a method for key verification through time varying item presentation based on a key hash result comprises generating a key hash result partially based on both a global identifier provided from a source and an estimated current time at that source . after generating the key hash result , a first time - varying item is produced using the key hash result as an index for a table lookup or generated based on certain bit patterns forming the key hash result . thereafter , the first time - varying item is presented for comparison with a second time - varying item being contemporaneously presented at the source . these computations are repeated , giving the impression of two views or instances of the same time - varying item . an attacker might be able to match one small portion of such a time sequence of presentations , by luck , but not any large portion of the sequence .