A system comprising: a Media Authenticity Server, comprising one or more processors that are configured: to receive a content item from a Content Creation Device, wherein the content item is a video file or an image file, to perform digital and cryptographic authentication of said Content Creation Device, and to generate for said content item a Unique Content-Item Identifier, UCII, that operably accompanies said content item, and that uniquely said content item, and that further indicates which modification operations are permitted with regard to said content item; subsequently, to receive from a Modification-Requesting Device, a Modification Request Descriptor which identifies said content item via said UCII, and which indicates a particular requested modification to video content or image content of said content item, wherein the particular requested modification includes at least one of: change or reduction of resolution, change or reduction or frame-rate, change or reduction of color depth, change of codec, video or image resizing, video or image cropping, video trimming, change of color palette, removing content from an image or video, adding content to an image or video, adapting a video or an image to a mobile electronic device, adapting a video or image to a particular screen size; to authorize said particular requested modification to said content item, only if said particular requested modification is indicated as permitted in said UCII. to receive a content item from a Content Creation Device, wherein the content item is a video file or an image file, to perform digital and cryptographic authentication of said Content Creation Device, and to generate for said content item a Unique Content-Item Identifier, UCII, that operably accompanies said content item, and that uniquely said content item, and that further indicates which modification operations are permitted with regard to said content item; subsequently, to receive from a Modification-Requesting Device, a Modification Request Descriptor which identifies said content item via said UCII, and which indicates a particular requested modification to video content or image content of said content item, wherein the particular requested modification includes at least one of: change or reduction of resolution, change or reduction or frame-rate, change or reduction of color depth, change of codec, video or image resizing, video or image cropping, video trimming, change of color palette, removing content from an image or video, adding content to an image or video, adapting a video or an image to a mobile electronic device, adapting a video or image to a particular screen size; change or reduction of resolution, change or reduction or frame-rate, change or reduction of color depth, change of codec, video or image resizing, video or image cropping, video trimming, change of color palette, removing content from an image or video, adding content to an image or video, adapting a video or an image to a mobile electronic device, adapting a video or image to a particular screen size; to authorize said particular requested modification to said content item, only if said particular requested modification is indicated as permitted in said UCII.