The invention comprises a computer-implemented method and system for controlling an exchange process, such as a loan, conducted between at least two parties via a blockchain such as the Bitcoin blockchain. A method according to the invention may comprise the step of generating a first blockchain transaction which includes a redeem script. The redeem script comprises a cryptographic public key associated with an initiating party and metadata which includes a hash of an exchange-related document; a redeem address; and an amount of digital currency. A second blockchain transaction is generated and published to the blockchain so as to spend the digital currency to the redeem address. This provides the advantage that the further transaction will be publically available and thus detectable by other parties once it has been published. The further transaction can provide the information necessary to trigger a response e.g. an offer from another (responding) party who publishes their response on the blockchain. Thus, the exchange process can be implemented via a multi-transaction mechanism on the blockchain rather than an alternative medium. The exchange related document may be an invitation (offer/request) which is stored off-block in a repository such as a Distributed Hash Table (DHT). The invitation may be an invitation to engage in a contract. A smart contract (and associated blockchain transaction) may be formed upon condition that a plurality of participants (e.g. lenders/borrowers) are matched with each other via one or more responses effected via transactions on the blockchain.