Is there a way to look inside and modify an adb backup created file?

I created a backup of my Galaxy Nexus with adb backup. The resulting file is named backup.db and it's somehow encrypted.

I wanted to restore the backup, but it stops when it comes to restoring com.android.providers.contacts. I used adb logcat to find out what's going on and found out that com.android.acore crashes during the restore process.

I'd like to gain access to the data in the backup and remove the contacts database to restore everything back to my phone. Are there any other ways restoring the data from the backup?

Answer

The file is not encrypted, unless your specify so when creating the backup. It is however compressed (using deflate). You can find out the exact format by looking at Android source (com/android/server/BackupManagerService.java) code, and, technically, should be able to extract specific data from it. However, IIRC, there are some file integrity checks in place, so it most probably won't work if you just delete a bunch of data from it. Unfortunately the restore command doesn't seem to have an option to restore a particular app/package only or exclude a package.