the DB have content hashes computed with the old
(incorrect) formula. Options:
1. **Recompute hashes** via migration (recommended for clean state)
2. **Dual lookup** — check both old and new hash on queries (backward
compat)
3. **Accept the break** — old hashes become stale, new packets get
correct hashes
Recommend option 1 (migration) as a follow-up. The volume of affected
packets depends on how many distinct route types were seen for the same
logical packet.
Fixes#786
---------
Co-authored-by: you <you@example.com>
Kpa-clawbot