In an exemplary method that is performed by a first computer system, one or more file blocks are identified within a target file. The one or more file blocks comprise a desired portion of the target file. At least one file block is downloaded from a second computer system. However, the entire target file is not downloaded. A previously calculated validation value is received for a file block that has been downloaded. A new validation value is calculated for the downloaded file block. If the new validation value for the file block does not match the previously calculated validation value for the file block, the file block is discarded.