Abstract: Techniques for authenticating a user of a computing device (e.g., handheld, desktop, laptop) are provided. In order to grant access to any of the services provided by the computing device, the computing device displays, to the user, one or more images that are stored on the computing device. The user is required to accurately identify one or more objects depicted in the one or more images in order to gain access to the computing device. The computing device is not required to be connected to any network at the time of authentication. Authentication data that is associated with object(s) in each displayed image may have been established previously by another computing device and then provided to the computing device.

Abstract: An authentication service provides authentication information to help potential buyers determine the legitimacy of an offer to sell a good or service. For example, an authentication service stores registration information that indicates (a) that a particular item was purchased by a particular party, and (b) contact information that indicates a particular address for communicating with the party. When the registered owner of an item wants to sell the item, the owner sends an intent-to-sell message to the authentication service. In response to receiving the intent-to-sell message, the authentication service sends access information to the particular address. The access information specifies a manner of receiving authentication information about the item from the authentication service. The registered owner includes the access information in his or her advertisements for the item.