The ZIP container of an XFL document

An XFL document is a standard ZIP archive, as PKWARE’s APPNOTE describes it. Flash uses only the plain parts of ZIP: no encryption, no ZIP64, no spanning across disks. All numbers are little-endian. The archive is read from the end: the end record gives the central directory, and the central directory gives each member’s local header.

Local file header

Each member starts with a local file header of 30 bytes, then its name and extra field, then its data.

Offset	Size	Field	Meaning
0	4	Signature	`50 4B 03 04` (“PK\3\4”).
4	2	Version needed	The ZIP version needed to extract the member, times ten: 10 for stored, 20 for Deflate.
6	2	Flags	Bit 3: the CRC and sizes follow the data, in a data descriptor. Bit 11: the name is UTF-8.
8	2	Method	0 stored, 8 Deflate. No other method occurs in XFL documents.
10	2	Time	MS-DOS time: hours (5 bits), minutes (6 bits), seconds ÷ 2 (5 bits).
12	2	Date	MS-DOS date: years since 1980 (7 bits), month (4 bits), day (5 bits).
14	4	CRC-32	The CRC-32 of the uncompressed data; zero here when flag bit 3 is set.
18	4	Compressed size	The size of the data as stored.
22	4	Uncompressed size	The size of the data once inflated.
26	2	Name length	`n`, in bytes.
28	2	Extra field length	`m`, in bytes.
30	`n`	Name	The member’s path, with `/` between folders, such as `LIBRARY/Tween 1.xml`.
30 + `n`	`m`	Extra field	Tagged blocks of extra data: a 2-byte tag, a 2-byte length and the data.

When flag bit 3 is set, a data descriptor follows the data: an optional signature 50 4B 07 08, then the CRC-32, the compressed size and the uncompressed size, four bytes each. The sizes in the central directory are then the ones to trust.

Members

The data of each member follows its local header. In the XFL documents examined for this site:

The XML files (DOMDocument.xml, the symbol files in LIBRARY/, PublishSettings.xml and the rest) and the PNG images kept in LIBRARY/ are compressed with Deflate (method 8, raw Deflate as in RFC 1951).
The members of bin/, the JPEG images in LIBRARY/, the mimetype member and the .xfl marker are stored (method 0), as they are.
Folders have members of their own, with no data and a name ending in /: LIBRARY/, META-INF/, LIBRARY/Assets/ and so on.

To read a member: find its central directory record, go to the local header at the offset it gives, skip the 30 bytes, the name and the local extra field (whose length can differ from the central one), and take the compressed size of bytes. Inflate them if the method is 8, and check the CRC-32. The explorer shows each member as a byte range in the file and shows what it holds, once inflated, in the model, with offsets in the decompressed data. Bytes that no record points to are shown as unlisted bytes; they are often an older copy of a member.

The member list is in the files page.

Central directory

After the last member comes the central directory: one record for each member, 46 bytes followed by the name, the extra field and a comment.

Offset	Size	Field	Meaning
0	4	Signature	`50 4B 01 02`.
4	2	Version made by	High byte: the system that made it (0 MS-DOS, 3 Unix, 19 macOS); low byte: the ZIP version times ten.
6	2	Version needed	As in the local header.
8	2	Flags	As in the local header.
10	2	Method	0 stored, 8 Deflate.
12	2	Time	MS-DOS time.
14	2	Date	MS-DOS date.
16	4	CRC-32	The CRC-32 of the uncompressed data.
20	4	Compressed size	The size as stored.
24	4	Uncompressed size	The size once inflated.
28	2	Name length	`n`.
30	2	Extra field length	`m`.
32	2	Comment length	`k`.
34	2	Disk number	The disk the member starts on: 0.
36	2	Internal attributes	Bit 0: the member is text.
38	4	External attributes	The system’s file attributes; for MS-DOS, `0x10` marks a folder.
42	4	Local header offset	Where the member’s local header starts, from the start of the file.
46	`n`	Name	The member’s path.
46 + `n`	`m`	Extra field	Tagged extra data.
46 + `n` + `m`	`k`	Comment	The member’s comment.

End of central directory

The file ends with a 22-byte record and an optional comment. A reader finds it by searching back from the end for its signature.

Offset	Size	Field	Meaning
0	4	Signature	`50 4B 05 06`.
4	2	Disk number	0.
6	2	Central directory disk	0.
8	2	Records on this disk	The number of central directory records on this disk: all of them.
10	2	Records	The number of central directory records, one a member.
12	4	Central directory size	Its size in bytes.
16	4	Central directory offset	Where it starts, from the start of the file.
20	2	Comment length	The length of the archive comment.
22	…	Comment	The archive comment.

A record count of FFFF or an offset of FFFFFFFF would mean a ZIP64 archive. XFL documents never are, and the explorer turns such a file down.

The mimetype member

Most XFL documents hold a stored member named mimetype whose 25 bytes are the text application/vnd.adobe.xfl, with no line break. Unlike in EPUB or OpenDocument files, it is not reliably the first member: in the files seen, all saved by Flash CS5.5, it comes first in some and last in others. So it cannot be used to recognise the format; DOMDocument.xml is used instead.

Oddities

In the files seen whose mimetype member comes last, the central directory size in the end record is exactly 54 bytes more than the records add up to (for example 1,474 bytes given for 1,420 bytes of records). 54 is the size of one central record for an 8-character name, 46 + 8, so it looks as if Flash counted the mimetype record twice; this is an inference. Info-ZIP’s unzip reports “missing 54 bytes” for such files and carries on. The explorer reads the records by their count, not by the size, and reports the mismatch.