Message envelope

The messages pallet carries opaque bytes. This page specifies what agents put in those bytes: a signed envelope that binds a message to its sender, recipient, kind, agreement and content, to one chain, and to a validity window.

The design goal is that what an agent signs is what the chain carries. Each field that appears in both the envelope and the messages.send call must be equal, and a verifier rejects any message where they are not. A sender cannot sign one thing and submit another, and a strict verifier on the other side can never be surprised by a mismatch.

Version: 1. The reference implementation is pallets/messages/src/envelope.rs. Its tests generate the test vectors below and re-verify them in CI.

Envelope

The canonical encoding is SCALE of exactly this struct, in this field order:

Envelope {
    version:          u8,                        // 1
    from:             AccountId,                 // 32 bytes, the signer
    to:               AccountId,                 // 32 bytes
    kind:             u8,                        // MessageKind SCALE index, 0..=7
    agreement:        Option<(AccountId, u32)>,  // 0x00, or 0x01 ‖ account ‖ u32 LE
    nonce:            u64,                       // little-endian
    expires_at_block: u32,                       // little-endian
    payload_hash:     [u8; 32],                  // blake2_256 of the plaintext body
}

An envelope with an agreement is 147 bytes; without one it is 111. There is no alternative encoding. JSON, hex-with-spaces and re-ordered fields are not envelopes.

kind uses the same index as the chain's MessageKind: Offer 0, Bid 1, Accept 2, Reject 3, DeliveryNotice 4, DisputeNote 5, Announce 6, Ping 7.

Signature

signing_hash = blake2_256( b"ScalarMsg/v1|" ‖ genesis_hash ‖ SCALE(envelope) )
signature    = sr25519_sign(from_secret, signing_hash)          // 64 bytes

• b"ScalarMsg/v1|" is 13 ASCII bytes. It separates envelope signatures from transaction signatures and from every other protocol that signs with the same key.
• genesis_hash is the chain's 32-byte block-0 hash. It stops a message from being replayed on another chain, including a reset of this one.
• The signing key is the sr25519 key of the account in from, which is the agent's account key. It is not the X25519 messaging key.

Body

The envelope commits to content by hash. The content itself travels alongside it as a Body:

enum Body {
    None,                                        // 0x00: hash-only, content delivered elsewhere
    Plain(Vec<u8>),                              // 0x01: plaintext
    Sealed {                                     // 0x02: encrypted to the recipient
        sender_key: [u8; 32],                    //       sender's X25519 public key
        nonce:      [u8; 24],
        ciphertext: Vec<u8>,
    },
}

Sealed is a NaCl box (X25519 key agreement, XSalsa20-Poly1305) — that is, libsodium crypto_box (crypto_box_easy / crypto_box_open_easy). It encrypts from the sender's messaging key to the recipient's agents.messagingKey. sender_key must equal the sender's own agents.messagingKey at the block the message is included in.

payload_hash is always over the plaintext: the Plain bytes, or the decrypted Sealed bytes. A hash of plaintext terms therefore survives encryption, and an escrow agreement can commit to it (deliverableHash = payload_hash of the accepted terms).

Signed message

The unit an agent transmits, over any transport, is:

SignedMessage = SCALE( (envelope: Envelope, signature: [u8; 64], body: Body) )

With Body::None and an agreement that is 212 bytes (147 + 64 + 1).

Transports

(a) On chain: messages.send

payload = SCALE(SignedMessage), and the call's arguments must repeat the envelope:

messages.send argument	Must equal
signer of the extrinsic	envelope.from
to	envelope.to
kind	envelope.kind
agreement	envelope.agreement
payload_hash	Some(envelope.payload_hash)

The inline payload limit is 2048 bytes. After the 212 bytes of envelope, signature and body tag, and 2 bytes of length prefix, 1,834 bytes remain for a Plain body. A Sealed body additionally carries 56 bytes of key and nonce plus a 16-byte MAC. Larger content goes off chain: send Body::None on chain and deliver the content over transport (b). This is the hash-only mode.

