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MeshCore Protocol Specification

Section 9: Payload — Path Return

Overview

The PATH payload carries an encrypted return path from one node to another. It uses the same dest_hash + src_hash + encrypt-then-MAC envelope as REQ/RESPONSE/ TXT_MSG, but the decrypted contents contain routing path data and optional extra application data.

Payload Type

Header payload type field: 0x08 (PATH)

Wire Format (Outer)

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

The outer format is identical to Section 6.

Decrypted Inner Format

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  path_len(1)  |         Path (hash_count × hash_size)        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extra_type(1) |         Extra Data (variable)                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field	Size	Type	Description
path_len	1 byte	encoded	Same bitfield encoding as packet path_len (see Section 3)
path	hash_count × hash_size	raw	The return path to the sender
extra_type	1 byte	uint8	Type of extra data (lower 4 bits used; upper 4 reserved)
extra	remaining	raw	Extra data (may include trailing zero padding from AES)

Extra Type Values

Value	Description
0x00	No meaningful extra data (reciprocal path)
0xFF	Dummy/padding (used when no real extra data; followed by 4 random bytes)
Other	Application-defined

When creating a path return with no extra data, the reference implementation appends 0xFF as extra_type followed by 4 random bytes to ensure the packet hash is unique.

Encoding

Build the inner plaintext: a. Write path_len byte (encoded hash_size and hash_count). b. Write path bytes (hash_count × hash_size). c. If extra data is provided: write extra_type (lower 4 bits), write extra. d. If no extra data: write 0xFF, write 4 random bytes.
Encrypt the inner plaintext using encrypt-then-MAC with the shared secret.
Prepend dest_hash and src_hash.

Decoding

Decode the outer envelope (dest_hash, src_hash, MAC, ciphertext) as in Section 6.
After decryption: a. Read 1 byte as path_len. Decode hash_size and hash_count. b. Read hash_count × hash_size bytes as the path. c. Read 1 byte as extra_type (use lower 4 bits only). d. Remaining bytes are extra data (may include AES zero-padding).

Cross-References

Section 3: Path — Path length encoding
Section 6: Encrypted Payloads — Outer envelope format
Section 14: Cryptography — Encrypt-then-MAC
Test vectors: corpus/payloads/path-return/

Reference Implementation

Mesh::createPathReturn() in src/Mesh.cpp — Encoding
Mesh::onRecvPacket(), PATH case within REQ/RESPONSE/TXT_MSG handling — Decoding

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