fix: reject EIP-1559 transactions missing chain_id

[BitHighlander]

Problem

hash_rlp_number(0) returns immediately without hashing any bytes:

static void hash_rlp_number(uint32_t number) {
  if (!number) { return; }   // hashes nothing
  ...
}

But rlp_calculate_number_length(0) returns 1 (for the 0x80 byte that should represent integer 0 in RLP).

For an EIP-1559 transaction sent without chain_id (has_chain_id = false), chain_id defaults to 0. The length calculation adds 1 byte to rlp_length for it, but hash_rlp_number(0) hashes nothing. The RLP list header claims N bytes but only N−1 are hashed — corrupting the keccak pre-image. The resulting signature recovers to a wrong address.

The legacy EIP-155 path is safe because send_signature() guards with if (chain_id) { hash_rlp_number(chain_id); }. The EIP-1559 path in ethereum_signing_init() had no such guard:

if (ethereum_tx_type == ETHEREUM_TX_TYPE_EIP_1559) {
    hash_rlp_number(chain_id);   // silent corruption if chain_id == 0
}

Fix

Fail fast at the input validation stage. EIP-1559 requires chain_id by spec (EIP-1559 §4.2: "chainId is the chain ID of the signing chain").

if (ethereum_tx_type == ETHEREUM_TX_TYPE_EIP_1559 && !msg->has_chain_id) {
    fsm_sendFailure(FailureType_Failure_SyntaxError, _("EIP-1559 requires chain_id"));
    ethereum_signing_abort();
    return;
}

Notes

• No host sends EIP-1559 without chain_id today, so this is not user-visible
• The companion fix for chain_id >= 256 wrong hash is PR fix: EIP-1559 chainId hashing broken for chainId >= 256 #427

[Copilot]

Pull request overview

This PR prevents malformed EIP-1559 Ethereum transactions (type 0x02) from being signed when chain_id is omitted, avoiding a mismatch between the computed RLP length and the bytes actually hashed (which can corrupt the keccak preimage and lead to signatures recovering to the wrong address).

Changes:

• Add an explicit input-validation check to reject EIP-1559 transactions when msg->has_chain_id == false.
• Return a Failure_SyntaxError with a specific message and abort signing early.

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