Improve GraphcastMessage typing and signing

[hopeyen]

Problem statement

• Grapchast message takes optional radio payload because of prost::Message implication to take an Optional field; this doesn't make sense since the point of Graphcast message is to send radio payload message. Investigate ways to provide payload without making it optional
• Graphcast agent wallet signs over radio payload field inside Graphcast message. Security can be improved by signing over Graphcast message fields such as block hash, block number, nonce, identifier, graph_account, etc.

Expectation proposal

• Require payload in GraphcastMessage
• Update GraphcastMessage structure and signed data types

Current Graphcast Message is structured as below such that only payload:T is signed by the signature

pub struct GraphcastMessage<T>
where
    T: Message
        + ethers::types::transaction::eip712::Eip712
        + Default
        + Clone
        + 'static
        + async_graphql::OutputType,
{
    pub identifier: String,                // not necessary as included in payload: T
    pub payload: T,                        // EIP712 signed
    pub nonce: i64,                        // not EIP712 signed, secure Graphcast level
    pub network: String,                   // not EIP712 signed, optional for radios
    pub block_number: u64,                 // not EIP712 signed, secure Graphcast level, may not be available for message sender
    pub block_hash: String,                // not EIP712 signed, secure Graphcast level, may not be available for message sender
    pub graph_account: String,             // not EIP712 signed, secure Graphcast level
    pub signature: String,                 // Signature field after signing over payload: T
}

Two options for Message layered structure 1.

• Thinking about other message types such as "upgrade_version_message" from subgraph developer, fields such as "block_number", "block_hash" shouldn't be needed in the message. These two fields only make sense in context for senders and receivers with valid block provider
• The check "valid_block_hash" might be on a message type specific level to poi-radio
• Move appropriate fields to Radio Payload message that're already of Eip712 types
  • Propose to move nonce, network, graph_account so that the fields can be securely signed .
  • Propose to move block_number, block_hash so that general operators of a radio operator might not need access from a block provider.
  • GraphcastMessage has identifier, payload, signature left 2.
• Enforce Eip712 message typing on Graphcast message,
• block_number, block_hash becomes optional,
• use graph_account or some constant as a placeholder on signature when building GraphcastMessage, sign over message and then replace the signature field
• require extra handling at decryption as well, it must read the message fields and replace graph_account before recovering sender address from the signature

Alternative consideration

• use Waku event timestamp for message nonce
• use Waku signatures

[pete-eiger]

Love it. Also think the first approach is better, it's cleaner in the sense that it doesn't leave the Optional fields in the top-level message and enables more freedom when defining the Radio-specific message types.