A user device may receive an identifier without prior provisioning from a server. The server may be configured to: receive, from a user device a first identifier, a first value of a second identifier, and an instruction to register the first value with the first identifier; register the first value with the first identifier based on determining that the second identifier is available for registration; receive request for subscription to receive a notification when a second user device registers with the first identifier; provide, to the user device, a list of unavailable or available second values of the second identifiers; receive, from the user device, a second instruction to register one of the available second values with the first identifier and with the user device; and receive, from the user device, a third instruction including an instruction to unregister the first value with the first identifier.