transaction input – Can signature be re-used to cash out everything in the address?

When you sign the transaction with your private key, you include the hash of the entire transaction data as a message. This means the signature which is generated is specific to that transaction itself and any modification to the transaction will render the signature invalid. In this case, the user who has signed the transaction has already specified the address to which the bitcoins have to be sent. Any modification in the address will change the message digest, thus invalidating the signature.

Moreover, Bitcoin operates on an UTXO model rather than an account/address based model. Everytime you send bitcoins you consume previously unspent outputs in its entirety and create new unspent outputs. For example, let us say you have 1BTC UTXO. When you want to spend for a 0.1 BTC coffee, you will consume the entire 1 BTC UTXO, send 0.1 BTC to the coffee shop and 0.9 BTC back to yourself. This 0.9 BTC output is different from the 1 BTC output that you controlled previously.

In a transaction, these unspent outputs are specified in the transaction with the identifiers: txid and vout. After the user’s transaction gets confirmed, the txid which was referenced in the previous transaction no longer remains unspent. Thus, it is not possible to re-use the same signature even if you are sending to the same addresses again.

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