Cyclic redundancy check (CRC) is an algorithm that can be used to detect changes in data. In a Zip or RAR archive, when a file item is archived into it, other than the compressed file data, the CRC value of the uncompressed file data is also calculated and stored together. Thus when the file item is extracted, the unzip or unrar program should also calculate the CRC value of the uncompressed data and compare it with the stored one. If they are the same, then the file data should be intact. However, if they are different, then this is called a CRC error, which means the file data has been changed. So, we use the CRC value to check whether the file data in the archive are corrupt or not.

CRC value is very strict. So even if one byte of the file data is changed, the CRC value will be inconsistent with the original one. In such a case, many Zip or RAR apps will refuse to unzip or unrar the file data. But actually, most of the bytes are still OK. Our DataNumen Zip Repair and DataNumen RAR Repair can recover these data from the archive, so to reduce the data losses.

Also sometimes, the file data are intact, but the CRC value itself is damaged. In such a case, when other Zip or RAR apps refuse to extract the file data, our DataNumen Zip Repair and DataNumen RAR Repair can also help you.