validate messageFrom to prevent identity spoofing

[paul-research]

Validate messageFrom to prevent spoofing

Problem

messageFrom in ReciboInfo is not validated. Anyone can set it to an arbitrary address:

recibo.sendMsg(ReciboInfo({
    messageFrom: PAUL_ADDRESS,
    messageTo: victim,
    message: encrypted_msg,
    metadata: "{}"
}))
// Event shows "Paul sent message" but actual sender was attacker

This will get flagged in audit.

Solution

require(info.messageFrom == msg.sender, "Recibo: messageFrom must match sender");

Applied to all functions in both Recibo.sol and ReciboToken.sol.

Bonus: Added missing spender != address(this) check in permitWithMsg.

Potential pushback & responses

"We're adding AA soon, won't this block relayers?"

Yes, but that's the point. Current design has no signature verification - anyone can spoof.

When we add AA, we'll add separate functions:

• sendMsg() - simple, direct calls (this PR)
• sendMsgWithSignature() - supports relayers (Phase 2)

Both need to be secure. Can't ship AA with an insecure fallback function.

"But calldata has tx.from, so we can verify off-chain"

Events are for indexing. Not every UI/indexer will parse calldata for tx.from.
Attack vector: phishing notifications ("Circle sent you a message!").

Arc Chain = financial messaging with USDC gas = higher security bar.

"messageFrom is for flexibility"

Without signature verification, it's for spoofing.

AA requires EIP-712 signatures anyway. This PR doesn't prevent AA

Breaking change

Impact: Minimal. Python client already uses sender_address correctly.
Only affects hypothetical relayer code (which doesn't exist yet and would need signatures anyway).