A method (400, 500) comprising: - transmitting a first challenge from a locker system to a mobile device of a first person; - receiving a first response from the mobile device of the first person at the locker system, wherein the first response has been generated by a backend system, which received the first challenge from the mobile device of the first person, at least based on the first challenge received by the backend system and on a first first key stored at the backend system and has been transmitted to the mobile device of the first person; - performing a first checking at the locker system whether the received first response has been generated under usage of at least the first challenge and a key that corresponds to a first second key stored at the locker system; - granting the first person access to a compartment of the locker system at least depending on a result of the first checking; - receiving, at the locker system, a second second key from the mobile device of the first person, wherein the second second key has been generated or selected by the backend system and has been transferred from the backend system to the mobile device of the first person; - storing, at the locker system, the second second key; - transmitting a second challenge from the locker system to a mobile device of a second person; - receiving a second response from the mobile device of the second person at the locker system, wherein the second response has been generated by the backend system, which received the second challenge from the mobile device of the second person, at least based on the second challenge received by the backend system and on a second first key stored at the backend system and has been transmitted to the mobile device of the second person; - performing a second checking at the locker system whether the received second response has been generated under usage of at least the second challenge and a key that corresponds to the second second key stored at the locker system; and - granting the second person access to the same compartment of the locker system at least depending on a result of the second checking; wherein the first second key is unique for the locker system; wherein the second second key is unique for the compartment or associated with a single use of the compartment only; wherein successful authentication of the first person vis-à-vis the backend system is a necessary condition for reception of the first response from the backend system; and wherein successful authentication of the second person vis-à-vis the backend system is a necessary condition for reception of the second response from the backend system. - transmitting a first challenge from a locker system to a mobile device of a first person; - receiving a first response from the mobile device of the first person at the locker system, wherein the first response has been generated by a backend system, which received the first challenge from the mobile device of the first person, at least based on the first challenge received by the backend system and on a first first key stored at the backend system and has been transmitted to the mobile device of the first person; - performing a first checking at the locker system whether the received first response has been generated under usage of at least the first challenge and a key that corresponds to a first second key stored at the locker system; - granting the first person access to a compartment of the locker system at least depending on a result of the first checking; - receiving, at the locker system, a second second key from the mobile device of the first person, wherein the second second key has been generated or selected by the backend system and has been transferred from the backend system to the mobile device of the first person; - storing, at the locker system, the second second key; - transmitting a second challenge from the locker system to a mobile device of a second person; - receiving a second response from the mobile device of the second person at the locker system, wherein the second response has been generated by the backend system, which received the second challenge from the mobile device of the second person, at least based on the second challenge received by the backend system and on a second first key stored at the backend system and has been transmitted to the mobile device of the second person; - performing a second checking at the locker system whether the received second response has been generated under usage of at least the second challenge and a key that corresponds to the second second key stored at the locker system; and - granting the second person access to the same compartment of the locker system at least depending on a result of the second checking; wherein the first second key is unique for the locker system; wherein the second second key is unique for the compartment or associated with a single use of the compartment only; wherein successful authentication of the first person vis-à-vis the backend system is a necessary condition for reception of the first response from the backend system; and wherein successful authentication of the second person vis-à-vis the backend system is a necessary condition for reception of the second response from the backend system.