A method (400; 500) comprising: - receiving (406; 507), at a mobile device (3; 4) and from a backend system, an identifier of a compartment of a locker system, wherein the compartment has been determined (406; 507) by the backend system from a plurality of compartments of the locker system, - transmitting (407; 508) a request for access to the compartment from the mobile device (3; 4) to the locker system, wherein the request comprises the identifier of the compartment and wherein a challenge comprising or being based on time-dependent and/or random or pseudo-random information and further comprising the identifier of the compartment is generated by the locker system and transmitted to the mobile device (3; 4) in response to the request; - receiving (409; 510), at the mobile device (3; 4), the challenge; - transmitting (410;511) the challenge from the mobile device (3; 4) to the backend system; - receiving (412; 514), at the mobile device (3; 4), a response from the backend system, wherein the response has been generated (411; 512) by the backend system at least based on the challenge received from the mobile device (3; 4) and on a first key stored at the backend system and wherein successful authentication (401;503) of a user (31; 41) of the mobile (3; 4) device vis-à-vis the backend system is a necessary condition for reception of the response from the backend system; - transmitting (413; 515) the response from the mobile device (3; 4) to the locker system, wherein the locker system is configured to grant access to the compartment at least depending on a result of a checking whether the response received at the locker system has been generated under usage of at least the challenge and a key that corresponds to a second key stored at the locker system. - receiving (406; 507), at a mobile device (3; 4) and from a backend system, an identifier of a compartment of a locker system, wherein the compartment has been determined (406; 507) by the backend system from a plurality of compartments of the locker system, - transmitting (407; 508) a request for access to the compartment from the mobile device (3; 4) to the locker system, wherein the request comprises the identifier of the compartment and wherein a challenge comprising or being based on time-dependent and/or random or pseudo-random information and further comprising the identifier of the compartment is generated by the locker system and transmitted to the mobile device (3; 4) in response to the request; - receiving (409; 510), at the mobile device (3; 4), the challenge; - transmitting (410;511) the challenge from the mobile device (3; 4) to the backend system; - receiving (412; 514), at the mobile device (3; 4), a response from the backend system, wherein the response has been generated (411; 512) by the backend system at least based on the challenge received from the mobile device (3; 4) and on a first key stored at the backend system and wherein successful authentication (401;503) of a user (31; 41) of the mobile (3; 4) device vis-à-vis the backend system is a necessary condition for reception of the response from the backend system; - transmitting (413; 515) the response from the mobile device (3; 4) to the locker system, wherein the locker system is configured to grant access to the compartment at least depending on a result of a checking whether the response received at the locker system has been generated under usage of at least the challenge and a key that corresponds to a second key stored at the locker system.