The invention provides a computer-implemented method (and corresponding system) for generating a blockchain transaction (Tx). This may be a transaction for the Bitcoin blockchain or another blockchain protocol. The method comprises the step of using a software resource to receive, generate or otherwise derive at least one data item; and then insert, at least once, at least one portion of code into a script associated the transaction. Upon execution of the script, the portion of code provides the functionality of a control flow mechanism, the behaviour of the control flow mechanism being controlled or influenced by the at least one data item. In one embodiment, the code is copied/inserted into the script more than once. The control flow mechanism can be a loop, such as a while or for loop, or a selection control mechanism such as a switch statement. Thus, the invention allows the generation of a more complex blockchain script and controls how the script will execute when implemented on the blockchain. This, in turns provides control over how or when the output of the blockchain transaction is unlocked.