Securely email files using encryption or password protection on a Mac

Files 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 to someone, work files or documents that should not be made public, and so on. A zip file that is password protected is one option, but there is no obvious way to create them in macOS and you need a third party utility. It is not always the best method and there is an alternative that works on the Mac without extra software.

Suppose you want to store files on a USB flash memory drive or thumbdrive. 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 as with email, but there are drawbacks and you cannot view or edit documents in a zip. without unzipping it and copying the files elsewhere.

The same problem occurs with files, documents, photos and so on, stored 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.

One 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 might get as much as 40 GB of files onto a 32 GB thumb drive for example. The amount of compression depends on the type of file and the .dmg file format adds a bit to the size. You get better compression with a lot of files rather than a few.

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

Create .dmg to encrypt files for emailing

So how do you create .dmg files? Go to the Applications/Utilities folder on your Mac’s disk and run Disk Utility. This is a multi-function tool with many uses relating to disks. One use is for making .dmg files.

Put one or more files you want to protect or email into a folder. Then go to the File menu and select New Image, Image from folder. (Blank Image works, too.)

Select the folder to turn into a .dmg file. Here I have put the files into a folder called Files in the desktop, but it could be anywhere, such as in the Documents folder. This will create a .dmg container with the same name as the folder.

Change the filename if you like, or leave it as it is. It is up to you. Click Encryption to show the menu. You don’t have to use any encryption with a .dmg disk image, 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-bit 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 take noticeably longer to create the encrypted .dmg.

You are then prompted to enter 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 carefull. If you are emailing this .dmg file to someone, they will need to know the password to open it and access the files it contains. Don't email the password with the .dmg file for obvious reasons.

At the bottom are a couple of useful options and in the Image Format menu the contents of the .dmg file can be made read-only or read/write (like a disk). This is useful for storing files on your Mac's drive, but if you will be emailing the file then the format to select is compressed. This reduces the size and is ideal for email and exchanging files with people over the internet using messaging image.

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 or they will be prompted for the password. Enter it and the .dmg file is added as a volume, like a disk, and is visible in Finder, apps and so on. The files are then easily browsed and accessed through finder and apps.

Create password protected zips on the Mac

Another way to protect files when emailing them, sending them via messaging or when carried on thumb drives, is to use a password protected zip archive. You can create unprotected zips on the Mac, just Ctrl+click one or more files and select Compress on the menu. However, there is no obvious way to add a password to stop unauthorised people from accessing the files if they got hold of the zip.

Keka is a Mac utility that enables you to create password protected zip files. It is free to download from the website, but $2.99 in the Mac App Store. I use the free version here.

It is a little odd and when it is run, it shows a window with settings. Choose the ZIP archive format in menu in the top right corner. Then enter a password to use with the zip you will create. Leave this window on the screen.

To actually create a zip, drag one or a whole bunch of files and drop them on the Keka icon in the Dock at the bottom of the screen. A window then pops up that enables you to set the filename and choose where to save the zip. The default save location is the same place you dragged the files from. That's it. You can do whatever you want with the zip, like email it, carry it on a thumb drive and so on.

Keka is not needed to access the files in a password protected zip. No software is needed and you just double click the zip file in Finder, macOS knows how to handle them.

You are prompted to enter a password for zip file that are password protected. Click OK and the files are extracted to a folder. I found another free utility that created password protected zip files, but it required the utilty to unzip them, Keka does not, so you can email the zips to people and they don't need anything special to unzip them.