Android - How do you extract an App's data from a full backup made through "adb backup"?

How to extract ab files

There is an open source project under the Apache 2.0 license, written by Nikolay Elenkov that will allow you to extract the .ab in to a tar file.

Usage:

java -jar abe.jar unpack <backup.ab> <backup.tar> <password>

Background

Just for reference of others, here is some background on the .ab file format.

The Android Backup (*.ab) file is a compressed TAR file. It is compressed using the DEFLATE algorithm. On top of that, there can be AES encryption used. This is determined when you create the backup, if you enter a password then the backup is encrypted, otherwise; there is no encryption, it is only compressed.

The HEADER of the file is a little different than a normal DEFLATE archive. It contains information about the backup and looks like the following:

ANDROID BACKUP
1
1
none

The first line is the "Magic" line. The next line is the version of the Android Backup file format. The next line is a boolean (true or false, 1 or 0) indicating if the file is compressed. The last line is the type of encryption. This example is not using any encryption. If there was a password, the line would read "AES-256". After that is the encryption cipher. If no password, then the DEFLATE "archive" starts.

It is compressed using the Java Deflater. Which, from a developers perspective, causes issues if you want to use anything besides Java to extract it. I haven't been able to find anything that can deflate it using the same algorithm, even though all that I have found (for like C#) are supposed to follow the "SPEC".

If you are not sure how to really use that (which is beyond the scope of this answer) Droid Explorer since v0.8.8.7 (available here) allows you to do exactly this, and more, right from Explorer. You can read more about the features on my blog (yes, i know, shameless plug. I do that when it fits the question)

Or with a one-liner:

( printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" ; tail -c +25 backup.ab ) |  tar xfvz -

One more option is to use bash, cat and gunzip (gzip).

The full process could be this (with an unencrypted backup):

1. backup one app's data (for example "Override DNS for KitKat"):

   $ adb backup -f net.mx17.overridedns.ab -noapk net.mx17.overridedns
   Now unlock your device and confirm the backup operation.
   

2. extract the compressed data

   $ dd if=net.mx17.overridedns.ab bs=1 skip=24 > compressed-data
   1285+0 records in
   1285+0 records out
   1285 bytes (1,3 kB) copied, 0,00745877 s, 172 kB/s
   

3. decompress the compressed data

   $ printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" \
       | cat - compressed-data | gunzip -c > decompressed-data.tar
   gzip: stdin: unexpected end of file
   

4. "untar" the tar file

   $ tar xf decompressed-data.tar