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spec: link corpus references to GitHub, version-aware

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>

2026-05-12 10:35:09 -06:00

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MeshCore Protocol Specification

Section 8: Payload — Group (GRP_TXT / GRP_DATA)

Overview

Group payloads enable encrypted communication within a channel. All members of a channel share a symmetric secret. The payload begins with a 1-byte channel hash (derived from the channel's shared key), followed by an encrypt-then-MAC blob.

Payload Types

Value	Name	Description
0x05	GRP_TXT	Group text message
0x06	GRP_DATA	Group datagram (binary data)

Wire Format

 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|channel_hash(1)|    Cipher MAC (2 bytes)       |               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
|              Ciphertext (N × 16 bytes, N ≥ 1)                |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields

Field	Offset	Size	Type	Description
channel_hash	0	1 byte	raw	First byte of SHA-256(channel_shared_key)
cipher_mac	1	2 bytes	raw	Truncated HMAC-SHA256 over ciphertext
ciphertext	3	variable (multiple of 16)	raw	AES-128-ECB encrypted data

Minimum Size

Minimum payload: 1 + 2 + 16 = 19 bytes

Channel Hash Derivation

The channel_hash is the first byte of the SHA-256 hash of the channel's 32-byte shared key (PUB_KEY_SIZE bytes):

channel_hash = SHA256(channel_shared_key)[0]

This means hash collisions are expected (1 in 256 chance). When a receiver finds a matching channel_hash, it attempts decryption with each matching channel's secret. Only a successful MAC verification confirms the correct channel.

Encryption

Encrypt plaintext using AES-128-ECB with first 16 bytes of the channel's shared key (GroupChannel.secret).
Compute HMAC-SHA256 over ciphertext using all 32 bytes of the channel's shared key. Truncate to 2 bytes.
Assemble: channel_hash || MAC || ciphertext.

Decryption

Read channel_hash (1 byte).
Search local channel database for channels with matching hash. Multiple channels may match (up to 4 in reference implementation).
For each matching channel, attempt MAC verification and decryption using the channel's shared key.
The first channel that produces a valid MAC yields the correct decryption.

Cross-References

Section 14: Cryptography — Encrypt-then-MAC
Test vectors: corpus/payloads/group/

Reference Implementation

Mesh::createGroupDatagram() in src/Mesh.cpp — Encoding
Mesh::onRecvPacket(), cases PAYLOAD_TYPE_GRP_TXT/GRP_DATA — Decoding
Mesh::searchChannelsByHash() — Channel lookup

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