A person's smartphone serves as a security token—used variously to establish the user's authorization, and to test the bona fides of the counter-party system. In one illustrative ATM arrangement, a bank's computer system and the user's smartphone share secret information across a network. The smartphone employs this shared secret to generate a corresponding image, which is displayed on its screen. The displayed image is sensed by a camera in the ATM, which sends corresponding data back to the bank computer system. The bank computer system analyzes the received data and responds by sending information to the ATM authorizing a transaction. By such arrangement, authentication of the transaction involves one-way optical communication of visual information between the ATM and the smartphone, and two-way network communication of information between the ATM and the bank computer system. A great number of other features and arrangements are also detailed.

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