IPSW files contain three disk image (.dmg) files in addition to the IMG3 images for the NOR and on older models, unencrypted files for the restore process. The three disk images are the update ramdisk, the restore ramdisk, and the main filesystem.
The main filesystem .dmg disk image (the largest .dmg file) is decrypted (with VFDecrypt) to an UDZO .dmg disk image containing additional checksums.
UDZO Image Format
The UDZO Image is an UDIF zlib-compressed read-only image which can be mounted on Mac OS X. UDZO Images can be converted to read/write UDRW Images with hdiutil [Archived 2017-10-07 at the Wayback Machine] on Mac OSX:
hdiutil convert -format UDRW udzo.dmg -o udrw.dmg
asr -imagescan udzo.dmg
UDRW Image Format
The UDRW Image is an UDIF read/write image which can be mounted on Mac OS X. UDRW Images can be converted to zlib-compressed read-only UDZO Images with hdiutil [Archived 2009-05-18 at the Wayback Machine] on Mac OS X:
hdiutil convert -format UDZO udrw.dmg -o udzo.dmg