Embodiments of a system that performs authentication during a financial transaction are described. During operation, this system asks a prospective customer a subset of questions from a set of questions, where the subset of questions has a probability of being different from those asked while previously or subsequently asking questions of the prospective customer or another prospective customer. Next, the system receives answers to the subset of questions from the prospective customer, thereby confirming that the prospective customer is a customer associated with a financial transaction. Then, the system provides transitory information associated with the financial transaction that, for the customer, uniquely identifies a provider of the financial transaction, thereby completing the authentication.