Securely email files in an encrypted dmg file on your Mac

Files on the disk drive, on USB disks, on thumb drives and attached to emails are easily accessed. How do you email them, transfer them, store them or carry them securely so no-one else can access them?

Emailing files is a problem if you want to keep them private, because anyone could intercept a message and read the contents. You might want to send personal information, secret files, or documents that should not be made public. A zip file with a password is one option, but it is not always the best one and there is an alternative.

Suppose you want to store files on a USB flash memory drive – thumb drive. You might worry about losing it and they are easily dropped, they can fall out of your bag or pocket, they are left on desks and they are easily stolen. Some method of securing the contents is required. Zip files with passwords are a possibility, but there are drawbacks and the contents of zip files are not easily accessed unless you unzip them, potentially leaving unprotected files around.

The same problem occurs with files, documents, photos and so on, on external drives. You want to protect them, but you still want easy access. Zip files is not the solution. OK, it is a solution, just not a very convenient one.

The best solution is a .dmg file. This is the file format that is commonly used for distributing Mac software on the internet, but it is not just for app downloads and you can create them and use them on your own Mac to store your own files.

There are several advantages of a .dmg file over a plain zip. One is that a .dmg file is mounted as a volume. Basically, it looks and works just like a disk drive, making it easy to browse and access the files it contains. You do not need to extract the contents and save them to disk, you access them directly.

A .dmg file can be compressed to make it smaller than the files it contains and this is useful for emailing or simply for squeezing more files onto your thumb drive. You could get 20GB of files onto a 16GB thumb drive for example. (Compression depends on the type of file though.)

A .dmg file can be encrypted with some of the most secure encryption around, making them impregnable to everyone who doesn’t have a supercomputer and about six months to spend trying to crack them.

Drag and drop to create .dmg

So how do you create .dmg files? Go to the Applications folder on your Mac’s disk and open the Utilities folder. Run Disk Utility. The window is not needed and it can be minimised to the Dock at the bottom of the screen.

Apple Mac Dock

Now drag a folder containing files to the Disk Utility icon in the Dock and drop it on it. This creates a .dmg container with the same name as the folder. Here a folder called Pics that contains lots of images was dragged from the Desktop and dropped onto Disk Utility in the Dock.

Create a dmg file

Change the name if you like, or leave it as it is. It is up to you. At the bottom are a couple of useful options and in the Image Format list are several choices.

Create a dmg file

The contents can be made read-only or read/write (like a disk), but here the format selected is compressed. This reduces the size and is ideal for email, exchanging files with people over the internet, and for thumb drives.

Next is the encryption to use. You don’t have to use any, but if you want to keep your files secure and prevent others from accessing them if they get hold of the .dmg file, then select either 128-but AES encryption or 256-bit. Choosing 256-bit is more secure, but it takes longer to create the .dmg file. If the files it contains are small,

you probably won’t notice, but if there are many megabytes of files, it will be noticeably longer to create the compressed and encrypted .dmg.

Create a dmg file on the Mac

The final step is to add a password. As with all passwords of course, you should make it hard to guess, long and complicated. If you ever forget it though, you will never be able to access the contents, so be careful!

Create an encrypted dmg file in OS X

The .dmg file is then created. Once it is finished, you can do whatever you want with it, such as email it, move it to a thumb drive and so on. If you or someone else tries to open it, you will be prompted for the password. Enter it and the .dmg file is added as a volume and is visible in Finder, in apps and so on.




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