SKP header (SketchUp 2017 to 2020)

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 0C 7B 00 31 00 38 00 2E 00 30 00 2E 00
00000030  31 00 36 00 39 00 37 00 35 00 7D 00 …

The two strings of a SketchUp 2018 file: “SketchUp Model” and “{18.0.16975}”; the GUID starts at 0x3C

Fields

The length of the version string varies, so the offsets after it are those of a twelve-character version such as {18.0.16975}. 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: `{18.0.16975}`. 28 bytes here.
60 (0x3C)	16	GUID	Model GUID	A random GUID, probably the model’s identity.
76 (0x4C)	4 or more	CString	Path	Usually empty. Meaning not known.
80 (0x50)	4	u32	Saved	The time of the save as a Unix time.
84 (0x54)			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 twelve characters takes 4 + 24 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. The major number is the last two digits of the release’s year: {17.1.174} is SketchUp 2017, and {18.0.16975} and {18.0.19877} are SketchUp 2018. The string’s length changes the offsets of everything after it: a ten-character version such as {17.1.174} puts the GUID at 0x38 and the version map at 0x50, a twelve-character one at 0x3C and 0x54.

Model GUID

Sixteen bytes in Microsoft’s byte order: a 32-bit number, two 16-bit numbers, then eight bytes as they are. For example, the bytes A8 35 DD B6 D6 6B 09 4A AD 8B 8A 4C C9 D8 C8 0E read as {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 almost every file seen. One classic file holds a Windows path to a .skp file here. 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. The times match the save dates of the files seen: a file whose name ends in _180522 holds EE 4C 04 5B, 1,527,008,494, which is 22 May 2018, 17:01:34 UTC.

.SKP File Format – Header