Works: Kindle app on Android 11 - key extraction from encrypted backup with Linux

[rwingnut]

I got a new Samsung S6 lite tablet and spent some time working out the workflow to extract the key using Ubuntu 20. I don't have any Windows or Mac machines so I couldn't pursue any of the other solutions.

Here is what worked:

Setting Up Ubuntu 20

• Install android tools with ADB
  sudo apt-get install android-tools-adb

If you have problems with only getting a 41 byte download, here is the discussion of the problem:
https://android.stackexchange.com/questions/143367/creating-a-backup-with-adb-results-in-41-byte-file
So, replace the adb binary with version 1.0.31. The above link has a link to the download and the path is `usr/lib/android-sdk/platform-tools/adb

Backup tablet

Standard instructions you might find elsewhere:

• enable developer mode, enable usb debug
• if kindle app > 4.17.0 is installed
  • uninstall kindle app > 4.17.0
  • remove all kindle data
• sideload kindle app 4.17.0
• adb install kindle417.apk

Standard instruction for creating the backup:

• adb backup com.amazon.kindle

When I run adb backup com.amazon.kindle it requires me to encrypt the backup since my tablet is encrypted. So, I need to decrypt the backup before I can use it with DeDRM_tools plugin in Calibre.

Decrypt Backup

ab_decrypt provides the code to decrypt the backup, but it writes the results as an uncompressed .tar rather than a compressed .ab.
https://github.com/lclevy/ab_decrypt/blob/a7cac6f17e06da67041d9af8a737f137178e5d42/ab_decrypt.py

So, using the file linked above, I modify it to write the compressed file and re-add the backup header.
Replace the content of line 165 til the end with

print('writing backup as .ab ... ', end='')
out = open(options.output, 'wb')
out.write(b'ANDROID BACKUP\n1\n1\nnone\n')
out.write(decryptedData)
print('OK. Filename is \'' + options.output + '\'')
out.close()

Now, you can decrypt your backup using:
python ab_decrypt.py -b backup.encrypted.ab -p password -v 2

Now, when you load the backup into the DeDRM plugin in Calibre to extract the key, its happy sailing.

Thanks to everyone who has worked to build these tools.

[ElleKayEm]

I'm surprised that version of Kindle for Android will still download books. Thanks for sharing this!