Archive Files

An archive is a special file used to store one or more files or folders. During the creation of an archive, compression is frequently used, reducing the amount of disk space required for the data and lessening the time taken to upload or download the material over the Internet.

The most common formats encountered on a Mac OS machine are as follows:-

Disk Image (Mac OS X)
.dmg devi

This special file stores the contents of a ‘virtual disk’ as a disk image. The ‘disk’ containing the original material is mounted on the Mac OS X desktop whenever you open such a file. These archives can be created using Apple’s Disk Utility (or Disk Copy in older systems) and other programs. These files are exclusive to Mac OS X, and can’t be opened in the Classic Mac OS. They don’t have a resource fork, so they can be sent over the Internet without any further encoding.

Disk Image (Classic Mac OS)
.img/.image rohd/dlmg/dimg

An older file format, similar to that described above, but in this instance produced by Disk Copy, ShrinkWrap and other applications in the Classic Mac OS. Documents of this kind contain a Mac OS resource fork and therefore must be encoded prior to transfer over the Internet.

Disk Self-Mounting Image
.smi APPL

A self-mounting image (SMI) is similar to the disk images described above, but is supplied in the form of a Mac OS application, allowing you to mount the image without any extra software. Modern SMIs contain Carbon code, so you can use them in both the Classic Mac OS and in OS X.

Tape Archive (TAR)
.tar TARF

An uncompressed format, native to the Unix operating system and Mac OS X, containing data from one or more files, as originally produced using tar archiver or similar software in Unix.

TAR with BZip Compression
.tbz Bzp2

This compressed form of tape archive (see above) is less common than the GZip variety (see below).

TAR with GZip Compression
.tgz Gzip

This compressed form of tape archive (see above) is often used in Mac OS X.

See below

A range of proprietary files produced by Aladdin, as described in the next section.

Unix Compress
.z/.Z/.taz ZIVU

A tape archive (see above) compressed using the Lempel-Ziv process.

.zip ZIP

The standard archive format for a PC running any version of Windows. This kind of archive can also be created in the Mac OS using Aladdin’s DropZip or similarly decompressed using StuffIt Expander or StuffIt Deluxe (see below).

The following outdated or less common formats may also be encountered:-

.arc mArc

An archive containing data from one or more files, as created with Arc or similar application.

Compact Pro
.cpt PACT

A Classic Mac OS archive containing one or more files, complete with Finder information and resource forks. These archives were originally created and expanded using Compact Pro or the older Compactor application, although freeware utilities such as cptExpand and Extractor, as well as other products from Aladdin, can also be used for decompression.

.dd DD01

A Classic Mac OS archive containing one or more files, complete with Finder information and resource forks. This type of file was originally created by Symantec’s DiskDoubler but can also be expanded with Aladdin products or with a freeware application such as DDExpand.

.pit PIT

A Classic Mac OS archive containing one or more files, complete with Finder information and resource forks: encryption is optional. These ancient files were originally created using PackIt, although they can also be expanded with Aladdin products or with the freeware Unpit application.

The following formats are often found on Windows machines:-

Microsoft Cabinet
.cab ????

A special format devised by Microsoft for storing files in the Windows operating system. The contents of such an archive can be viewed and extracted using a Windows application called PicoZip.

Windows Installation File
.??_ ????

These files, identified by a filename extension containing any two characters followed by _ (underscore), are often compressed but are only used for the installation of Windows software.


This family of files are found on Mac OS computers, although they can also be used on Windows machines. The creation of a StuffIt archive is known as stuffing and can be achieved by using Aladdin’s DropStuff, although those with an eye for complexity might like to try StuffIt Deluxe.

StuffIt archives come in one of four basic forms:-

Type ​of ​ArchiveExtension
Mac ​OS ​X ​StuffIt ​file ​*.sitx
Standard ​StuffIt ​file.sit
Classic ​Mac ​OS ​self-extracting ​archive ​(SEA).sea
Windows ​self-extracting ​archive ​(SEA).exe
* Requires StuffIt 7.x or later software in the Classic Mac OS or Mac OS X

The self-extracting archive (SEA) formats are useful in that they don’t need an application for decompression: you simply double-click the archive, choose where you want the decompressed items to go and the job’s done. However, a Classic Mac OS SEA contains a resource fork, is larger than a standard archive and won’t decompress itself on a Windows machine. In other words, a Classic SEA is fine for sharing with other Classic Mac OS users but not for other platforms or the Internet.

