The Nero BackItUp Compressed file, NCO, is commonly used to create a compressed backup for CD or DVD storage. Although the file format is only used by Nero, the actual file format uses ZIP to perform the file compression. NCO files may also be password protected and may be encrypted using a 56-bit key. Because of the way that these files are created, a non-encrypted NCO file may also be opened and viewed by standard unzipping programs such as WinZip.