swaitsandClaude Opus 4.7
f83d4088e3
Every "Test vectors: \`corpus/...\`" mention in the spec is now a markdown link pointing at the corresponding file or directory on GitHub. Each version build resolves its links to the correct git ref: - /latest/ -> https://github.com/swaits/meshcore-spec/tree/main/corpus/... - /v0.1.0/ -> https://github.com/swaits/meshcore-spec/tree/v0.1.0/corpus/... - future /vX.Y.Z/ -> /tree/vX.Y.Z/corpus/... Two changes: 1. src/*.md (18 files): the inline-code corpus paths in each "Cross-References" section are now markdown links. Directories point at /tree/main/corpus/..., individual .json files point at /blob/main/corpus/.... CHANGELOG and other non-corpus main links are intentionally left pointing at main — the canonical history lives there regardless of which version you're reading. 2. justfile: `cut-version` now also runs a sed over the snapshot's *.md files replacing /tree/main/corpus -> /tree/<VERSION>/corpus and /blob/main/corpus -> /blob/<VERSION>/corpus. So a freshly-cut version is self-consistent. versions/v0.1.0/* regenerated by re-running `just cut-version v0.1.0` after the new src/ links + recipe sed landed. Tagging the release commit (`git tag v0.1.0`) is now REQUIRED — not "optional" — because the rendered /v0.1.0/ pages reference /tree/v0.1.0/corpus/... on GitHub. Without the tag the links 404. cut-version's output now says so. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
82 lines
2.8 KiB
Markdown
82 lines
2.8 KiB
Markdown
# MeshCore Protocol Specification
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## Section 8: Payload — Group (GRP_TXT / GRP_DATA)
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### Overview
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Group payloads enable encrypted communication within a channel. All members of a
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channel share a symmetric secret. The payload begins with a 1-byte channel hash
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(derived from the channel's shared key), followed by an encrypt-then-MAC blob.
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### Payload Types
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| Value | Name | Description |
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|-------|------|-------------|
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| 0x05 | GRP_TXT | Group text message |
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| 0x06 | GRP_DATA | Group datagram (binary data) |
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### Wire Format
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```
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0
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|channel_hash(1)| Cipher MAC (2 bytes) | |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
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| Ciphertext (N × 16 bytes, N ≥ 1) |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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```
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### Fields
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| Field | Offset | Size | Type | Description |
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|-------|--------|------|------|-------------|
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| channel_hash | 0 | 1 byte | raw | First byte of SHA-256(channel_shared_key) |
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| cipher_mac | 1 | 2 bytes | raw | Truncated HMAC-SHA256 over ciphertext |
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| ciphertext | 3 | variable (multiple of 16) | raw | AES-128-ECB encrypted data |
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### Minimum Size
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- Minimum payload: 1 + 2 + 16 = 19 bytes
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### Channel Hash Derivation
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The channel_hash is the first byte of the SHA-256 hash of the channel's 32-byte
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shared key (PUB_KEY_SIZE bytes):
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```
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channel_hash = SHA256(channel_shared_key)[0]
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```
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This means hash collisions are expected (1 in 256 chance). When a receiver finds
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a matching channel_hash, it attempts decryption with each matching channel's
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secret. Only a successful MAC verification confirms the correct channel.
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### Encryption
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1. Encrypt plaintext using AES-128-ECB with first 16 bytes of the channel's
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shared key (GroupChannel.secret).
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2. Compute HMAC-SHA256 over ciphertext using all 32 bytes of the channel's
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shared key. Truncate to 2 bytes.
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3. Assemble: channel_hash || MAC || ciphertext.
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### Decryption
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1. Read channel_hash (1 byte).
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2. Search local channel database for channels with matching hash. Multiple
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channels may match (up to 4 in reference implementation).
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3. For each matching channel, attempt MAC verification and decryption using the
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channel's shared key.
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4. The first channel that produces a valid MAC yields the correct decryption.
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### Cross-References
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- [Section 14: Cryptography](14-crypto.md) — Encrypt-then-MAC
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- Test vectors: [`corpus/payloads/group/`](https://github.com/swaits/meshcore-spec/tree/v0.1.0/corpus/payloads/group/)
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### Reference Implementation
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- `Mesh::createGroupDatagram()` in `src/Mesh.cpp` — Encoding
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- `Mesh::onRecvPacket()`, cases `PAYLOAD_TYPE_GRP_TXT/GRP_DATA` — Decoding
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- `Mesh::searchChannelsByHash()` — Channel lookup
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