A computer-implemented security may be implemented on a blockchain comprising applying a one-way function to a first secret value to create a first veiled secret value; communicating the first veiled secret value to a user; receiving a second veiled secret value from the user, wherein the second veiled secret value is created by applying a one-way function to the second secret value; and constructing a first blockchain transaction comprising the first veiled secret value and the second veiled secret value, the first blockchain transaction arranged to be unlockable to transfer control of a first resource upon provision of both the first secret value and the second secret value to a respective blockchain transaction.