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91 lines
3.5 KiB
Markdown
91 lines
3.5 KiB
Markdown
# MeshCore Protocol Specification
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## Section 9: Payload — Path Return
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### Overview
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The PATH payload carries an encrypted return path from one node to another. It
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uses the same dest_hash + src_hash + encrypt-then-MAC envelope as REQ/RESPONSE/
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TXT_MSG, but the decrypted contents contain routing path data and optional extra
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application data.
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### Payload Type
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Header payload type field: `0x08` (PATH)
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### Wire Format (Outer)
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```
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0
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| dest_hash(1) | src_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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The outer format is identical to [Section 6](06-payload-encrypted.md).
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### Decrypted Inner Format
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```
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| path_len(1) | Path (hash_count × hash_size) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| extra_type(1) | Extra Data (variable) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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```
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| Field | Size | Type | Description |
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|-------|------|------|-------------|
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| path_len | 1 byte | encoded | Same bitfield encoding as packet path_len (see [Section 3](03-path.md)) |
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| path | hash_count × hash_size | raw | The return path to the sender |
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| extra_type | 1 byte | uint8 | Type of extra data (lower 4 bits used; upper 4 reserved) |
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| extra | remaining | raw | Extra data (may include trailing zero padding from AES) |
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### Extra Type Values
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| Value | Description |
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|-------|-------------|
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| 0x00 | No meaningful extra data (reciprocal path) |
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| 0xFF | Dummy/padding (used when no real extra data; followed by 4 random bytes) |
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| Other | Application-defined |
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When creating a path return with no extra data, the reference implementation
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appends `0xFF` as extra_type followed by 4 random bytes to ensure the packet
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hash is unique.
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### Encoding
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1. Build the inner plaintext:
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a. Write path_len byte (encoded hash_size and hash_count).
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b. Write path bytes (hash_count × hash_size).
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c. If extra data is provided: write extra_type (lower 4 bits), write extra.
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d. If no extra data: write `0xFF`, write 4 random bytes.
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2. Encrypt the inner plaintext using encrypt-then-MAC with the shared secret.
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3. Prepend dest_hash and src_hash.
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### Decoding
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1. Decode the outer envelope (dest_hash, src_hash, MAC, ciphertext) as in
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[Section 6](06-payload-encrypted.md).
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2. After decryption:
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a. Read 1 byte as path_len. Decode hash_size and hash_count.
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b. Read hash_count × hash_size bytes as the path.
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c. Read 1 byte as extra_type (use lower 4 bits only).
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d. Remaining bytes are extra data (may include AES zero-padding).
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### Cross-References
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- [Section 3: Path](03-path.md) — Path length encoding
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- [Section 6: Encrypted Payloads](06-payload-encrypted.md) — Outer envelope format
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- [Section 14: Cryptography](14-crypto.md) — Encrypt-then-MAC
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- Test vectors: [`corpus/payloads/path-return/`](https://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/v0.1.0/corpus/payloads/path-return/)
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### Reference Implementation
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- `Mesh::createPathReturn()` in `src/Mesh.cpp` — Encoding
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- `Mesh::onRecvPacket()`, PATH case within REQ/RESPONSE/TXT_MSG handling — Decoding
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