swaits
a031c6e266
Rewrites lingering github.com / *.sr.ht references to this Gitea instance, translating GitHub and Sourcehut URL shapes to Gitea's (/blob/<ref>/ and sr.ht /tree/<ref>/item/ -> /src/branch/<ref>/, /tree/<tag> -> /src/tag/<tag>, /actions/workflows/X -> /actions?workflow=X, /edit/ -> /_edit/). Left pointing at the old hosts on purpose: pull-request links (PRs were not migrated), forks that were never migrated, and GitHub-only surfaces Gitea lacks. OpenPGP identity proofs are untouched by design.
68 lines
2.2 KiB
Markdown
68 lines
2.2 KiB
Markdown
# MeshCore Protocol Specification
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## Section 16: Packet Hash
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### Overview
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MeshCore uses SHA-256 hashing for packet deduplication. Each node maintains a
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table of recently seen packet hashes and discards packets it has already
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processed. The hash is computed over the payload type and payload contents, with
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a special case for TRACE packets.
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### Hash Computation
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The packet hash is computed as:
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```
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hash = SHA-256(payload_type_byte || payload_data)[0..MAX_HASH_SIZE-1]
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```
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Where:
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- `payload_type_byte` is a single byte: `(header >> 2) & 0x0F`
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- `payload_data` is the raw payload bytes (payload_len bytes)
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- The SHA-256 output is truncated to MAX_HASH_SIZE (8) bytes
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### TRACE Packet Special Case
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For TRACE packets (payload_type = 0x09), the path_len byte is included in the
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hash to distinguish trace packets that revisit the same node on their return
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path:
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```
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hash = SHA-256(payload_type_byte || path_len_byte || payload_data)[0..MAX_HASH_SIZE-1]
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```
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### Algorithm
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```
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function calculatePacketHash(packet):
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sha = SHA256_init()
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type_byte = getPayloadType(packet.header) // single byte
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SHA256_update(sha, type_byte)
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if type_byte == 0x09: // TRACE
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SHA256_update(sha, packet.path_len) // 1 byte (as uint8)
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SHA256_update(sha, packet.payload, packet.payload_len)
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hash = SHA256_finalize(sha)
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return hash[0..7] // first MAX_HASH_SIZE bytes
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```
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### Properties
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- The hash does NOT include the header byte, path, or transport codes. This
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means the same logical message received via different routes produces the
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same hash.
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- The hash does NOT include the route type or version. Only the payload type
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and payload content determine the hash.
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- For TRACE packets, including path_len ensures that the same trace payload
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at different stages of traversal produces different hashes.
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### Cross-References
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- [Section 10: Trace](10-payload-trace.md) — TRACE packet special handling
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- Test vectors: [`corpus/crypto/sha256/packet-hash.json`](https://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/v0.1.0/corpus/crypto/sha256/packet-hash.json)
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### Reference Implementation
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- `Packet::calculatePacketHash()` in `src/Packet.cpp`
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- `MeshTables::hasSeen()` — Deduplication table lookup
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