The present disclosure provides methods and systems for ensuring the security of a blockchain and associated network, and for enabling the establishment of consensus regarding the state of the blockchain. A method of the disclosure may be implemented by one or more nodes on a blockchain network, using a non-parallelisable algorithm to calculate an output based on a computational difficulty parameter, a hash of at least one blockchain transaction; and/or a hash of at least one blockchain block header. The non-parallelisable, inherently sequential algorithm comprises at least one of the following operations or a combination thereof: a recursive operation, a modular exponentiation and/or a repeated squaring operation.