Tuesday, January 20, 2015

Redacted font on Android emulator

I wanted to show the Gmail app in my partial SlidingPaneLayout article. However, I do not want to show my actual emails. Normally I would just blur out the sensitive data, but I want to show the cross-fade animation in a video, so that doesn't really work.

The easy way to get around this is to create a throw-away email account and show the fake data, but I came up with something more fun: replace the system font to show squiggly lines instead of words! I will be using Redacted, a beautiful squiggly font. I don't want to muck with my actual device, so I went with modifying an emulator.

yaff2 vs ext4

At first I followed this tutorial: How to pack and unpack system.img and userdata.img from an Android factory image.

The first step should be a giveaway that it may not work:

git clone https://android.googlesource.com/platform/system/extras
cd extras
git checkout android-4.1.1_r1

The tools described in the tutorial has been removed in the recent releases, so you have to check out an older tag.

In any case, I proceeded to mount system.img, replace Roboto-Regular.ttf with redacted-script-regular.ttf, and repackage the image with make_ext4fs. To compile make_ext4fs I needed to checkout platforms/system/core as well, and add its includes:

gcc -o make_ext4fs -I../../core/include -DANDROID \
  make_ext4fs_main.c make_ext4fs.c ext4fixup.c \
  ext4_utils.c allocate.c backed_block.c output_file.c \
  contents.c extent.c indirect.c uuid.c sha1.c \
  sparse_crc32.c wipe.c -lz

Next I created a new AVD, and copied the modified system.img into its folder. Alas, it did not work.

ko:Invalid cache partition image type: ext4 (expected yaffs2)

Turns out the Android 5.0 filesystem its in ext4, but the tutorial produces a yaff2 image.

Mount and sync

This tutorial has steps to make an ext4 image: How to create EXT4 images

But instead of creating a new image, I modified a copy of system.img like this:

cp ${ANDROID_HOME}/system-images/android-21/google_apis/\
armeabi-v7a/system.img .
mkdir my_system
sudo mount -t ext4 -o loop system.img my_system
cp redacted-script-regular.ttf my_system/fonts/Roboto-Regular.ttf

I copied the modified system.img into the AVD folder and booted. Yes, squiggly!


However I had two problems:

  1. How do I add a Gmail account in squiggly?
  2. Some strings were in squiggly but others were still in English

To solve the first problem, I moved the modified system.img out of the AVD folder and rebooted the emulator to configure Gmail in English. This works because the changes are stored in user data, which will not be overridden when I swap back in the modified system.img.

To solve the second problem, I overwrote more fonts.


Everything with Black or Bold got redacted-script-bold.ttf. Everything with Light or Thin got redacted-script-light.ttf. The rest got redacted-script-regular.ttf.


In case you are wondering, I did everything on my ubuntu machine. To record the video, I used SimpleScreenRecord. I chose "Select window..." to pick the emulator window, and adjusted top and height to crop out the window title. The height is the height of my emulated device, and I add the delta from height to top to compensate.

With all that, I have Gmail in squiggly, recorded to share!

1 comment:

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