To make a payment, a smartphone presents artwork for a payment card (e.g., a Visa card) that has been selected by a user from a virtual wallet of such cards. Encoded in the displayed artwork is payment information that has been encrypted with a context-dependent session key. A cooperating system (e.g., a retailer's point of sale system) uses a camera to capture an image of the artwork, and independently creates the session key from its own context sensor(s), enabling decryption of the payment information. Such technology provides a superior transaction security model at a fraction of the cost of competing chip card payment systems (which require, e.g., expensive physical cards, and single-purpose reader hardware). A great variety of other features and arrangements are also detailed.