meshcore-spec/versions/v0.1.0/07-payload-anon-req.md

MeshCore Protocol Specification

Section 7: Payload — Anonymous Request

Overview

The anonymous request payload allows a node to send an encrypted message to a recipient without the recipient needing to know the sender in advance. Instead of a 1-byte source hash, the full 32-byte Ed25519 public key of the sender is included, enabling the recipient to compute the shared secret for decryption.

Payload Type

Header payload type field: 0x07 (ANON_REQ)

Wire Format

 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| dest_hash(1)  |                                               |
+-+-+-+-+-+-+-+-+                                               |
|                   Sender Public Key (32 bytes)                |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Cipher MAC (2 bytes)       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|              Ciphertext (N × 16 bytes, N ≥ 1)                |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields

Field	Offset	Size	Type	Description
dest_hash	0	1 byte	raw	First byte of recipient's Ed25519 public key
sender_pub_key	1	32 bytes	raw	Full Ed25519 public key of sender
cipher_mac	33	2 bytes	raw	Truncated HMAC-SHA256 over ciphertext
ciphertext	35	variable (multiple of 16)	raw	AES-128-ECB encrypted data

Minimum Size

• Minimum payload: 1 + 32 + 2 + 16 = 51 bytes

Encryption

1. Sender computes ECDH shared secret using its private key and the recipient's public key.
2. Encrypt plaintext using AES-128-ECB with first 16 bytes of shared secret.
3. Compute HMAC-SHA256 over ciphertext with full 32-byte shared secret. Truncate to 2 bytes.
4. Assemble: dest_hash || sender_pub_key || MAC || ciphertext.

Decryption

1. Read dest_hash. If it does not match this node, the packet is not for us (but MAY be forwarded).
2. Read 32-byte sender_pub_key. Construct an Identity from it.
3. Compute ECDH shared secret between this node's private key and sender's public key.
4. Verify the MAC and decrypt as in Section 14.

Cross-References

• Section 6: Encrypted Payloads — Standard encrypted format
• Section 14: Cryptography — Encrypt-then-MAC
• Section 15: Identity — ECDH key exchange
• Test vectors: corpus/payloads/anon-req/

Reference Implementation

• Mesh::createAnonDatagram() in src/Mesh.cpp — Encoding
• Mesh::onRecvPacket(), case PAYLOAD_TYPE_ANON_REQ — Decoding