A method comprising: receiving, by a server computer and at a first time, device information and a resource provider identifier from a portable communication device of a user, wherein the resource provider identifier was received by the portable communication device from a base station that is located at a resource provider location identified by the resource provider identifier; electronically searching, by the server computer, a database for an account identifier associated with the portable communication device using the device information; hashing, by the server computer, at least the resource provider identifier and the account identifier to generate a cryptographic pattern; transmitting, by the server computer, the generated cryptographic pattern via the portable communication device and then the base station to an access device that stores the cryptographic pattern; receiving, by the server computer from the access device involved in a transaction conducted with a portable transaction device of a user at a second time that is later than the first time, an authorization request message comprising an account identifier received by the access device from the portable transaction device at the second time, the authorization request message being generated in the event that the access device identifies a match between the generated and stored cryptographic pattern and a cryptographic pattern generated at the second time by the access device hashing the resource provider identifier and the account identifier that it received from the portable transaction device of a user at the second time, whereby the matching of said cryptographic pattern generated at the second time and said stored cryptographic pattern confirms the presence of the portable communication device at the access device at the second time; and determining, by the server computer, based on the authorization request message that the user of the portable communication device is conducting the transaction at the access device with the portable transaction device at the second time. receiving, by a server computer and at a first time, device information and a resource provider identifier from a portable communication device of a user, wherein the resource provider identifier was received by the portable communication device from a base station that is located at a resource provider location identified by the resource provider identifier; electronically searching, by the server computer, a database for an account identifier associated with the portable communication device using the device information; hashing, by the server computer, at least the resource provider identifier and the account identifier to generate a cryptographic pattern; transmitting, by the server computer, the generated cryptographic pattern via the portable communication device and then the base station to an access device that stores the cryptographic pattern; receiving, by the server computer from the access device involved in a transaction conducted with a portable transaction device of a user at a second time that is later than the first time, an authorization request message comprising an account identifier received by the access device from the portable transaction device at the second time, the authorization request message being generated in the event that the access device identifies a match between the generated and stored cryptographic pattern and a cryptographic pattern generated at the second time by the access device hashing the resource provider identifier and the account identifier that it received from the portable transaction device of a user at the second time, whereby the matching of said cryptographic pattern generated at the second time and said stored cryptographic pattern confirms the presence of the portable communication device at the access device at the second time; and determining, by the server computer, based on the authorization request message that the user of the portable communication device is conducting the transaction at the access device with the portable transaction device at the second time.