What Is ZIP CRC?
zip-crc
CRC, or ZIP CRC, is a metadata value you will often find in archive files. Read more on what CRC stands for and what the value tells you about your own file.
The CRC is a short number worked out from a file's contents at the moment it was added to an archive. It acts as a fingerprint, so that the file can later be checked to confirm it came back out of the archive exactly as it went in. CRC is short for cyclic redundancy check.
Where it comes from, and which files have it
Archives carry it. That means ZIP files and the many formats built on ZIP, including .docx, .xlsx, .epub, .jar and .apk, while RAR, 7z and other formats keep checksums of their own.
The compression program calculates the value over the file's original, uncompressed content and stores it alongside that entry, so every member of an archive has its own CRC. You cannot choose it or type it, and it is not information about you or your computer. It is derived purely from the bytes of the file.
The point of storing it is the comparison that happens later. When the archive is unpacked, the program calculates the CRC again from what it has just extracted and compares the two. If they differ, you get the familiar CRC error message in WinZip, 7-Zip or unzip, which means the extracted file does not match the one that was originally added. That points to damage somewhere along the way: an interrupted download, a failing disk, a copy that was cut short. One damaged member is enough to produce the message, even if everything else in the archive is fine.
What the value looks like
ZIP uses a variant called CRC-32, which produces a 32-bit number. It is usually displayed as eight hexadecimal digits, for example 0x8f3d2b1c, and some tools print exactly the same value as a long decimal number instead. There is no unit, and no scale on which a value is better or worse. An empty file, and a folder entry that holds no data, shows 0x00000000.
Why it matters
This is an integrity field, and it is the archive's own built-in proof that content survived storage and transfer. Finding a CRC in a metadata report does not mean anything is wrong with your archive. It is simply the value that was recorded when the archive was made; the actual check happens when you extract, not when you look.
It has a handy side use. Identical content always produces an identical CRC, so two entries with the same CRC and the same size are almost certainly the same file, which makes it a quick way to spot duplicates inside a large archive.
What it is not is a security feature. A CRC is designed to catch accidental damage, and it is short enough that a matching value can be arranged on purpose, so a CRC that checks out is not proof that nobody has altered a file. Cryptographic hashes such as SHA-256 exist for that job. Since the value is derived from content, it can also be used to tell whether two different archives contain the same file, which is occasionally useful and occasionally worth remembering before you hand an archive over.