Heartbeat

Heartbeat

The library establishes expected format for heartbeat messages signed by wallet ECDSA signing group. Heartbeat messages are constructed in such a way that they can not be used as a Bitcoin transaction preimages.

The smallest Bitcoin non-coinbase transaction is a one spending an OP_TRUE anyonecanspend output and creating 1 OP_TRUE anyonecanspend output. Such a transaction has 61 bytes (see BitcoinTx documentation): 4 bytes for version 1 byte for tx_in_count 36 bytes for tx_in.previous_output 1 byte for tx_in.script_bytes (value: 0) 0 bytes for tx_in.signature_script 4 bytes for tx_in.sequence 1 byte for tx_out_count 8 bytes for tx_out.value 1 byte for tx_out.pk_script_bytes 1 byte for tx_out.pk_script 4 bytes for lock_time

The smallest Bitcoin coinbase transaction is a one creating 1 OP_TRUE anyonecanspend output and having an empty coinbase script. Such a transaction has 65 bytes: 4 bytes for version 1 byte for tx_in_count 32 bytes for tx_in.hash (all 0x00) 4 bytes for tx_in.index (all 0xff) 1 byte for tx_in.script_bytes (value: 0) 4 bytes for tx_in.height 0 byte for tx_in.coinbase_script 4 bytes for tx_in.sequence 1 byte for tx_out_count 8 bytes for tx_out.value 1 byte for tx_out.pk_script_bytes 1 byte for tx_out.pk_script 4 bytes for lock_time

A SIGHASH flag is used to indicate which part of the transaction is signed by the ECDSA signature. There are currently 3 flags: SIGHASH_ALL, SIGHASH_NONE, SIGHASH_SINGLE, and different combinations of these flags.

No matter the SIGHASH flag and no matter the combination, the following fields from the transaction are always included in the constructed preimage: 4 bytes for version 36 bytes for tx_in.previous_output (or tx_in.hash + tx_in.index for coinbase) 4 bytes for lock_time

Additionally, the last 4 bytes of the preimage determines the SIGHASH flag.

This is enough to say there is no way the preimage could be shorter than 4 + 36 + 4 + 4 = 48 bytes.

For this reason, we construct the heartbeat message, as a 16-byte message. The first 8 bytes are 0xffffffffffffffff. The last 8 bytes are for an arbitrary uint64, being a signed heartbeat nonce (for example, the last Ethereum block hash).

The message being signed by the wallet when executing the heartbeat protocol should be Bitcoin's hash256 (double SHA-256) of the heartbeat message: heartbeat_sighash = hash256(heartbeat_message)

isValidHeartbeatMessage

function isValidHeartbeatMessage(bytes message) internal pure returns (bool)

Determines if the signed byte array is a valid, non-fraudulent heartbeat message.

Wallet heartbeat message must be exactly 16 bytes long with the first 8 bytes set to 0xffffffffffffffff.

Parameters

Name	Type	Description
message	bytes	Message signed by the wallet. It is a potential heartbeat message, Bitcoin transaction preimage, or an arbitrary signed bytes.

Return Values

Name	Type	Description
[0]	bool	True if valid heartbeat message, false otherwise.