Mobile electronic device, comprising - an interface to a short-range wireless network, - another interface to another network, - a digital key stored in the mobile electronic device for locking or unlocking an assigned lock, - an ownership tag assigned to the digital key, - a processing unit configured to, in response to a lock operation trigger, send, via the interface, a token generated dependent on the digital key, and configured to, in response to the token being sent, receive, via the interface, a lock status confirmation from the lock when present in the range of the wireless network, - wherein the processing unit is configured to, in response to an access grant trigger, send an identifier of another user to a server registering users (U,V) of the locking system including an associate encryption key, - wherein the processing unit is configured to, in response to sending the identifier to the server, receive from the server, via the other interface, an encryption key assigned to the other user of another mobile electronic device, - wherein the processing unit is configured to, in response to the receipt of the encryption key, generate another token dependent on the digital key and the encryption key, - wherein the processing unit is configured to, send the other token, via the other interface, via the server, to another mobile electronic device to enable the assigned lock to be locked or unlocked at least temporarily by the other mobile electronic device, - wherein the digital key remains stored in the mobile electronic device as digital key operable to lock or unlock the assigned lock and transmittable to the other mobile electronic device as long as the ownership tag - digital key assignment is not released, - wherein the processing unit is configured to, in response to a release trigger, send, via the interface, a release request including an ownership identification to the lock to have the lock to verify the ownership identification with the ownership identification stored in the lock, and in case of a match to delete the digital key from its storage or to mark it invalid, and to set a status of the lock to yet unassigned, and to return a release confirmation to the Z mobile electronic device provided the ownership verification has a positive result, - wherein the processing unit is configured to, in response to the receipt of the release confirmation, release the ownership tag - digital key assignment by one or more of: - marking the digital key as inactive; - removing the digital key from the mobile electronic device; - removing the ownership tag. - an interface to a short-range wireless network, - another interface to another network, - a digital key stored in the mobile electronic device for locking or unlocking an assigned lock, - an ownership tag assigned to the digital key, - a processing unit configured to, in response to a lock operation trigger, send, via the interface, a token generated dependent on the digital key, and configured to, in response to the token being sent, receive, via the interface, a lock status confirmation from the lock when present in the range of the wireless network, - wherein the processing unit is configured to, in response to an access grant trigger, send an identifier of another user to a server registering users (U,V) of the locking system including an associate encryption key, - wherein the processing unit is configured to, in response to sending the identifier to the server, receive from the server, via the other interface, an encryption key assigned to the other user of another mobile electronic device, - wherein the processing unit is configured to, in response to the receipt of the encryption key, generate another token dependent on the digital key and the encryption key, - wherein the processing unit is configured to, send the other token, via the other interface, via the server, to another mobile electronic device to enable the assigned lock to be locked or unlocked at least temporarily by the other mobile electronic device, - wherein the digital key remains stored in the mobile electronic device as digital key operable to lock or unlock the assigned lock and transmittable to the other mobile electronic device as long as the ownership tag - digital key assignment is not released, - wherein the processing unit is configured to, in response to a release trigger, send, via the interface, a release request including an ownership identification to the lock to have the lock to verify the ownership identification with the ownership identification stored in the lock, and in case of a match to delete the digital key from its storage or to mark it invalid, and to set a status of the lock to yet unassigned, and to return a release confirmation to the Z mobile electronic device provided the ownership verification has a positive result, - wherein the processing unit is configured to, in response to the receipt of the release confirmation, release the ownership tag - digital key assignment by one or more of: - marking the digital key as inactive; - removing the digital key from the mobile electronic device; - removing the ownership tag.