swaits
a031c6e266
Rewrites lingering github.com / *.sr.ht references to this Gitea instance, translating GitHub and Sourcehut URL shapes to Gitea's (/blob/<ref>/ and sr.ht /tree/<ref>/item/ -> /src/branch/<ref>/, /tree/<tag> -> /src/tag/<tag>, /actions/workflows/X -> /actions?workflow=X, /edit/ -> /_edit/). Left pointing at the old hosts on purpose: pull-request links (PRs were not migrated), forks that were never migrated, and GitHub-only surfaces Gitea lacks. OpenPGP identity proofs are untouched by design.
75 lines
2.9 KiB
Markdown
75 lines
2.9 KiB
Markdown
# MeshCore Protocol Specification
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## Section 7: Payload — Anonymous Request
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### Overview
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The anonymous request payload allows a node to send an encrypted message to a
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recipient without the recipient needing to know the sender in advance. Instead of
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a 1-byte source hash, the full 32-byte Ed25519 public key of the sender is
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included, enabling the recipient to compute the shared secret for decryption.
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### Payload Type
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Header payload type field: `0x07` (ANON_REQ)
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### Wire Format
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```
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0
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| dest_hash(1) | |
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+-+-+-+-+-+-+-+-+ |
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| Sender Public Key (32 bytes) |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 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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### Fields
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| Field | Offset | Size | Type | Description |
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|-------|--------|------|------|-------------|
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| dest_hash | 0 | 1 byte | raw | First byte of recipient's Ed25519 public key |
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| sender_pub_key | 1 | 32 bytes | raw | Full Ed25519 public key of sender |
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| cipher_mac | 33 | 2 bytes | raw | Truncated HMAC-SHA256 over ciphertext |
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| ciphertext | 35 | variable (multiple of 16) | raw | AES-128-ECB encrypted data |
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### Minimum Size
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- Minimum payload: 1 + 32 + 2 + 16 = 51 bytes
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### Encryption
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1. Sender computes ECDH shared secret using its private key and the recipient's
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public key.
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2. Encrypt plaintext using AES-128-ECB with first 16 bytes of shared secret.
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3. Compute HMAC-SHA256 over ciphertext with full 32-byte shared secret.
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Truncate to 2 bytes.
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4. Assemble: dest_hash || sender_pub_key || MAC || ciphertext.
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### Decryption
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1. Read dest_hash. If it does not match this node, the packet is not for us
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(but MAY be forwarded).
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2. Read 32-byte sender_pub_key. Construct an Identity from it.
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3. Compute ECDH shared secret between this node's private key and sender's
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public key.
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4. Verify the MAC and decrypt as in [Section 14](14-crypto.md).
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### Cross-References
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- [Section 6: Encrypted Payloads](06-payload-encrypted.md) — Standard encrypted format
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- [Section 14: Cryptography](14-crypto.md) — Encrypt-then-MAC
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- [Section 15: Identity](15-identity.md) — ECDH key exchange
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- Test vectors: [`corpus/payloads/anon-req/`](https://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/v0.1.0/corpus/payloads/anon-req/)
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### Reference Implementation
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- `Mesh::createAnonDatagram()` in `src/Mesh.cpp` — Encoding
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- `Mesh::onRecvPacket()`, case `PAYLOAD_TYPE_ANON_REQ` — Decoding
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