SKP header (SketchUp 3 to 2016)

The file starts with five plain values: two strings that name the file and the SketchUp that saved it, a GUID, a third string and the time of the save. The version map follows straight after them. All numbers are little-endian.

00000000  FF FE FF 0E 53 00 6B 00 65 00 74 00 63 00 68 00
00000010  55 00 70 00 20 00 4D 00 6F 00 64 00 65 00 6C 00
00000020  FF FE FF 0A 7B 00 38 00 2E 00 30 00 2E 00 33 00
00000030  31 00 31 00 37 00 7D 00 A8 35 DD B6 D6 6B 09 4A
00000040  AD 8B 8A 4C C9 D8 C8 0E FF FE FF 00 F6 09 64 4D
00000050  FF FF 00 00 0B 00 43 56 65 72 73 69 6F 6E 4D 61
00000060  70

The header of a SketchUp 8 file: “SketchUp Model”, “{8.0.3117}”, the GUID, an empty path and the save time, then the start of the version map at 0x50

Fields

The length of the version string varies, so the offsets after it are those of the example. Read each string’s length to find the next field.

Offset	Size	Type	Name	Meaning
0	32	CString	Signature	The text `SketchUp Model`.
32 (0x20)	4 + 2 × length	CString	Version	The version of SketchUp that saved the file, in braces: `{8.0.3117}`. 24 bytes here.
56 (0x38)	16	GUID	Model GUID	A random GUID, probably the model’s identity.
72 (0x48)	4 or more	CString	Path	Usually empty. Meaning not known.
76 (0x4C)	4	u32	Saved	The time of the save as a Unix time.
80 (0x50)			Version map	The first object of the archive.

Strings

Every string in the file is an MFC CString as a Unicode CArchive writes it:

Bytes	Meaning
`FF`	A byte FF.
`FE FF`	The word FFFE. Together with the FF before it, this marks a Unicode string.
length	The length in UTF-16 code units: one byte if below 255; otherwise `FF` and a 16-bit length if below FFFE; otherwise `FF FF FF` and a 32-bit length.
text	The text, length × 2 bytes of UTF-16LE. There is no terminating zero.

So the empty string is FF FE FF 00, and a string of ten characters takes 4 + 20 bytes. The files seen only use the one-byte length. That longer strings escape to a word and a dword follows MFC’s CArchive, and OpenSKP’s reader describes the same three forms.

To read a string: check for FF FE FF; read a byte; if it is below FF it is the length; otherwise read a 16-bit word, and if that is below FFFF it is the length; otherwise read a 32-bit length. Then read twice that many bytes as UTF-16LE.

Signature

The text SketchUp Model as a CString: FF FE FF, the length 14 (0E) and 28 bytes of UTF-16LE. These 32 bytes are the same in every SKP file of every version, and are what identifies the file as a SketchUp model.

Version

The version of SketchUp that saved the file, as a CString in braces: the major version, the minor version and the build, separated by dots, such as {8.0.3117}, {15.3.329} or {16.1.1449}. From SketchUp 2013 the major number is the last two digits of the release’s year. The string is 24 bytes long for {8.0.3117}, and its length changes the offsets of everything after it: with the nine characters of {6.4.112} the version map starts at 0x4E, not 0x50.

Fab Lab Iceland’s 2008 notes on a SketchUp 6 file (6.4.112) place the name CVersionMap at offset 85, counted from 1. That is byte 84 (0x54) counted from 0, which is exactly where it falls with a nine-character version and the fields on this page, so the SketchUp 6 header has the same fields.

Model GUID

Sixteen bytes in Microsoft’s byte order: a 32-bit number, two 16-bit numbers, then eight bytes as they are. So A8 35 DD B6 D6 6B 09 4A AD 8B 8A 4C C9 D8 C8 0E is {B6DD35A8-6BD6-4A09-AD8B-8A4CC9D8C80E}.

In every file seen the GUID is different and is a random (version 4) GUID: the third group starts with 4 and the fourth with 8, 9, A or B. That it identifies the model is an inference.

Path

A CString that is empty (FF FE FF 00) in all but one of the files seen. That one holds a Windows path to a .skp file. What it means is not known; it may be where the model was saved or where it came from.

Saved

A 32-bit Unix time: seconds since 1 January 1970, UTC. In the example F6 09 64 4D is 1,298,401,782, which is 22 February 2011, 19:09:42 UTC. The times match the save dates of the files seen.

.SKP File Format – Header