A computer system for controlling delivery of media assets encrypted according to a common encryption, CENC, format in an over-the-top service which governs the delivery of a set of media assets to a group of authorised users, the computer system comprising: a store configured to: store a plurality of environment identifiers, each environment identifier identifying an environment, at least one environment being associated with an over-the-top service, each environment defined by configuration data which defines: a) multiple DRM technologies for decrypting the same asset at multiple end user platforms, each DRM technology being associated with its own set of default license properties; b) at least one software plug-in to be instantiated to perform a verification method to verify if an end user for delivery of an asset is valid; store a plurality of environment rules, each environment rule being specific to one of the plurality of environments, each rule defining an association of an asset to be delivered to one or more of the defined environments; and store a key identifier for each media asset to be delivered, the key identifier uniquely identifying the media asset and an identity of each environment associated to that asset, a license server (15, 28) operable to: receive a request from an end user device (14, 16, 18) identifying a key identifier and an environment identifier, the environment identifier identifying an environment at the end user device; identify one or more environments associated with the media asset identifier based on the key identifier; check if said one or more environments correspond to the environment identifier; and responsive to the check being satisfied, return a response to the end user, the response controlling the delivery of the media asset identified in the end user request. a store configured to: store a plurality of environment identifiers, each environment identifier identifying an environment, at least one environment being associated with an over-the-top service, each environment defined by configuration data which defines: a) multiple DRM technologies for decrypting the same asset at multiple end user platforms, each DRM technology being associated with its own set of default license properties; b) at least one software plug-in to be instantiated to perform a verification method to verify if an end user for delivery of an asset is valid; store a plurality of environment rules, each environment rule being specific to one of the plurality of environments, each rule defining an association of an asset to be delivered to one or more of the defined environments; and store a key identifier for each media asset to be delivered, the key identifier uniquely identifying the media asset and an identity of each environment associated to that asset, a) multiple DRM technologies for decrypting the same asset at multiple end user platforms, each DRM technology being associated with its own set of default license properties; b) at least one software plug-in to be instantiated to perform a verification method to verify if an end user for delivery of an asset is valid; store a plurality of environment rules, each environment rule being specific to one of the plurality of environments, each rule defining an association of an asset to be delivered to one or more of the defined environments; and store a key identifier for each media asset to be delivered, the key identifier uniquely identifying the media asset and an identity of each environment associated to that asset, store a plurality of environment rules, each environment rule being specific to one of the plurality of environments, each rule defining an association of an asset to be delivered to one or more of the defined environments; and store a key identifier for each media asset to be delivered, the key identifier uniquely identifying the media asset and an identity of each environment associated to that asset, a license server (15, 28) operable to: receive a request from an end user device (14, 16, 18) identifying a key identifier and an environment identifier, the environment identifier identifying an environment at the end user device; identify one or more environments associated with the media asset identifier based on the key identifier; check if said one or more environments correspond to the environment identifier; and receive a request from an end user device (14, 16, 18) identifying a key identifier and an environment identifier, the environment identifier identifying an environment at the end user device; identify one or more environments associated with the media asset identifier based on the key identifier; check if said one or more environments correspond to the environment identifier; and responsive to the check being satisfied, return a response to the end user, the response controlling the delivery of the media asset identified in the end user request.