How to create an encrypted file container with dm-crypt

October 1, 2013

Here are some instructions on how to create an encrypted filesystem on a file.

Create an empty file with the size of your container. Here I’ll use a 100MB container. The file is created with dd which reads chunks from an input device and writes the contents to a file or another device.

dd if=/dev/zero bs=1M count=100 of=~/my-container.img

This command means the following: read 100 chunks of one megabyte from the zero device /dev/zero and write them to the file ~/my-container.img. This will create a file named my-container.img in your home directory that will be about 100MB of zeros.

Next, we’ll initialise the LUKS partition on the file and set the initial passphrase.

sudo cryptsetup luksFormat ~/mycontainer

Note that you need to type “YES” (i.e. ‘yes’ in uppercase to confirm the operation; there is no error message when you fail this step which may be confusing. Make sure that the file you want to format is your container file or an empty partition’s device file. Input your passphrase when prompted. You will have to input this passphrase whenever you mount the container unless you decide to store the passphrase with the container (which obviously is not very safe). Note that you have to run this as root, because cryptsetup must access the loopback device. (On the Hurd this would not be necessary, I think.)

Now, we’ll open the container. Opening the container creates a kernel device file which can then be mounted.

sudo cryptsetup luksOpen ~/mycontainer secret-device

This command will prompt for the container’s passphrase and then create a device file with the name /dev/mapper/secret-device. You may choose another name than “secret-device”.

The container is now decrypted. Since the device has no filesystem yet we still cannot put any data on it. Use mkfs.ext4 to create an ext4 filesystem on the decrypted container:

sudo mkfs.ext4 /dev/mapper/secret-device

Now the filesystem can be mounted like a filesystem on a regular block device.

mkdir ~/my-mount-point
sudo mount /dev/mapper/secret-device ~/my-mount-point

The first command creates a new mount point (an empty directory) named “my-mount-point” in your home directory. The second command mounts the decrypted device at this location.

You can now write to the directory as usual. Once you are done follow these steps to unmount the device and close (= re-encrypt) the container:

sudo umount ~/my-mount-point
sudo cryptsetup luksClose secret-device

To access the container again only these two commands are required:

sudo cryptsetup luksOpen ~/mycontainer secret-device
sudo mount /dev/mapper/secret-device ~/my-mount-point
  1. You could use /dev/random as the input device if you wanted to, but that would be considerably slower and wouldn’t help you much. Later commands will initialise the file/partition, so you don’t need to initialise it manually with random numbers.

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