The use of compression and encryption precludes the possibility of performing file delta calculation at the server. Performing the delta calculation at the client also increases server scalability and efficiency. As each block is stored on the server the client stores a 64-bit CRC (two 32-bit CRCs generated from different polynomials) associated with that block. During the next backup if a file has been changed (its modified date/time stamp has changed or its size has changed) then the client will compare the new 64-bit CRC with the stored CRC. If the CRC has changed the client will upload the new compressed and encrypted block. Otherwise the client will tell the server that block has not changed. The CRC block fingerprints and other data are protected by transactions such that if the backup of a file fails, all 64-bit CRCs and other information are rolled back to the consistent state.