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Cross-Chain Announcement Payload (96 bytes)

Status: Draft · Version: 1 · License: CC0-1.0

The fixed 96-byte body carried inside a Universal Announcement Bus (UAB) message. It is the chain-neutral, self-describing form of a CSAP stealth payment announcement, sized so a scanner can run the EIP-5564 view-tag pre-filter without parsing chain-specific event encodings. See UAB.md for how the body is transported (Wormhole) and re-emitted.

Layout

All multi-byte integers are big-endian. The body is exactly 96 bytes.

Offset Size Field Description
0 1 view_tag EIP-5564 view tag = MSB of Keccak-256(shared_secret). The cheap pre-filter.
1 33 ephemeral_pubkey Compressed secp256k1 ephemeral public key.
34 32 stealth_address Recipient one-time address. EVM 20-byte addresses are left-padded to 32 bytes; Solana pubkeys are 32 bytes as-is.
66 2 source_chain_id Wormhole chain id of the origin chain (Ethereum = 2, Solana = 1).
68 4 scheme_id CSAP / ERC-5564 stealth scheme. 1 = secp256k1 with view tags.
72 24 metadata Sender-defined bytes (e.g. an encrypted payment id). The view tag is not repeated here.

1 + 33 + 32 + 2 + 4 + 24 = 96.

Reconstructing a native announcement

A UAB receiver reconstructs the local EIP-5564 / CSAP announcement so existing scanners need no change:

  • schemeId = scheme_id
  • stealthAddress = stealth_address (EVM consumers take the low 20 bytes)
  • ephemeralPubKey = ephemeral_pubkey
  • metadata = view_tag ‖ metadata (25 bytes, view tag first — the EIP-5564 convention)

Scope and versioning

This v1 format carries CSAP payment announcements. The 24-byte metadata budget is deliberately small so the body stays fixed-size for the scanner fast path; it does not carry large PSR V2 attestation metadata (130 bytes: schema id / issuer / uid / nonce), which remains chain-local in v1. A future length-delimited body is reserved for cross-chain attestation relay; until then UAB messages are exactly 96 bytes and consumers MUST reject any other length.

Chain id note

source_chain_id is the Wormhole chain id of the emitting chain — not the EVM block.chainid, and not an ASCII tag. (An earlier draft wrote 0x534F for Solana; the correct value is 1.) The Wormhole VAA envelope independently carries emitterChainId; receivers MUST treat the VAA's emitterChainId as authoritative and MAY cross-check it against source_chain_id.

Copyright

Copyright and related rights waived via CC0-1.0.