All StuffIt files can be unstuffed using StuffIt Expander, which is often the default helper application for handling StuffIt files over the Internet. Alternatively, you can use StuffIt Deluxe to expand an entire archive or to extract individual items from within an archive.At the time of writing, the full StuffIt Expander package on the Mac OS can also handle the following file formats, which include the most common archive formats:-

Type ​of ​ArchiveExtensions
BZip.bz2, ​.bz, ​.tbz
Compact ​Pro.cpt
GZip.gz, ​.tgz
LHa.lha, ​.lhz
MIME/Base64.mime, ​.mim
Private ​
Self-extracting ​archive ​(SEA).sea
StuffIt ​X.sitx
Unix ​Compress.Z, ​.taz
UU.uu, ​.uue

as well as .img and .smi varieties of disk images. In addition, some versions of StuffIt Expander can open older files produced using DiskDoubler.

StuffIt Files in Detail

The various kinds of StuffIt file are described below, complete with each filename extension, as used on most computer platforms, and type code, as used by the Classic Mac OS.

Self-Extracting Archive (Classic Mac OS)
.sea APPL

A Classic Mac OS application program created with a StuffIt application, although a similar kind of application can also be created with Compact Pro or DiskDoubler (see below). It contains one or more Mac OS files, complete with Finder information and resource forks. This kind of file ‘self-extracts’ its contents when you double-click on it from within the Classic Mac OS environment.

Self-Extracting Archive (Windows)
.exe PCFA

A Windows application created with a StuffIt application, containing one or more files, complete with the Finder information and resource forks used in the Classic Mac OS. This kind of file ‘self-extracts’ its contents when you double-click on it from within the Windows environment.

StuffIt 1.5.1
.sit SIT!

An early form of Classic Mac OS archive containing one or more Mac files, complete with Finder information and resource forks. This kind of file can still be unstuffed using a modern application, although most recent applications can only create the newer types of StuffIt files (see below).

StuffIt 2/3/4
.sit SITD

These updated StuffIt formats (see above) can be unstuffed by modern applications, although they can’t be created by StuffIt 5.x applications. You can also process StuffIt files on a Windows machine with the appropriate version of DropStuff or the Aladdin Expander application.

StuffIt 5.x
.sit SIT5

A later version of the StuffIt format (see above) which can only be created with StuffIt 5.x or later applications. StuffIt 5.x software is also required for unstuffing. So check that any recipients of such an archive have the necessary software to decode it. This type of file doesn’t contain a resource fork, so you don’t have to encode it into a text file before sending it over the Internet.

StuffIt X
.sitx ????

An advanced version of StuffIt designed for use with Mac OS X, accommodating 512-bit encryption, 30% improvement in compression and support for long filenames. This format is supported by StuffIt Deluxe 7.x or higher and also by newer versions of StuffIt Expander in the Classic Mac OS.


StuffIt applications can also be used to create an encrypted archive, whilst other applications can be used to encrypt a single file. Encrypted items can’t be restored without entering the same password as presented during encryption. The level of security provided often doesn’t match that provided by the Data Encryption Standard (DES) used by RSA for use on the Internet. One exception to this rule is the Apple File Security application, as supplied with Mac OS 9.x.

Choosing a Password

Inevitably, encryption involves the use of a password. In choosing a password you should always avoid obvious words, such as a relative’s name or any other known connections. Below are shown the generally accepted tricks for creating passwords. Remember, a password that contains a mixture of letters, numbers and non-alphabetical characters is much harder to crack.

Whatever you do, avoid writing down the password but don’t forget it. Some people put their password in a false entry in an address book, which isn’t very secure since this trick is well known!

©Ray White 2004.