BNK Format (Halloween Harry)

BNK Format (Halloween Harry)

Format type	Archive
Max files	Unlimited
File Allocation Table (FAT)	Embedded + External
Filenames?	Yes, 8.3
Metadata?	None
Supports compression?	No
Supports encryption?	No
Supports subdirectories?	No
Hidden data?	Yes
Games	Halloween Harry

The BNK (Bank) archive format is the format used by Halloween Harry to store a number of related game files. The format is trivial, being a collection of concurrent file chunks. Each BNK file has an associated FAT (File Allocation Table) file that provides quick access to the filenames, offset in the BNK file and size of the files stored in the BNK file. The FAT file however is usually not necessary to read data from the BNK file, all required information can be obtained by simply scanning said BNK file.

Files in the BNK file may or may not be compressed. This depends on what version of the BNK format is being used.

File format

Signature

Each file entry begins with a signature, so the very first file entry's signature can be used to verify the main file in in .BNK format. This means at offset zero, there should be a character of code 0x04, followed by the four characters "-ID-" (see file entry below for more details.)

Data type	Name	Description
BYTE[5]	signature	"\x04-ID-" (not null-terminated)

Note that there are two versions of the .BNK format - the original Halloween Harry version (with no compression) and the later Alien Carnage version (with compression.) The version can be detected by using the following method:

1. Read in the first file header (see file entry below), in Halloween Harry format (i.e. pretend the iDecompressedSize field was not present.)
2. Use that information to jump to the second file within the main .BNK file.
3. Check for the "-ID-" signature at the start of the second file entry. If it's there (at the expected offset), then it's a non-compressed Halloween Harry .BNK file.
4. If the "-ID-" signature is missing, try again four bytes further in (making up for the iDecompressedSize field that we ignored in the first step.)
5. If the "-ID-" signature is now present, it's a compressed Alien Carnage .BNK file.
6. If the "-ID-" signature still can't be found, it's not a valid .BNK file.

If there is a .FAT file and it is a multiple of 21, then it is in Halloween Harry format. Alien Carnage no longer uses the .FAT file.

FAT format

For Halloween Harry (not Alien Carnage), an external file (with the same name as the BNK file but with a .FAT extension) stores a summary of the files in the BNK file. It also stores file offsets for quick access. This file is not necessary to read the BNK archive (since the filenames are stored in both places) however it can provide a faster alternative for listing files compared to scanning through the entire BNK file. Of course if a BNK file is to be modified, then the FAT file will also need to be updated.

Alien Carnage no longer uses the FAT file. ! Is this data stored somewhere else instead or is it no longer required?

A FAT entry consists of a filename, offset and size.

Data type	Name	Description
UINT8	lenName	Length of the filename
char[12]	filename	Filename, 12 chars long, but the name itself is only lenName characters long. Not null-terminated, but null-padded if needed.
UINT32LE	fileOffset	Offset of the file from the start of the BNK
UINT32LE	fileSize	Size of the file in bytes

The very first file entry is at offset zero, and the above structure is repeated back-to-back until there are no more files. The offset given for each file is to the start of the file data itself, just after the header (in other words, the offset points to cData in the structure below.)

File entry

Each file entry in the BNK is similar to the entry in the external FAT file, with the addition of a signature and the omission of the offset, and of course the actual file data as well.

Data type	Name	Description
BYTE[5]	signature	"\x04-ID-" (not NULL-terminated)
UINT8	lenName	Length of the filename
char[12]	filename	Filename, 12 chars long, but the name itself is only lenName characters long. Not null-terminated, but null-padded if needed.
UINT32LE	fileSize	Size of the file in bytes
UINT32LE	decompressedSize	Alien Carnage only: Size of the file in bytes, once it has been decompressed (this field is not present in Halloween Harry)
BYTE[iFileSize]	data	File data

The very first file entry is at offset zero, and the above structure is repeated back-to-back until there is no more data in the .BNK file.

Compression format

The files are compressed with the PKWare DCL Implode algorithm. Note that this is different to the PKZip Implode algorithm.

Sample code:

• Go
  • blast (compress and decompress)
• C
  • ACXTRACT

Tools

The following tools are able to work with files in this format.

Name	Platform	Extract files?	Decompress on extract?	Create new?	Modify?	Compress on insert?	Access hidden data?	Edit metadata?	Notes
ACXTRACT	Any	Yes	Yes	No	No	No	N/A	N/A	Algorithm is decompiled so only works on little-endian machines
Camoto	Linux/Windows	Yes	No	Yes	Yes	No	N/A	N/A
Camoto/gamearchive.js	Any	Yes	No	Yes	Yes	No	N/A	N/A
Wombat	Windows	Yes	Yes	No	No	No	N/A	N/A

Credits

This file format was reverse engineered by Malvineous. If you find this information helpful in a project you're working on, please give credit where credit is due. (A link back to this wiki would be nice too!)