DISCLAIMER
This tutorial is for educational purposes only. The writer does NOT held any responsibility for any damages caused by this article and/or it’s implementation.

Hidden OS? Yup, just as the name suggest you can hide your operating system completely from anyone who don’t have the password to your system, as the original TrueCrypt documentation stated, provided that certain guidelines are fulfilled.

Technically speaking, Hidden OS is an OS which installed in a hidden volume of a partition which encrypted with TrueCrypt. This following picture illustrate ‘what’ is a Hidden OS is actually looks like:

Process of installing a Hidden OS is actually copying CURRENT system partition into a hidden volume. So if you’re hoping that you can make a new installation into the hidden volume, then I’m sorry to say that this is not really exactly what you’re looking about (perhaps future version of TrueCrypt will support this?)

For more information about TrueCrypt and Hidden OS, you can visit http://www.truecrypt.org.

PREPARATION
There are certain requirement must be met before installation of Hidden OS can be done, i.e:
1. The outer volume (Partition 2 at above illustration) must reside at the first position AFTER the system partition (Partition 1) of the disk.
2. The outer volume must be AT LEAST 5% larger than the system partition IF formatted as FAT, and 110% larger than the system partition IF formatted as NTFS.
3. By the way, installing a Hidden OS at the moment still can only be done from Windows , since the Linux version of TrueCrypt (latest version I used is 0.6) didn’t have “System” menu to start creating Hidden OS (I’ve spent 3 days reading carefully through the docs, verifying that it is really “System” menu they wrote on it since I can’t find it anywhere in the TrueCrypt GUI nor the command line version ).

Okay, here is a screen capture of my partition I prepared to install a Hidden OS:

As you can see my system partition is ~10 GiB and I’ve planned to use NTFS so the outer volume (labeled ‘HiddenPart’) need at least ~21 GiB. In this example I used ~24 GiB.

CREATING HIDDEN OS
1. From TrueCrypt GUI, select “System – Create Hidden Operating System”.
2. The next window will inform you the conditions which must be met in order for the Hidden OS creation to be done. Click “Yes” if you’re sure your system met the requirements.
3. Then choose the number of operating system. Since in this tutorial I used only one, I choose the “Single-boot” and then click “Next”.

4. Next is to create the outer volume (Partition 2 in above illustation). Click “Next” to move to the next step.
5. TrueCrypt will automatically recognize the first partition after the current system partition. This partition will be used as outer volume. Click “Next” to move to the next step.
6. Enter the password for the outer volume. Note that I intentionally check the “Display password” to show you what password I used for this tutorial. For your case, you probably want to keep it masked

7. The next step is to choose whether you want to format the outer volume to support files larger than 4 GiB. In this tutorial I choose “No”. You can choose whatever suits your need. Click “Next” when done.
8. Click “Format” whenever you are ready to format the outer volume. WARNING: all the data in the partition which will be formatted will be lost! Make sure you click “Format” button when there are no more important data in it. This formatting may take several minutes to hours, depending on the size of partition and the speed of the computer.
9. After the formatting complete, TrueCrypt will then asks you to define the password for the hidden operating system. Click “Next” after you done. Remember to make this password ‘exclusive’ since this password is the one you’ll use to activate the hidden OS when booting.

10. Next is to define the password for the decoy operating system. The password for the decoy OS is exact opposite to the password for hidden OS whereas the password for hidden OS should be held securely, while for the decoy OS is ’safe’ to reveal to anyone else.

11. After defining the passwords, TrueCrypt will begin to collect random data which will be used in encryption process (more detail about what is this pool of data means can be found in TrueCrypt docs). Move your cursor as random as possible to increase the ‘randomness’ of the data. Click “Next” when done.

12. The next window informs you about the generated key from previous step. Click “Next”.
13. TrueCrypt demands the user to create a Rescue Disk when creating a hidden OS as backup if somehow the bootloader is damaged or if you cannot boot into your system. Click “Next” and TrueCrypt will save the .iso file of the Rescue Disk in selected folder.

14. Burn the .iso into a CD/DVD and when you done, reeenter the CD/DVD so that TrueCrypt can verify whether the .iso already burned or not.
HINT: This step cannot be bypassed, so if you want to burn the .iso later, you can use some tools which allows loading an .iso to a virtual drive, so TrueCrypt can verify that the .iso is ‘already burned’.
15. Click “Next” on the next Window.
16. Click “Start” to start to copy the current system into the hidden volume. In order for doing this, system requires you to restart you computer. Save all of your work, and click “Yes” in the next Window asking “Do you want to restart it now?”.
17. After restarting, you will be bringed to a DOS-like screen at boot. Enter you HIDDEN OS password to start copying the OS in system partition into the hidden volume.

18. Then TrueCrypt will copy your OS in the system partition into the hidden volume. Depending on the size of partition, this can take a while.

19. After TrueCrypt done copying entire system partition, you will be then asked to boot into DECOY OS. Enter the password of your decoy OS in the screen like below, to boot into your decoy OS.

20. When the first time your decoy OS started, TrueCrypt will prompt you to encrypt the entire partition of decoy system.
Click “Encrypt” to start encrypting the decoy OS partition.

21. Done Your decoy OS and hidden OS are now ready to use. Now, whenever you boot your computer, you will be prompted at boot screen to enter a password. Whichever password you entered will define which system to start, e.g: if you enter the password for hidden OS then system will boot the hidden OS, same thing goes for decoy OS too.