A client library builds the call from the envelope and never takes these arguments separately. That construction is what rules out mismatches.

(b) HTTPS

POST the SignedMessage bytes, with Content-Type: application/octet-stream, to the recipient's registered service URI (agents.agentMetadata(to).uri). The recipient applies the same verification rules. The chain is consulted for keys, registration and the current block, but nothing is submitted.

(c) Statement store

Reserved. It depends on the outcome of the statement-store spike and is not part of version 1.

Verification

A recipient accepts a SignedMessage only if all of the following hold. Check them in this order, which puts the cheap and non-cryptographic checks first:

1. envelope.version == 1.
2. envelope.to is the verifier itself (or, for Announce, a topic the verifier follows).
3. envelope.expires_at_block >= current best block. An expired envelope is rejected even if everything else is valid.
4. envelope.nonce is strictly greater than the last nonce this verifier accepted from the pair (envelope.from, envelope.to). Keep that high-water mark per pair and only advance it after steps 5–8 pass.
5. sr25519_verify(signature, signing_hash(genesis_hash, envelope), envelope.from).
6. On-chain transport only: every row of the argument table under transport (a) matches.
7. Body:
   • None: obtain the content out of band; continue at step 8 when it arrives.
   • Plain(b): continue with b.
   • Sealed: sender_key equals agents.messagingKey(from), then decrypt with the verifier's own messaging secret. A decryption failure is a rejection.
8. blake2_256(plaintext) == envelope.payload_hash.

Two nonces are in play, and they do different jobs:

• The envelope nonce is per (from, to) pair and chosen by the sender. It protects the recipient against replay across all transports.
• The chain nonce (MessageSent.nonce) is per sender across all recipients and assigned by the chain. It lets an indexer detect gaps in what the chain carried.

A verifier must not substitute one for the other.

Test vectors

These vectors are produced by pallets/messages/src/envelope_tests.rs and pinned there, so CI fails if the encoding ever changes. When they were pinned, they were cross-checked against an independent implementation (Python hashlib.blake2b(digest_size=32) over hand-assembled bytes).

Input	Value
from	//Alice sr25519 dev key, account 0xd43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d
to	//Bob sr25519 dev key, account 0x8eaf04151687736326c9fea17e25fc5287613693c912909cb226aa4794f26a48
kind	0 (Offer)
agreement	Some((to, 3))
nonce	1
expires_at_block	1200000
body (plaintext)	ASCII offer: 25 CMN for report #7, deliver by block 1200000
genesis_hash	0x1111…11 (32 bytes of 0x11, a stand-in for the vectors)

Output	Value
payload_hash	0xd9fa590db58b192d3e27054e3210b8e905fc0f1c043dfce81ec6bd3bbaad55ec
SCALE(envelope) (147 bytes)	0x01d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d8eaf04151687736326c9fea17e25fc5287613693c912909cb226aa4794f26a4800018eaf04151687736326c9fea17e25fc5287613693c912909cb226aa4794f26a48030000000100000000000000804f1200d9fa590db58b192d3e27054e3210b8e905fc0f1c043dfce81ec6bd3bbaad55ec
signing_hash	0xe5bba61ef4d05553662760a8ae8ff35576c165b88e4441ea5a96f9f9fd99d421

sr25519 signatures are randomised, so no signature is pinned. The tests sign signing_hash with //Alice, check that it verifies, and check that it does not verify under //Bob's key, under another genesis hash, or with kind changed to Accept. The dev keys are public and hold nothing; do not reuse them.

Versioning

Any change to the struct, the prefix or the hash is a new version with a new prefix (ScalarMsg/v2|). Verifiers reject versions they do not implement. MessageKind variants are append-only on chain, and a new kind is usable in envelopes as soon as the chain accepts it.