Abstract: A set of user accounts are maintained, each associated with a corresponding mobile device. Each user account comprises a set of user-identifying features, including at least one of: an image of a user-identifying asset, and information that is derivable from an image of a user-identifying asset. Each user account also comprises the location of the associated mobile device and transactional data, such as payment card data, for performing a transaction. An image of a user-identifying asset is captured at a known service location, and a match is determined with one of the user accounts dependent on image and location similarity. The use of transactional data from the matched user account is therefore authorized, and so service provision at the service location is permitted.