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Initially UltraISO was shareware however since 2006 it has turned into commercial software. The 'Free Trial' version is limited to ISO images of 300 MB or less, effectively making it Crippleware.
ISZ format
UltraISO uses a proprietary format known as ISZ. The format is advertised as "ISO Zipped", even though it is not a simple zip archive. The format uses zlib or bzip2 to compress the data, and may use AES-128,192 or 256 encryption in the CBC mode (note that this provides no integrity protection and is vulnerable to the padding oracle attack). The file format specification is available publicly on EZB Systems's website. The format is now supported by third-party applications such as Daemon Tools, Alcohol 120% and CDemu.
Convention: Any item in this table that has the form of "A+B" or "A+B+C" indicates a disk format that spans multiple files, where A contains the bulk of the data, and B and C are sidecar files.
A screenshot of UltraISO