StuffItFormat

Archive header

All types larger than one byte in size are stored in big-endian order, as expected from a Mac format.

Offset	Size	Meaning
0	4	Magic number 1 (Can have various values, see table below)
4	2	Number of files (Meaning not quite clear, perhaps top-level files only)
6	4	Total size of archive
10	4	Magic number 2 (0x724c6175)
14	1	Version (Meaning not quite clear)
15	7	Unknown

Known values for "Magic number 1":

SIT!
ST46
ST50
ST60
ST65
STin
STi2
STi3
STi4

The archive header is followed by the first file header.

File headers

Offset	Size	Meaning
0	1	Resource fork compression method
1	1	Data fork compression method
2	1	File name length
3	63	File name
66	4	Mac OS file type
70	4	Mac OS file creator
74	2	Mac OS Finder flags
76	4	Creation date in classic Mac OS format (seconds since 1904)
80	4	Modification date in classic Mac OS format (seconds since 1904)
84	4	Resource fork uncompressed length
88	4	Data fork uncompressed length
92	4	Resource fork compressed length
96	4	Data fork compressed length
100	2	Resource fork CRC-16
102	2	Data fork CRC-16
104	6	Unknown
110	2	Header CRC-16

The "compression method" fields use the lowest four bits to encode the actual compression method, from 0 to 15 (0 means no compression, and see StuffItSpecs for a list of the other algorithms used). Bit 4 indicates whether the data is encrypted (the algorithm used is unknown).

If the compression method field is 32, this indicates that the entry marks the start of a folder. The folder name is taken from the file name field as usual.

If the compression field is 33, this indicates the end of a folder. The size of the header is 112 bytes in all cases.

The compressed data follows each file header. The resource fork data is first, follwed by the data fork data. The next file header is at "Resource fork compressed length" plus "Data fork compressed length" bytes from the end of the last header.

CRC algorithm

The CRC-16 algorithm used is the CCITT one with polynomial 0x1021 and no pre- or post-conditioning.