How are apk files compressed and how can they be manually decompressed

darksky · November 23, 2024, 8:24pm

I tried to decompress one of the .apk files from my buildsystem using gzip, xz, zstd, bsdtar, and tar, but none of these can recognize the format. Does anyone know what compression is applied and how to manually decompress an OpenWrt build-system generated .apk file?

brada4 · November 23, 2024, 8:33pm

Try decompression tools as a first effort. libmagic file can teach you.
https://wiki.alpinelinux.org/wiki/Apk_spec

darksky · November 23, 2024, 8:41pm

% file shonenjump-0.8.0-r1.apk
shonenjump-0.8.0-r1.apk: data

And

% gzip -l shonenjump-0.8.0-r1.apk
gzip: shonenjump-0.8.0-r1.apk: not in gzip format

Yet from the link you posted

This is a set of commands to partially build a package. DO NOT DO THIS , it's mainly an example to see how this all fits together. Use the official build tools to build packages.

tar -c .PKGNIFO .pre-install | abuild-tar --cut | gzip -9 > $controldir/control.tar.gz
cd $pkgdir; tar -c * | abuild-tar --hash | gzip -9 > $controldir/data.tar.gz
cat $controldir/control.tar.gz $controldir/data.tar.gz > mypackage-1.0-r0.apk

So should be just two gz files catted together. I am not sure how to break them apart for decompression.

appelsin · November 23, 2024, 9:14pm

Magic is ADBd:

# curl -s 'https://downloads.openwrt.org/snapshots/packages/x86_64/packages/acme-4.0.0.apk' | dd bs=1 count=4 status=none | hexdump -C
00000000  41 44 42 64   |ADBd|

Does not exactly match magic from "adb.h" in apk-tools:

github.com

alpinelinux/apk-tools/blob/master/src/adb.h#L47


      
          #define ADB_VAL_TRUE		0x00000001
          #define ADB_VAL_FALSE		0x00000002
          
          #define ADB_NULL		ADB_VAL(ADB_TYPE_SPECIAL, ADB_VAL_NULL)
          
          /* Generic */
          #define ADBI_NUM_ENTRIES	0x00
          #define ADBI_FIRST		0x01
          
          /* File Header */
          #define ADB_FORMAT_MAGIC	0x2e424441	// ADB.
          #define ADB_SCHEMA_ANY		0
          #define ADB_SCHEMA_IMPLIED	0x80000000
          
          struct adb_file_header {
          	uint32_t magic;
          	uint32_t schema;
          };
          
          /* Blocks */
          #define ADB_BLOCK_ALIGNMENT	8

But does match this file:

github.com

alpinelinux/apk-tools/blob/master/src/adb_comp.c#L137


      
          		*spec = (struct adb_compression_spec) {
          			.alg = ADB_COMP_DEFLATE,
          		};
          	}
          
          	switch (spec->alg) {
          	case ADB_COMP_NONE:
          		return os;
          	case ADB_COMP_DEFLATE:
          		if (spec->level != 0) break;
          		if (apk_ostream_write(os, "ADBd", 4) < 0) goto err;
          		return apk_ostream_deflate(os, 0);
          	}
          
          	ci = compression_info_by_alg(spec->alg);
          	if (!ci) goto err;
          	if (spec->level < ci->min_level || spec->level > ci->max_level) goto err;
          
          	if (apk_ostream_write(os, "ADBc", 4) < 0) goto err;
          	if (apk_ostream_write(os, spec, sizeof *spec) < 0) goto err;
          	return ci->compress(os, spec->level);

So probably a DEFLATE-compressed ADB file.

(Contrarily ADB. might indicate uncompressed and ADBc might indicate zstd-compressed.)

For decompression, I would try apk-tools.

EDIT: manpage for apk extract is here:

EDIT2: Cannot figure out why "apk" was chosen as the project name when there is already another package format (android packages) with the same name.. Near impossible to google for apk commands.

EDIT3: Oh, I see they also use the YAML file format in some of their tools, so they are clearly just insane.

hnyman · November 23, 2024, 10:07pm

Note that Alpine .APK has been there since at least 2005, well before Android was initially launched...

appelsin · November 23, 2024, 10:12pm

Makes sense then, thanks! I'll direct my frustrations towards android next time hah