A method of generating a plurality of different one time tokens, the method comprising: acquiring a first numerical value from a predetermined sequence of consecutive numerical values; applying format preserving encryption to the acquired first numerical value to generate a first numerical one time token; acquiring a second numerical value from the predetermined sequence of consecutive numerical values, the acquired second numerical value being different to the acquired first numerical value; and applying the format preserving encryption to the acquired second numerical value to generate a second numerical one time token.