One way is to use a generator that.. (who would've guessed that) generates the tan, another one (though no longer in use at most banks) are indexed tans – a list of one time codes that is enumerated. you are then prompted to enter "code 71" or something, you're getting the idea. the third, and currently widely used variant is mobile tan, where the transaction number is sent to your cell phone, along with some additional information like the amount to be transferred. Further reading: http://en.wikipedia.org/wiki/Transaction_authentication_numb...