A computer-implemented method, comprising: receiving from a client a request for a download of a digital item, wherein the digital item includes existing data portions (122, 124, 126; 302-316); and characterised by: creating a modified digital item from the digital item, wherein the modified digital item includes N existing data portions replaced with N invalid data portions (304, 314), wherein N is an integer greater than or equal to one; providing M existing data portions (302, 306-312, 316) not replaced with the N invalid data portions and the N existing data portions replaced with the N invalid data portions (304, 306) to the client; receiving from the client an indication of receipt of a download of the M existing data portions not replaced with the N invalid data portions and the N existing data portions replaced with the N invalid data portions; for each of the N existing data portions replaced with the N invalid data portions: providing to the client a location of the invalid data portion; providing to the client a respective valid data portion to replace the invalid data portion; and receiving from the client an indication (132, 136) of receipt of each of the respective valid data portions; and providing a notification to the client indicating that the digital item has been downloaded. receiving from a client a request for a download of a digital item, wherein the digital item includes existing data portions (122, 124, 126; 302-316); and characterised by: creating a modified digital item from the digital item, wherein the modified digital item includes N existing data portions replaced with N invalid data portions (304, 314), wherein N is an integer greater than or equal to one; providing M existing data portions (302, 306-312, 316) not replaced with the N invalid data portions and the N existing data portions replaced with the N invalid data portions (304, 306) to the client; receiving from the client an indication of receipt of a download of the M existing data portions not replaced with the N invalid data portions and the N existing data portions replaced with the N invalid data portions; for each of the N existing data portions replaced with the N invalid data portions: providing to the client a location of the invalid data portion; providing to the client a respective valid data portion to replace the invalid data portion; and receiving from the client an indication (132, 136) of receipt of each of the respective valid data portions; and providing a notification to the client indicating that the digital item has been downloaded. creating a modified digital item from the digital item, wherein the modified digital item includes N existing data portions replaced with N invalid data portions (304, 314), wherein N is an integer greater than or equal to one; providing M existing data portions (302, 306-312, 316) not replaced with the N invalid data portions and the N existing data portions replaced with the N invalid data portions (304, 306) to the client; receiving from the client an indication of receipt of a download of the M existing data portions not replaced with the N invalid data portions and the N existing data portions replaced with the N invalid data portions; for each of the N existing data portions replaced with the N invalid data portions: providing to the client a location of the invalid data portion; providing to the client a respective valid data portion to replace the invalid data portion; and receiving from the client an indication (132, 136) of receipt of each of the respective valid data portions; and providing to the client a location of the invalid data portion; providing to the client a respective valid data portion to replace the invalid data portion; and receiving from the client an indication (132, 136) of receipt of each of the respective valid data portions; and providing a notification to the client indicating that the digital item has been downloaded.