Cyclic Redundancy Check. The CRC is a 32-bit checksum system that is used to verify the integrity of data. The zip file format contains the 32-bit CRC of each file's original uncompressed data. When a file is unzipped, the CRC is calculated on the file's newly unzipped data and compared with the stored CRC of the original file. If the CRCs are the same, then the unzipped file is very probably identical to the original file, because the 32-bit CRC is able to detect single bit variations even in the largest files.