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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://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/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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